Silicic Magma Reservoirs in the Earth’s Crust · 1 Silicic Magma Reservoirs in the Earth’s Crust 2 3 ... 69 postulate causes for magma migration, storage, and interaction at different

ReviewMUSH 4/19/2016 1

Silicic Magma Reservoirs in the Earth’s Crust 1

2

3

Olivier Bachmann, Institute of Geochemistry and Petrology, ETH Zurich, 80924

Switzerland5

Christian Huber, School of Earth and Atmospheric Sciences, Georgia Institute of6

Technology,GA30332,USA7

Outline (for reviewing purpose only) 8

1Abstract9

2Introduction10

3Howcanwestudymagmareservoirs11

3.1Samplingvolcanicandplutoniclithologies12

3.2Studyingactivevolcanoes,gasandgeophysicalmeasurements13

3.3Reproducingtheconditionsprevailinginmagmachambersinthe14

laboratory15

Experimentalpetrology:16

a) Phaseequilibriumexperiments17

b) Rheologicalexperiments18

Rheologicalstudies19

Tankexperimentsformagmachamberprocesses20

3.4Overviewofmagmareservoirdynamicsfromtheoreticalmodels21

4Modelofmagmareservoir:focusingonthe“Mush”22

4.1Presenceofcrystal-poorrhyolites,andzonedignimbrites23

4.2The“Monotonousintermediates”-remobilizedmushes24

4.3Theunzoned,crystal-poortointermediatedeposits25

4.4Chemicalfractionationinmushyreservoirs:thedevelopmentof26

compositionalgapsinvolcanicsequences27

4.5Plutondegassingandvolatilecycling28


4.6Thehigheruptabilityofrhyoliticmeltpockets29

4.7TheExcessSparadox30

4.8Calderacycles31

5.Timescalesassociatedwithmagmareservoirs32

6.Frontiersinourunderstandingofmagmachamberevolution33

7Outlook34

8Acknowledgement35

9References36

37

Totalof15figures.38

39

1 Abstract 40

Magma reservoirs play a key role in controlling numerous processes in planetary41

evolution, including igneous differentiation and degassing, crustal construction and42

volcanism.Fordecades,scientistshavetriedtounderstandwhathappensinthesereservoirs,43

usinganarrayoftechniquessuchasfieldmapping/petrology/geochemistry/geochronology44

onplutonicandvolcaniclithologies,geophysicalimagingofactivemagmaticprovinces,and45

numerical/experimentalmodeling.This reviewpaper tries to follow thismulti-disciplinary46

frameworkwhilediscussingpastandpresent ideas.Wespecifically focuson recent claims47

thatmagma columnswithin the earth’s crust aremostly kept at high crystallinity (“mush48

zones”), and that the dynamicswithin thosemush columns, albeitmodulated by external49

factors(e.g.,regionalstressfield,rheologyofthecrust,pre-existingtectonicstructure),play50

an important role in controlling how magmas evolve, degas, and ultimately erupt. More51

specifically,weconsiderhowthechemicalanddynamicalevolutionofmagmaindominantly52

mushy reservoirs provides a framework to understand (1) the origin of petrological53

gradientswithindepositsfromlargevolcaniceruptions(“ignimbrites”),(2)thelinkbetween54

volcanic and plutonic lithologies, (3) chemical fractionation of magmas within the upper55

layersofourplanet,includingcompositionalgapsnoticedacenturyagoinvolcanicseries(4)56

volatilemigrationandstoragewithinmushcolumnsand(5)theoccurrenceofpetrological57

cyclesassociatedwithcaldera-formingeventsinlong-livedmagmaticprovinces.Therecent58


advances inunderstanding the innerworkingsof silicicmagmatismarepaving theway to59

exciting future discoveries, which, we argue, will come from interdisciplinary studies60

involving more quantitative approaches to study the crust-reservoir thermo-mechanical61

couplingaswellasthekineticsthatgoverntheseopensystems.62

63

2 Introduction 64

Determiningtheshape,size,depth,andstateofmagmasbodiesintheEarth’scrustas65

well as how they evolve over time remain key issues for a number of Earth Science66

communities.With a better determination of these variables, petrologists could construct67

more accurate chemical models of differentiation processes; magma dynamicists could68

postulatecausesformagmamigration,storage,andinteractionatdifferentlevelswithinthe69

crust; volcanologists could better predict the style and volume of upcoming volcanic70

eruptions; and geochronologists could construct an evolutionary trend with greater71

precision.Althoughourknowledgeonmagmaticsystemshasexpandedgreatlyoverthepast72

decades,muchremains tobedone toconstrain themulti-phase,multi-scaleprocesses that73

areatplayinthosereservoirsandtheratesatwhichtheyoccur.74

75

Previousattemptstosummarizethestateofknowledgeinthisfield(e.g.,Smith1979;76

Lipman 1984; Sparks et al. 1984; Marsh 1989a; de Silva 1991) have greatly helped77

crystallizingideasonthecontemporaneousstateofknowledgeandpossiblealleysforfuture78

directions to explore. Since then,manymorepublicationshaveattempted todrawmagma79

reservoirs geometries and internal complexities (see Figure 1 for a potpourri of such80

schematicdiagramsforseveralexamplesoflargecalderasystemsintheUSA,aswellasthe81

morerecentreviewsbyPetfordetal.2000;Bachmannetal.2007b;Lipman2007;Cashman82


andSparks2013;CashmanandGiordano2014;deSilvaandGregg2014).Atthisstage,itis83

clear that magmas are complex mixtures of multiple phases (silicate melt, H2O-CO2-84

dominatedfluid,andupto10differentmineralphasesinsomesystems)withverydifferent85

physical properties. For example, viscosity contrast betweenwater vapor and crystals can86

spanmorethan20ordersofmagnitude,andevenvarysignificantlywithinonegivenphase87

(e.g.,severalordersofmagnitudeforsilicatemelt),ascomposition-P-T-fH2O-fO2vary(Mader88

et al. 2013). Theway thesemagmasmove and stall in the crustwill therefore reflect the89

complexinteractionbetweenallthesephasesandthetypicallycolderwallrocks.90

91

Over the last decades, an impressive body of work has accumulated on the92

characterizationofmagmamainly through (1) high-precision, high-resolution geochemical93

work (with ever more powerful instruments, including the atom-probe, e.g., Valley et al.94

2014 and nanoSIMS in the recent past; e.g., Eiler et al. 2011; Till et al. 2015) and (2)95

experimental petrology (e.g., phase diagrams, trace element partitioning between phases,96

isotopic fingerprinting), culminating in powerful codes for thermodynamic modeling97

(GhiorsoandSack1995a;Boudreau1999;Connolly2009;Gualdaetal.2012).Thisworkwill98

continue to greatly improve the constraints thatwe place on thesemagmatic systems for99

years to come, but we argue that the field will have to take into account kinetic,100

disequilibrium processes, which are commonplace, and can be severe in the world of101

magmas.102

103

Thermodynamics andkinetics areboth valuable tounderstandmagmaticprocesses104

andkineticsisboundtobecomeevenmoreimportantinthefollowingdecadesaswestrive105


tobetter constrain the timescaleofdynamicalprocessesassociatedwithmagma transport106

andevolutioninthecrust.ThisCentennialVolumeofAmericanMineralogistappearsasan107

appropriate landmark to summarize the recent advances on the subject, and bring the108

multiplefacetsofthisproblemtogethertobetterframethefuturedirectionsofresearch.109

110

Figure1:Recent schematicdiagrams for the threebigactive caldera systems in theWestern111

USA.Therearemanyothercalderasystemsaroundtheworld(e.g.,HughesandMahood2008),112

andthefocusonexamplesfromtheUSAhereissolelyduetopersonalacquaintance.(a)Valles113

caldera;Wilcock et al. 2010, (b) LongValley caldera,Hildreth 2004, (c) Yellowstone,Official114

websiteandLowensternandHurvitz2008.115

The most exciting scientific challenges are, we believe, those that are driven by116

enigmas arising by observations ofNature. Below are a few examples of key questions or117

long-standingobservationsthatrelatetomagmareservoirsandmagmaticdifferentiation:118

119


1. Thefundamentaldichotomyinsupervolcanicdeposits(alsocalledignimbritesorash-120

flow tuffs), which show, in most cases, (A) compositionally and thermally zoned121

dominantlycrystal-poorunits(sometimesgradingtocrystal-richfaciesattheendof122

the eruption, Lipman 1967; Hildreth 1979; Wolff and Storey 1984; Worner and123

Schmincke1984;deSilva andWolff1995)and (B) strikinglyhomogeneous crystal-124

rich dominantly dacitic units (the “Monotonous Intermediates” of Hildreth 1981),125

known tobesomeof themostviscous fluidsonEarth (WhitneyandStormer1985;126

Francis et al. 1989; de Silva et al. 1994; Scaillet et al. 1998b; Lindsay et al. 2001;127

Bachmannetal.2002;Bachmannetal.2007b;Huberetal.2009;Folkesetal.2011b;128

seealsoBachmannandBergantz2008b;Huberetal.2012aforreviews).129

2. The striking resemblance in chemical composition (for major and trace elements)130

between the interstitial melt in crystal-rich silicic arc magmas and the melt-rich131

rhyoliticmagmas(BaconandDruitt1988b;HildrethandFierstein2000;Bachmannet132

al.2005).133

3. Theubiquitousobservationofmagmarecharges fromdepthprior toeruptions,and134

their putative role on the remobilization of resident (often crystal-rich) magmas135

(“rejuvenation”),forlargeandsmalleruptions(e.g.,Sparksetal.1977;Pallisteretal.136

1992;Murphyetal.2000;Bachmannetal.2002;BachmannandBergantz2003;Wark137

etal.2007;Molloyetal.2008;Bachmann2010a;CooperandKent2014;Klemettiand138

Clynne2014;WolffandRamos2014;Wolffetal.2015).139

4. The long-noticed compositional gap in volcanic series, coined the Bunsen-Daly Gap140

afterR.BunsenandR.Daly’searlyobservations(Bunsen1851;Daly1925;Daly1933),141

and confirmed in many areas worldwide since (Chayes 1963; Thompson 1972;142


Brophy1991;Thompsonetal.2001;Deeringetal.2011a;Szymanowskietal.2015)143

evenwherecrystalfractionation,whichshouldleadtoacontinuousmeltcomposition144

atthesurface,dominates(Bonnefoietal.1995;Geistetal.1995;Peccerilloetal.2003;145

Macdonaldetal.2008;DufekandBachmann2010;Sliwinskietal.2015).146

5. The complex relationship between silicic plutonic and volcanic units with possible147

hypotheses (not mutually exclusive in plutonic complexes) ranging from plutons148

being“failederuptions”(crystallizedmelts;Tappaetal.2011;Barbonietal.;Glazner149

et al. 2015; Keller et al. 2015) to plutons being “crystal graveyards” (crystal150

cumulates; Bachl et al. 2001; Deering and Bachmann 2010; Gelman et al. 2014;151

Putirka et al. 2014; Lee and Morton 2015). Related to this volcanic-plutonic152

connection is the corollary that granites sensu stricto (evolved, high-SiO2, low Sr153

composition)appeartobelesscommonthantheirvolcaniccounterparts(e.g.,Navdat154

database,Hallidayetal.1991;Hildreth2007;Gelmanetal.2014).155

6. Theobservationthatsilicicplutons,whenfullycrystallized,containlittlevolatileleft156

(typically less than 1wt%H2O that is bound up in theminerals; e.g., Caricchi and157

Blundy2015)despitethefacttheystartedwithvolatileconcentrationsabove6wt%,158

implyingsignificantlossofvolatileatdifferentstagesofthemagmabodies’evolution159

(e.g., by first and second boiling, pre-, syn-, and post-eruption degassing, see for160

example Anderson 1974; Wallace et al. 1995; Candela 1997; Lowenstern 2003;161

Webster2004;Wallace2005;Bachmannetal.2010;Blundyetal.2010;Sillitoe2010;162

BakerandAlletti2012;HeinrichandCandela2012).163

7. Thewell-known“ExcessS”,highlightedbythemuchhigherScontentreleasedduring164

eruptions(measuredbyremotesensingorestimatedfromicecorerecords)thatcan165


beaccountedforbythemeltinclusiondata(i.e.petrologicestimate,see,forexample,166

reviewsbyWallace2001;Costaetal.2003;Shinohara2008).167

8. The remarkable cyclic activity observed large-scale volcanic systems, which168

culminates in caldera-forming eruptions (“caldera cycles”) that can repeat itself169

severaltimes,e.g.,themulti-cycliccalderasystemssuchasYellowstone(Hildrethetal.170

1991;BindemanandValley2001;Christiansen2001;LowensternandHurvitz2008),171

Southern RockyMountain Volcanic field (e.g., Lipman 1984; Lipman 2007; Lipman172

and Bachmann 2015), Taupo Volcanic Zone (e,g,,Wilson 1993;Wilson et al. 1995;173

Suttonetal.2000;Deeringetal.2011a;Barkeretal.2014),HighAndes(e.g.Pitcheret174

al. 1985;Petfordet al. 1996;Lindsayet al. 2001; Schmitt et al. 2003;deSilvaet al.175

2006;KlemettiandGrunder2008),CampiFlegreii(e.g.Orsietal.1996;Civettaetal.176

1997;Gebaueretal.2014)andAegeanarc(e.g.Bachmannetal.2012;Degruyteretal.177

2015).178

179

Allthosequestions/observationsrequireanoverarchingconceptofmagmachamber180

growth and evolution that ties together these seemingly disparate concepts. Before181

attemptingsuchatask,thesectionbelowbrieflyoutlinesthemethodsthataretypicallyused182

toinferthestateofthesereservoirs,highlightingsomeoftheirstrengthsandlimitations.183

184

3 How can we study magma reservoirs? 185

3.1 Sampling volcanic and plutonic lithologies 186

The foundation of volcanology is motivated by observations of ongoing and past187

volcanic activity,with the purpose of understanding the underlying processes that govern188


the evolutionofmagmatic systemsonEarth andotherbodies of the solar system (Wilson189

andHead1994).However, large-scaleeruptionsarerare(Simkin1993;Masonetal.2004)190

and as volcanologists/petrologists we rely much on a forensic approach: studymagmatic191

rocks (bothplutonicandvolcanic) that formed/erupted in thepast, and try to reconstruct192

the conditions (P, T, fO2, fH2O, fSO2, …) that prevailed in the reservoirs at the time of193

formationoreruption(see, forexample,areviewoftechniquesbyPutirka2008orBlundy194

and Cashman 2008). Of course, volcanic and plutonic lithologies do not provide the same195

type of information, and could/should be complementary. Volcanic rocks carry limited196

spatialcontextforthereservoir,butprovideaninstantaneoussnapshotintothestateofthe197

magmabody justbefore theeruption. Incontrast,plutonicrockspresenta time-integrated198

image of themagma accumulation zone, oftenwith a history spanningmulti-million years199

(e.g.,Greeneetal.2006;Walkeretal.2007;Schoeneetal.2012;Cointetal.2013),forwhich200

wecanteaseoutsome informationaboutsizesandshapesofmagmasbodies.Muchof the201

geochemicaldataacquiredoverthelastcenturyisnowtabulatedinlargedatabasessuchas202

Georoc (http://georoc.mpch-mainz.gwdg.de/georoc/), EarthChem203

(http://www.earthchem.org),andNavdat(http://www.navdat.org),providingaremarkable204

resource toanalyzeglobalgeochemicalproblems(seerecentpapersofKellerandSchoene205

2012;Chiaradia2014;Gelmanetal.2014;Glazneretal.2015;Kelleretal.2015).206

207

3.2 Studying active volcanoes, gas and geophysical measurements 208

Measuring gases coming out of active volcanoes provides us with some key209

information about the state ofmagmatic systems (e.g., Giggenbach 1996; Goff et al. 1998;210

Edmondsetal.2003;Burtonetal.2007;Humphreysetal.2009).Due to their lowdensity211


andviscosity,volcanicgasescanescapetheirmagmatictrapsandprovide“atelegramfrom212

theEarth’sinterior”,aslaidoutbyoneofthepioneersofgasmeasurements,SadaoMatsuo213

(e.g.,Matsuo1962).Forexample, theabundanceofchemicalspecies likeHe,CO2,orClcan214

helpdeterminethetypeofmagmathatisdegassing(Shimizuetal.2005),andthepotential215

volumethatistrappedinthecrust(LowensternandHurvitz2008).Asmentionedabove,the216

amountofSreleasedduringeruptionsisalsokeyinestimatingthepresenceofanexsolved217

gasphaseinthemagmareservoirpriortoaneruption(Scailletetal.1998a;Wallace2001;218

Shinohara 2008). As discussed by Giggenbach (1996), the composition of volcanic gases219

measuredat thesurface integratesbothdeep, i.e.magmareservoir,processesandshallow220

processes within the structure of the edifice and possibly the interaction with a221

hydrothermalsystem.Assuchitisoftenchallengingtodirectlyrelategascompositiontothe222

conditions of shallow magma storage (e.g., Burgisser and Scaillet 2007) and a better223

monitoring strategy is generally to look for relative changes in gas compositions and224

temperatureratherthanfocusingonabsolutevalues.225

226

With the exception of gas measurements, imaging active magma reservoirs mainly227

involves geophysicalmethods. Several techniques arenowavailable, andprovidedifferent228

pictures of activemagmas bodies. Such techniques include (1) seismic tomography (both229

activeandpassivesources;seeforexampleDawsonetal.1990;Lees2007;WaiteandMoran230

2009; Zandomeneghi et al. 2009; Paulatto et al. 2010, ambient noise tomography, e.g.,231

Brenguieretal.2008;Fournieretal.2010;Jayetal.2012),(2)Magneto-Telluricsurveys(e.g.,232

Hill et al. 2009; Heise et al. 2010), (3) deformation studies using strain-meter, GPS, and233

satellitedata(e.g.,Massonnetetal.1995;Hooperetal.2004;Lagiosetal.2005;Newmanet234


al.2006;Hautmannetal.2014),(4)muontomographyonsteepvolcanicedifices(Tanakaet235

al.2007;Gibertetal.2010;Marteauetal.2012;Tanakaetal.2014).236

237

Geophysicalmethods arebasedon the inversionof signals transmitted through the238

crustandmeasuredfromorclosetothesurface.Thechoiceofmethodsand,inseveralcases,239

the frequencybandof thesignalsstudieddependson the targetedresolutionanddepthof240

interest. Geophysical methods probe variations in physical properties caused by the241

presenceofpartiallymoltenreservoirsinthecrustandprovide2or3dimensionalimagesof242

these systems. These inversions however do not provide an unequivocal picture of the243

thermodynamicalstateofthemagmastorageregion,becauseseveralvariables(meltfraction,244

temperature, exsolved gas content, composition, connectivity of the various phases) affect245

the elastic, magnetic and electric properties of magmas. Similarly, surface deformation246

signalsarenotonly functionof thestate,shape,orientationandsizeofamagmabody,but247

also depend on the crustal response to stresses around the body (through anelastic248

deformationandmovementalongweaknessplanessuchasfracturesandfaults;Newmanet249

al.2006;Greggetal.2013).250

251

Insummary,geophysicalsurveysmainlyprovideconstraintsaboutthedepth,shape252

andextentofapartiallymolten(orhot)regionunderanactivevolcanicedifice.Themany253

surveys using inSAR, GPS field campaigns or ambient noise tomographywhere, as for gas254

monitoring,thefocusliesonrelativechangesratherthantheeffectivestateofthereservoir,255

have significantly increased our ability to monitor volcanic unrest (see recent review by256

Acocella et al. 2015). They efficiently highlight rapid (decadal or less) changes in the257


mechanical state of magmatic systems, which is generally difficult to achieve using258

petrologicaldatasets.However,thespatialscalesprobedbygeophysicalimagingtechniques259

are limited by the resolution of the method considered. Under most circumstances, the260

spatial resolution remains greater or equivalent to the kilometer scale (Waite and Moran261

2009;Farrelletal.2014;Huangetal.2015),which is instarkcontrastwith theextremely262

highresolutionofgeochemicalandpetrologicaldata.Assuch,geophysicalinversionscanbe263

thought of as upscaling filters of the state of heterogeneous multiphase reservoirs. A264

fundamental assumption underlying these inversions is that spatial resolution is265

commensurable with a Representative Elementary Volume (REV) where the physical266

propertiesoftheheterogeneousmediumcanbe(linearly)averagedoutintoasingleeffective267

value(amoredetaileddiscussionofthelimitationsimposedbythisassumptionisprovided268

insection6).269

270

3.3 Reproducing the conditions prevailing in magma chambers in the laboratory 271

Experimentalpetrology:272

273

a) Phaseequilibriumexperiments274

275

Starting with N.L. Bowen at the turn of the 20th century, many geoscientists have276

applied experimental studies to study the thermodynamics that control the assemblage of277

phasesinmagmaswithdifferentbulkcompositions.Fornearlyacentury,alargeamountof278

data has been collected on crystal-melt stability and composition as a function of several279

variables(P,T,fugacitiesofvolatileelementsandoxygen,compositionsofmagmas,cooling280


rate,…).Muchofthisdataisnowavailableindatabases,someinweb-basedportals,asisthe281

case for thegeochemicaldata(see forexampleBerman1991;HollandandPowell2011or282

LEPR, http://lepr.ofm-research.org/YUI/access_user/login.php). The data sets range from283

crystal-melt equilibria in the mantle melting zones (e.g., see Poli and Schmidt 1995, and284

pMELTSdatabase,http://melts.ofm-research.org/Database/php/index.php)tolowercrustal285

(e.g.,MuntenerandUlmer2006;Alonso-Perezetal.2009)anduppercrustal conditions in286

manydifferentsettings(e.g.,JohnsonandRutherford1989;JohannesandHoltz1996;Scaillet287

and Evans 1999; Costa et al. 2004; Almeev et al. 2012; Gardner et al. 2014; Caricchi and288

Blundy2015).289

290

For shallow magma reservoirs in arcs, a third phase, volatile bubbles, is also291

commonly present. This exsolved volatile phase does not only affect the thermo-physical292

properties of magmas (see below) but can also impact greatly the partitioning of some293

volatiletraceelementsandthereforeinfluencethechemicalfractionationthattakesplacein294

magma reservoirs. Laboratory experiments have played a major role in determining the295

solubilityofvolatilephases(H2O,CO2,S,CL,F,…)asafunctionofpressure,temperatureand296

meltcomposition(seethereviewbyBakerandAlletti2012andreferencestherein).Several297

species, such as Cl and S compounds, have a strong influence on the fractionation and298

transportofpreciousmetalsoutofmagmareservoirs, to the siteoforedepositsandhave299

therefore receiveda thoroughattention (Scaillet et al. 1998a;Williams-Jones andHeinrich300

2005;Zajaczetal.2008;Sillitoe2010;Fiegeetal.2014).301

302


Chemicalequilibriumbetweenthemeltandnewlyformedcrystalsorcrystalrimsis303

oftenadecentassumption,but thisassumptionbreaks loosewhenchanges takingplace in304

the reservoir aremore rapid than the equilibration timescale (see Pichavant et al. 2007).305

Suchrapidchangesinreservoirs’conditionscaninclude:(1)magmarechargesandpartial306

307

308

309

310

311

(incomplete)mixing,(2)efficientphysicalseparationofmineraland/orbubble-melt,312

and (3) during rapid pressure drops associated with mass withdrawal events (eruptions,313

dikepropagationoutof the chamber). In theseparticular cases,kinetics controls themass314

balancebetweenthedifferentphases;onethereforeneedstoconsiderdiffusionofelements315

in silicate melts and minerals, as well as mineral and bubble growth or dissolution. The316

diffusive transportof chemical species in silicatemelts isa strong functionof (a)viscosity317

(whichisaproxyforthedegreeofpolymerizationofthemelt),(b)dissolvedwatercontent,318

and(c)toalesserextent,oxygenfugacity(redoxconditionsinthemagmaaffectspeciation).319

Webringtheattentionof the interestedreadersto thereviewofY.ZhangandD.Cherniak320

(ZhangandCherniak2010)foradditionalinformationonthistopic.321

322

Thepresenceofkineticsisreadilyobservedinmineralzoningpatternsbecausesolid-323

statediffusionisgenerallyordersofmagnitudeslowerthandiffusioninsilicatemelts.Over324

thelastdecade,severalmodelshavebeendevelopedtousethediffusionofcationsinsilicate325


minerals and retrieve timescales relevant to thedynamic evolutionofmagmasbefore and326

duringeruptions.Thesemodelsrestheavilyonexperimentswherethekineticsofdiffusion327

asfunctionoftemperatureandcompositionismeasured(SeereviewofZhangandCherniak328

2010 and reference therein). Diffusionmodeling inminerals informs us on the interval of329

timebetweenasignificantchangeinthermodynamicconditionsofthemagma(mixingfrom330

recharges for example) and the time atwhich theminerals transitions across the closure331

temperature (e.g., eruption) where subsequent diffusion can be ignored. To date, studies332

have explored the diffusion of a growing quantity of cations in hosts such as quartz,333

plagioclase,pyroxene,biotiteandolivines(formoremaficsystemsthantheonesconsidered334

here;SeeFigure2forpublishedexamples).335

336

ReviewMUSH 4/19/2016 16337


338

339

Figure2: Examples of diffusionalprofiles inquartz crystals fromTaupo,NZ (Matthews et al.340

2012), in pyroxene crystals from the Bishop Tuff, Ca, USA (Chamberlain et al. 2014a) and341

plagioclasecrystalsfromCosigüina,Nicaragua(Longpreetal.,2014).342

343

Stableisotopescanalsoprovideinformationregardingthethermodynamicconditions344

(equilibrium fractionationbetweendifferentphases) andkinetics (kinetic fractionation) in345

themagma.Becauseofimprovedaccuracyandspatialresolutionofanalyticalmeasurements,346

thereisagrowinginterestinusingdifferentstableisotopesystems,suchasO,S,Caaswellas347

othermajorelements,tofingerprinttheoxidationstateofmagmas(MetrichandMandeville348

2010; Fiege et al. 2015), crustal assimilation (e.g., Friedman et al. 1974; Taylor 1980;349

Hallidayetal.1984;Eileretal.2000;BindemanandValley2001;Boroughsetal.2012),or350

mixingprocessesbetweenmagmaswithdifferentcompositions(Richteretal.2003;Watkins351

etal.2009a).352

353

b) Rheologicalexperiments354

355

Themigration, convection anddeformationofmagmaticmixtures in the crusthave356

directimplicationsontherateandefficiencyoffractionationprocesses,onthecoolingrateof357

magmareservoirsanderuptiondynamics(e.g.,GonnermannandManga2007;Lavalléeetal.358

2007;Cordonnieretal.2012;GonnermannandManga2013;Parmigianietal.2014;Pistone359


et al. 2015). The response of magmas to differential stresses is complex, owing to the360

multiphase nature of these systems. Several research groups have performeddeformation361

experiments to better constrain rheological laws pertinent tomagmas (Spera et al. 1988;362

Lejeune andRichet 1995; Caricchi et al. 2007; Ishibashi and Sato 2007; Champallier et al.363

2008; Cimarelli et al. 2011;Mueller et al. 2011; Vona et al. 2011; Pistone et al. 2012;Del364

Gaudio et al. 2013; Laumonier et al. 2014;Moitra and Gonnermann 2015). These studies365

have focused on diverse aspects of the rich non-linear dynamics of the rheology of366

suspensions,suchas(1)theeffectofthecrystalvolumefraction,(2)theeffectiveshearrate,367

(3) the effect of polydisperse crystal size distributions and various crystal shapes. These368

results clearly show that suspensions become very stiff as crystal content approach or369

exceed40to50percentofthevolumeofthemagma,althoughtheyprovidelittleinformation370

about the onset and magnitude of a yield stress for crystal-rich suspensions. Even more371

challenging is the development of physically consistent rheology laws for three-phase372

magmas because of the additional complexity of the capillary stresses (coupling bubbles,373

melt, and crystals), bubble deformation, three phases lubrication effects and possible374

coalescence(Pistoneetal.2012;Trubyetal.2014).375

376

Magma rheology experiments have been synthesized to yield different empirical377

(Lavalléeetal.2007;Costaetal.2009)andsemi-empirical(Petford2009;Maderetal.2013;378

Faroughi and Huber 2015) models for the deformation of magmas under shear flow379

conditions.Themechanicalbehaviorofmagmasremainsarichfieldforfutureinvestigations,380

and several important challenges remain to constrain the rate at which magma flows in381

reservoirsanduptothesurface.Theyinclude:382


• How to treat multiphase rheology when phase separation takes place383

concurrently?384

• Howdobubblesinteractwithcrystalssuspendedinviscousmelts,howdoes385

thisbehaviordependonthestrainrateandvolumefractionsofeachphases?386

• Can we constrain the yield strength of crystal-rich magmas under magma387

chamberconditions(imposedstressesratherthanimposedstrain-rate)?388

• Byrheology,weoftenrestrictourselvestomeasuretheshearviscosityusing389

experimentsormodelsthatrelatestressanddeformationunderverysimple390

geometries.Asviscosity is a tensor, is itpossible tomeasure its anisotropy391

fornon-linearmultiphasesystemswithcomplexcrystalshapesundervarious392

strainratesandconditionspertinenttomagmachambers?393

394

The rheology of a magma body should therefore be considered as a dynamical395

quantity, which can vary significantly (orders ofmagnitude) because of changes in stress396

distributions, variations in crystal-melt-bubble phase fractions and potentiallymany other397

factors.398

399

Several other physical properties have a resounding impact on the evolution of400

magmasinthecrust.Magmasarefarfromthermalequilibriuminthemidtouppercrustand401

heattransferinandoutofthesebodiescontrolstheirchemicalanddynamicevolutiontoa402

greatextent(Bowen1928;McBirneyetal.1985;Nilsonetal.1985;McBirney1995;Reiners403

et al. 1995; Thompson et al. 2002; Spera and Bohrson 2004). In that context, the heat404

capacity,latentheat,andthermalconductivityofsilicatemelts/mineralsandCO2-H2Ofluids405


areimportanttoestablishthetimescaleoverwhichthesereservoirscrystallizeinthecrust406

and by extension the vigor of convective motion in these systems. It is important to407

emphasizethatthermalconvectionintheclassicalsenseisnotdirectlyapplicabletomagma408

chambersundermostconditions,becausedensitycontrastsbetweenthedifferentphasesfar409

exceeds those by thermal expansion (e.g.,Marsh andMaxey 1985; Bergantz andNi 1999;410

Dufek and Bachmann 2010). Additionally, the rate of crystallization directly impacts the411

coolingrateofmagmas(duetonon-linearreleaseof latentheatduringcrystallization,e.g.,412

Huberetal.2009;Morse2011), and this latentheatbuffering becomesdominantas silicic413

magmas approach eutectic behavior near their solidus (Gelman et al. 2013b; Caricchi and414

Blundy2015).Somerecentstudieshavetestedhowthetemperaturedependenceofthermal415

properties influence the cooling and crystallization timescales ofmagmas in the crust and416

showedthat theseeffectsaresometimesnon-trivial(Whittingtonetal.2009;Gelmanetal.417

2013b;deSilvaandGregg2014).418

419

Tankexperimentsformagmachamberprocesses:420

421

Anelegantapproachtostudythecomplexmultiphasefluiddynamicsthattakesplace422

inmagmareservoiristoresorttolaboratorytankexperiments(seeforexample,McBirneyet423

al.1985;Nilsonetal.1985;JaupartandBrandeis1986;MartinandNokes1988;Shibanoetal.424

2012).Experimentsprovidethemeanstoinvestigatesomeaspectsofthecomplexnon-linear425

dynamicsthatprevailinthesesystems,however,inmostcases,itisdifficultorimpossibleto426

satisfydynamical similitudewith realmagmatic systems.Nevertheless, experiments reveal427

interesting feedbacks that may have been overlooked and can serve as a qualitative428


assessmentofhowprocessescoupletoeachother.Oneofthemaindirectionsforlaboratory429

fluiddynamicsexperimentsistostudyhowmagmaticsuspensionsbehaveat lowReynolds430

number during convection, phase segregation and magma recharges with mixing (e.g.,431

McBirney 1980; Sparks et al. 1984; McBirney et al. 1985; Davaille and Jaupart 1993;432

Koyaguchi et al. 1993). Here, the term suspension is used loosely to describe as both433

suspensionwithsolidparticles(crystals)andbubblyemulsions.434

435

Asdiscussedthroughoutthisreview(andclearlyalludedtobyearlypetrologistssuch436

asR.Dalyasfarbackasthe1910s,e,g,Daly1914),magmabodiesareopenreservoirsthat437

exchangemassandenergywiththeirsurroundings.Chemicalheterogeneitiesandzonations438

thenreflecteitherphaseseparationbygravity(e.g.,McBirney1980;Hildreth1981;Sparkset439

al. 1984; McBirney 1993) or incomplete mixing between magmas with different440

compositions(Eichelberger1975;Dunganetal.1978;BlakeandIvey1986;Eichelbergeret441

al.2000).Becausebuoyancyforcesrelatedtocompositionaldifferences(includingpresence442

ofvariousphasesofvariabledensities)exceedthermalbuoyancyeffectbyseveralordersof443

magnitude,mechanicalconvectionandstirringaredominatedbychemicalvariations,bubble444

rise and/or crystal settling. Strong spatial contrasts in bulk viscosity because of mixing445

between magmas with different compositions, contrasts in crystal content or thermal446

gradientsalsotendtostabilizeagainsthybridizationormagmamixing(SparksandMarshall447

1986; Turner and Campbell 1986; Koyaguchi and Blake 1989; Davaille and Jaupart 1993;448

Jellinek and Kerr 1999). Mixing in magma reservoirs can also be accelerated during449

eruptions,wheresubstantialstressdifferentialscanbegeneratedbydecompressionorroof450

collapse(BlakeandCampbell1986;BlakeandIvey1986;Kennedyetal.2008).451


452

Figure3.Injectionofnegativelybuoyantdropletsof(lowviscosity)waterdyedingreenintoa453

tankfilledwith(moreviscous)siliconoil(fromFaroughiandHuber2015).Dropletinteractions,454

eveninthepresenceofdroplettrains,cansignificantlyreducethesettlingvelocityanddecrease455

therateofphaseseparation.456

Convection strongly influences phase separation, which in turns controls chemical457

differentiation. Several groups have investigated the effect of crystals (Jaupart et al. 1984;458

BrandeisandJaupart1986;BrandeisandMarsh1990;Koyaguchietal.1993;Shibanoetal.459

2012)bubbles(Huppertetal.1982;Thomasetal.1993;CardosoandWoods1999;Phillips460

andWoods2002;Namikietal.2003),andshapesofreservoirs(e.g.,deSilvaandWolff1995)461

onconvectivemotion.Thepresenceofcrystalsorbubblescontrolsthegravitationalenergy462

potentialthatisconvertedtokineticenergyduringconvection,asexperimentsconfirmthat463


they impact convection even at relatively small volume fractions. Obviously, suspended464

phases affect convection, but convection itself also influences crystal settling or bubble465

migration. For example,Martin andNokes 1988;Martin andNokes 1989 used a series of466

experimentstoquantifythetimescaleoverwhichaconvectingsuspensionofcrystalsclears467

outbecauseofStokessettling.Hydrodynamicinteractionsbetweenthedifferentphasescan468

significantly reduce the settling velocity and affect phase separation even at low particle469

(bubblesorcrystals)volumefractions(FaroughiandHuber2015,seeFig.2).470

471

3.4 Overview of magma reservoir dynamics from theoretical models 472

473

Numericalstudiesenableustoisolatespecificdynamicalfeedbacksandtoprovidea474

frameworktounderstandthecomplextemporalevolutionofthesesystems,whichcanoften475

be challenging to infer from natural samples or laboratory experiments where dynamic476

similarity is difficult to satisfy. Numerical approaches based on different sets of starting477

assumptionsandgoverningequationshavebeenusedtoaddressquestionssuchas:478

• The chemical evolution of open-system magma reservoirs undergoing479

assimilation,crystalfractionationandmagmaevacuation.480

• Thethermalevolutionandlongevityofmagmareservoirsinthecrust.481

• The relative importance of crustal melting vs. crystal fractionation of more482

maficparentsindrivingtheevolutionofmagmastowardssiliciccompositions.483

• The pressure variations in and around magma chambers and stability of484

reservoirsfollowingrecharges.485

• Gasexsolutionandhowitimpactsthedynamicsinshallowmagmareservoirs.486


487

a)Boxmodels(nospatialdimensions)488

The first category of studies summarized below is based on boxmodels where the489

reservoirsareconsideredhomogeneousandinternalstructuresarenotexplicitlyaccounted490

for. These models involve ordinary differential equations that depend solely on time (no491

spatialdependency)andbyextensionassumethathomogenizationtakeplaceoverashorter492

timescale than the processes considered. In other words, in these models, the different493

phases(melt,crystals,sometimesbubbles)areassumedtoremaininthermalandchemical494

equilibriumwitheachotheroverthescaleofthewholereservoir.495

496

Since the seminal work of H. Taylor and D. DePaolo (Taylor 1980; DePaolo 1981),497

where a box model was constructed to infer the relative importance of assimilation and498

crystalfractionationprocessesonthetraceelementsandisotopicrecord,manystudieshave499

expandedon themethod to relate chemical tracers to the actualdynamicalprocesses that500

control the evolution of magmatic systems. The scope of these models ranges from non-501

linear reactor models (Bonnefoi et al. 1995) investigating the development of bimodal502

compositionsfromasinglemagmaticsystemtoconstrainingtheeffectofmeltextractionon503

thechemicalsignatureoftheresidualcumulateandhigh-silicamelts(Gelmanetal.2014;Lee504

andMorton 2015) aswell as focusing on the development of crystal zonations in awell-505

mixed reservoir (Wallace and Bergantz 2002; Nishimura 2009). These models provide506

insights into thechemicalresponseofreservoirs tovarious fractionationprocesses,butdo507

notexplicitlytreatthesourceoftheseprocesses(e.g.,coolingandcrystallizationleadingto508


gravitational settling or the reheating of the surrounding crust and its partial509

melting/assimilation).510

511

As a first step towards answering these limitations, W. Bohrson and F. Spera512

developed,inaseriesofpapers(SperaandBohrson2001;SperaandBohrson2004;Bohrson513

and Spera 2007; Bohrson et al. 2014), a model that couples the mass balance of trace514

elements and isotopes in a reservoir where cooling and assimilation processes are515

parameterized. They also added a thermal energy equation for the reservoir. The energy516

balanceinopenmagmachambersisquitecomplexandinvolvesmorethanjustsensibleand517

latentheatcontributionsinresponsetotheheatlosstothesurroundingcrust.Evenwithin518

the assumption that the reservoir remains homogeneous and the phases remain in519

equilibriumatalltimes,enthalpyisconstantlytransferredfrommechanicalworktoheatand520

vice-versa.Asanexample, the injectionof freshmagma intoa reservoiraffects theenergy521

budget not only by providing a possible heat source but also exerts mechanical work522

(pressure changes) that affect the stability of the phases present in the magma. These523

feedbacks become even more important when shallow volatile-saturated systems are524

considered(HuppertandWoods2002;DegruyterandHuber2014;Degruyteretal.2015).It525

is therefore important to extend the energy conservation statement in these models to526

includemechanicalworkandphasechangeswhenpossible(henceintroducingaconsistent527

twowaycouplingbetweenthehotmagmareservoirandthesurroundingcrust;deSilvaand528

Gregg2014;DegruyterandHuber2014).529

530

b)Modelswithspatialdimensions531


532

Theassumptionofthermalandchemicalequilibriumatthescaleofthereservoirisuseful533

and allows us to draw interesting and informative conclusions on the general trends that534

govern magma chamber evolution. However, this assumption is untenable under most535

conditions.Introducingspatialheterogeneitiesinthemagmabodyandthereforeallowingfor536

somedegreeofdisequilibriumbetweenthephases(atleastdowntothescaleofthemodel’s537

resolution,wherelocalequilibriumisgenerallystillassumedbetweenphases)isnecessary538

to get realistic estimates of the temporal scales over which magma bodies evolve. More539

specifically, a spatiotemporaldescriptionofmagmareservoirsprovides theopportunity to540

testfortheconditionsunderwhichefficientmixingisnolongerpossible(Huberetal.,2009)541

and chemical differentiation by mechanical separation (flow) occurs. We will divide the542

spectrumofnumericalmodelsgearedtoaddressthesequestionsinto(1)thermalmodels543

ononehandand(2)coupledthermalandmechanicalmodelsontheotherhand.544

545

b.1Thermalmodels546

547

Thermalmodelsareconcernedonlywithoneaspectoftheenergybalanceandconsider548

only sensible and latent heat. These models involve various levels of complexity, from549

equilibrium crystallization, i.e. the amount of crystallization over each time step is that550

predicted from equilibrium thermodynamics (e.g. Annen et al. 2008; Leeman et al. 2008;551

Annen2009;Gelmanetal.2013b;KarakasandDufek2015)tothemodelofC.Michautand552

coworkerswherecrystallizationisconsideredkineticallylimited(MichautandJaupart2006).553

Equilibriumcrystallizationmodels relyonaconstitutiveequation that relates temperature554


and crystallinity for a given choice of magma composition and pressure, generally555

constrained experimentally or using thermodynamicmodels such asMELTS (Ghiorso and556

Sack1995b).Becausenomomentumconservationisconsidered,crystalsandmeltarestatic557

and bound to each other, i.e. no phase separation and by extension no chemical558

differentiation by crystal fractionation is possible. Heat is therefore transported only by559

diffusionwithinandoutofthemagmareservoir.Thesemodelshavebeenusedextensivelyto560

study the longevity of activemagma bodies in the crust as function of the rate ofmagma561

injections.Whiletheysuggesthighaverageratesofmagmainjectionstocounteracttheheat562

lost to thecrust, theyoverlook theroleofmechanicalprocessesandvolatileexsolutionon563

the energy balance in response to repeated magma recharges and sporadic evacuation564

events(Degruyteretal.2015).565

566

b.2Thermo-mechanicalmodels567

568

Another type of model accounts for the coupled momentum and enthalpy balance in569

magma reservoirs and has been used tomodel the thermal evolution aswell as chemical570

differentiation (by crystal fractionation and assimilation) in magmatic systems. In these571

models,themomentumbalanceiseitherparameterized(Huberetal.2009)orrequiresaset572

of coupled continuum equations that allows the different phases to separate from one573

another (Dufek and Bachmann 2010; Gutierrez and Parada 2010; Lohmar et al. 2012;574

SimakinandBindeman2012;Solanoetal.2012).Inthoselattermodels,thedifferentphases575

are coupled in the momentum equation through drag terms. The first numerical models576

appliedtomixingofchemicalheterogeneities(singlephasefluids)inmagmasbyconvective577


stirringwheredevelopedbyC.Oldenburgandcolleagues(Oldenburgetal.1989;Speraetal.578

1995), and highlighted the different rates of mixing by normal and shear strains. Later,579

Huberetal.,(2009)discussedamethodtocharacterizetheefficiencyofmixingbyconvective580

stirringintime-dependentsystemswherecoolingandcrystallizationdampentheefficiency581

of convection over time.Multiphasemixing by sinking crystal plumes initiated at thermal582

boundary layers has been studied extensively as the source of convection in magmas583

(BergantzandNi1999;DufekandBachmann2010;SimakinandBindeman2012).Thefocus584

of these models is to look at crystal fractionation (Bergantz and Ni 1999; Dufek and585

Bachmann2010;Solanoetal.2012)andcrustalassimilation(SimakinandBindeman2012)586

assourcesofchemicaldifferentiationformagmasemplacedinthecrust.587

588

At this stage, amodel that involves a thermo-mechanical solution to the crust-magma589

bodysystemsandthatalsosolvesfortheinternalfluidmechanicsandchemicalevolutionof590

thereservoir isyet tobecompleted (seeGreggetal.2013;DegruyterandHuber2014 for591

preliminary attempts to do this). Some of the processes associated with the mechanical592

response to thesereservoirs tomassadditionorwithdrawalcanoperateovermuch faster593

timescales than the thermal and chemical evolution of the reservoir and coupling these594

variousprocessesprovestobechallenging.595

596

The mechanical and chemical interaction between the many phases that coexist in597

magma reservoirs is complex. It is generally parameterized with simple constitutive598

empirical relationships (e.g. drag terms, interphase heat transfer coefficients). Because599

modelsareonlyasgoodastheassumptionstheyrelyon,itisimportanttorevisitthevalidity600


of these constitutivemodels that introduce the dynamical coupling between the different601

phasesincontinuummodels.Intruth,theseinteractiontermsinvolvelengthscalesthatare602

farbeyondtheresolutionofcontinuummodelsanditistheinterplaybetweencrystals,melt603

andpossiblygasbubblesthatdrivemostofthedynamicsinmagmabodies.Oneexamplefor604

suchanempiricallawisthemechanicalseparationbetweencrystalsandthemeltbygravity,605

whichcontrolschemicaldifferentiation.Belowathresholdcrystallinity,gravitationalsettling606

laws based on Stokes settling are generally assumed valid (Martin and Nokes 1989).607

However,itassumesthatcrystalsarefarapartandneverinteracthydrodynamicallyandthat608

thereisnoreturnflowofmelttoconservethemassfluxoveranyhorizontalsurfaceinthe609

magmabody, i.e. themagmabodyhasan infinitevolume.Themechanicsofmulti-particles610

gravitationalsettlingisquitecomplexthoughandrequiresanaccountofbothshortandlong-611

rangehydrodynamiccorrectionsevenat crystalvolume fractions significantlybelow10%612

(Koyaguchietal.1990;Koyaguchietal.1993;FaroughiandHuber2015).Anotherexample613

lies in a proper way to account for the thermal energy balance in multiphase media614

(crystals+meltforexample).Atthecontinuumscale,thetwogeneralapproachescommonly615

usedassumeeither thatover the resolutionof thenumericalmodel the twophasesare in616

thermal equilibrium (Local Thermal Equilibrium conditions or LTE) as in Simakin and617

Bindemann (2012) or that the contrast in thermal properties requires a thermal lag or618

inertiabetweenthephasesparameterizedwithinter-phaseheattransfercoefficients(Local619

ThermalNonEquilibriumconditionsorLTNE)asinDufekandBachmann(2010).Although620

the latter thermalmodel (LTNE) ismore realistic andprovidesbetter results (the current621

state-of-the-art), ithasseveral issues thatsometimesprecludes itsuse formultiphaseheat622

transfer (e.g.,KaraniandHuber, submitted).There is thereforeaneed to studymultiphase623


magmasatthediscretescaleanddevelopnewcontinuum-scaleconservationlawsthatdoa624

morecarefuljobwhenfilteringoutheterogeneitiesduringvolumeaveraging(seeFrontiers625

section).626

627

c)Futuredevelopmentinmodeling628

629

Apossible solution to resolve the actual thermal andmechanical interactions inmulti-630

phase magmas is to resort to granular (discrete) scale models as a complement to these631

continuumscalestudies.Thefundamentaldifferencebetweencontinuumandgranularscale632

models is that the latter explicitly solve for the mass (including chemical exchanges),633

momentum (e.g. drag) and energy (e.g. heat transfer) exchange between the different634

component through their interfaces,while the former involvesaparameterizationof these635

interactionsor, insomecases (e.g., effectivemediumtheory), relieson thedefinitionofan636

effective medium with hybrid (parameterized) properties. The granular-scale approach637

offers a significant advantage by solving a more realistic model and allows us to study638

complex feedbacksarising fromthe interactionofmultiplephases,butat thecostofheavy639

computationalrequirements.640

Several“granular”multiphasemodelshavebeendevelopedoverthelastdecadetostudy641

magma dynamics. For example, multiphase fluid dynamics modeling where crystals are642

introduced using the Discrete ElementMethod (DEM) coupled to Stokes flow solvers has643

been applied to magma chamber dynamics recently by Furuichi and Nishiura 2014 and644

Bergantz et al. 2015, consideringmostly how crystal melt mechanical interactions affects645

settling inmagmachambers.Granularscalemodels,usingthe latticeBoltzmanntechnique,646


havealsopermittedtostudythemigrationofexsolvedvolatilebubblesinmagmachambers647

athighcrystalcontent(seeFigure4;Parmigianietal.2011;Huberetal.2012b;Parmigianiet648

al.2014).Asmentionedabove,discretemodels(forthedispersedphase)arevaluableinthat649

theyprovideanaccuratedescriptionoftheactualphysicsthatgovernsthemass,momentum650

andenergyexchangebetweenphases,buttheyaregenerallylimitedbycomputerpowerto651

smallcomputationaldomainsandrequireupscalingstrategiestoextrapolatethedynamicsto652

thescaleofthereservoir.Upscalingmultiphasedynamicsisandwillremainagreatchallenge653

in the years to come to better constrain the chemical and dynamical evolution ofmagma654

bodiesinthecrust(seeFrontierstopicattheendofthisreview).655

656

657


658

Figure4.Exampleofgranularscalecalculationformultiphasemagmachamberdynamics.(a-659

c) modified from from Furuichi and Nishiara (2014) for crystal settling from a stratified660

chamberwithacrystal-freeregionatthebottomandadenser,crystal-richhorizon,atthetop,661

and(d)Parmigianietal.,2011forthemigrationofabuoyantvolatilephase inacrystal-rich662

rigidmagma.663

664


4 Model of magma reservoir: focusing on the “Mush” 665

Afterreviewingmostofthetechniquesandtoolsusedtostudymagmareservoirsin666

theEarth’scrust,thenextsectionwillfocusonaspecificmodelofmagmareservoirevolution,667

the so-called “Mush Model” (Bachmann and Bergantz 2004; Hildreth 2004; Huber et al.668

2009), strongly influenced by the pioneering efforts of many previous researchers (e.g.,669

Smith 1960; Smith 1979; Hildreth 1981; Lipman 1984; Bacon and Druitt 1988b; Brophy670

1991; Sinton and Detrick 1992 to cite a few). Althoughwe fully admit that this choice is671

stronglydictatedbyourpreviousexperience,webelieveithelpsprovidingaframeworkto672

many of the questions we listed in the introduction. Obviously, the model needs many673

refinements,andwewilltrytobringoutthecaveatsasweseethem.Wearealsoawareof674

the controversial aspects that remain tobe resolved (e.g.,Glazner et al. 2008;Tappaet al.675

2011; Simakin andBindeman 2012; Rivera et al. 2014; Streck 2014;Wotzlaw et al. 2014;676

Glazneretal.2015;Kelleretal.2015),buthopethatsuchareviewcanprovideastepping677

stonetobringthediscussionforward.678

679

Since volcanic rocks are, at times, relatively crystal-poor (particularly the680

compositionalextremes,basaltsandrhyolites,whereasintermediatemagmasareoftenmore681

crystal-rich; Ewart 1982), researchers have tended to draw magmas reservoirs as682

dominantly liquids (big pools of liquid magmas for both mafic and silicic units, see683

YellowstonereservoirdepictedonFigure1;figure9-14ofMcBirney1993;Irvineetal.1998;684

Maughan et al. 2002), which easily caught on in the general public’s views of magma685

reservoirs.However,asmagmasareemplacedincontactwithacoldercrust,heatlosstothe686

surroundingcrustlimitsthetimemagmasremaininamostlyliquidstateinthesereservoirs.687


Hence,manyresearchersarguedthatsolidificationzones(orfronts,(Marsh1981;Marshand688

Maxey1985;Marsh1989b;Marsh2002;Gutierrezetal.2013,Figure5)arelikelytodevelop689

quickly,andformacrystal-richbufferzonebetweenthehottestpart,mostliquidpartofthe690

reservoirand the subsoliduswall rocks. Since,many linesofevidencehavesuggested that691

magmas are dominantly kept as high-crystallinity bodies in the Earth’s last few tens of692

kilometers.Thesehigh-crystallinitybodiesareoftenreferred toas “crystalmushes”.Miller693

andWark (2008) defined a crystalmush as “Amixtureofcrystalsandsilicate liquidwhose694

mobility,andhenceeruptibility, is inhibitedbyahigh fractionof solidparticles.” (Miller and695

Wark 2008). This is the case for subvolcanic reservoirs (Hildreth 2004; Bachmann et al.696

2007b)inlargesilicicsystems,butalsoatMidOceanRidges(SintonandDetrick1992).The697

supporting evidences for the importance of crystal mushes stem from petrological,698

geophysical, geochemical and geological observations, aswell as thermalmodeling. These699

aredetailedbelow.700

701


702

703

Figure5:Crystallinityvariationsinmagmas,fromsolidustoliquidus,whichcanspanupto250704

ºC(modifiedfromMarsh1996).Thetypicalamountofheatliberatedby1kgofmagmafrom705

liquidustosolidusis~600’000Joules,ofwhich~½isfromlatentheat.706

Asthetemperaturecontrastwiththesurroundingcrustdiminishes, thecoolingrate707

alsodecreaseandmagmasspendmore timesat temperature close to their solidus (Marsh708


1981;KoyaguchiandKaneko1999;Huberetal.2009).Thewaningofconvectionalsoplaysa709

role decreasing the cooling rate with decreasing T. As stirring occurs, it steepens the710

temperature gradient at the edge of the body, and tend to hasten the cooling. Since711

convection only occurs at low crystallinity, it enhances the cooling near the liquidus (see712

figure 6 of Huber et al. 2009). Finally, latent heat released during crystallization provides713

energythatneedstoberemovedbeforesubsequentcoolingispossible.Ifthetemperaturevs.714

crystallinity diagram is relatively linear, latent heat is released equally over the cooling715

intervalbetweentheliquidusandsolidus,anddoesnotfavoraprolongedstateateitherthe716

low or high crystallinity range. However, the temperature vs. crystallinity relationship is717

quite complex and non-linear for many types of magmas, particularly those with near-718

eutectic points. For example, evolved high-SiO2 magmas rapidly reach the haplogranite719

eutectic and are thus characterized by a strongly non-linear phase diagram (e.g., dacitic720

magmasreachnear-eutecticconditionsat40-50vol%crystals;Bachmannetal.2002).The721

latent heat is mostly released when the magma approaches this eutectic condition and722

providesanadditionalthermalbufferingeffect,i.e.slowsdownthecoolingratesignificantly723

from that point onward (a process called “mushification” by Huber et al. 2009). Magma724

recharges canprovidea sufficient amountof enthalpy to slowor even reverse the cooling725

trendgenerallyobservedinthesereservoirs,whichcanprolongtheirexistenceatlowmelt726

fraction (Annen and Sparks 2002; Annen et al. 2006; Leeman et al. 2008; Annen 2009;727

Gutierrez et al. 2013; Gelman et al. 2014), see Figure 6 for an attempt to illustrate the728

temperature-time evolution in the warmest parts of the reservoirs. The maturation of a729

magmaticfieldthroughrepeatedintrusionsofmagmasappearstoalsoplayanimportantin730


priming the thermal state of the crust to host long-lived active magma reservoirs (e.g.,731

Lipmanetal.1978;deSilvaandGosnold2007a;Lipman2007;Grunderetal.2008).732

733

734

Figure 6: Schematic temperature-time diagram for a part of mature magma reservoirs735

situatedinthehottest,corezone.Notetheslowercoolingratesasthecrystalcontentincreases736

(magmaapproachingtheirsolidus).737

Evidencefromeruptedrocks:738

739

Theeruptionof large,homogeneouscrystal-richunits, suchas theFishCanyonTuff740

(Whitney and Stormer 1985; Lipman et al. 1997; Bachmann et al. 2002), the Lund Tuff741

(Maughanetal.2002)orLaPacanaTuff(Lindsayetal.2001)havealsobeenpivotalinthe742

developmentoftheMushModel(seesection4.2below).Such“MonotonousIntermediates”743

(Hildreth1981)arecommoninlargemagmaticprovinces,andunambiguouslydocumentthe744

presenceoflarge,homogeneouszonesofcrystal-richmagmasintheuppercrust,potentially745

remobilizablebyeruption.Theirhomogeneityisstriking,andrequiresarelativelythorough746


stirringpriortoeruption,likelytriggeredbyarechargeandremobilizationpriortoeruption747

(seebelow,Huberetal.2012aandParmigianietal.2014forreviews).748

749

Evidencefromgeophysics:750

751

Large,lowseismicvelocityzonesandconductiveMTareasbeneathactivevolcanoes752

orvolcanicprovincesalsosuggestthepresenceofnear-solidusmagmasbodies.Althoughthe753

resolutionremainspoor(hundredsofmeterstokilometers),theanomalies(bothintermsof754

electricconductivityandseismicvelocities)andthesizeofthosebodiesarenotsupportiveof755

dominantlymoltenbodies(e.g.,Dawsonetal.1990;Lees1992;Weilandetal.1995;Stecket756

al.1998;Zolloetal.1998;Masturyonoetal.2001;Zandtetal.2003;DeNataleetal.2004;757

Husenet al. 2004;Hill et al. 2009;Waite andMoran2009;Heise et al. 2010; Farrell et al.758

2014;Wardetal.2014).Estimatesofmeltrangefromafewpercenttonearly50%meltin759

some areas of the anomalies (Heise et al. 2010; Farrell et al. 2014). In addition to those760

seismic and MT anomalies, Bouguer gravity anomalies under older volcanic fields also761

suggesttheaccumulationof lowdensity,silicicrocksintheuppercrust(e.g,SRMVF,Plouff762

andPakiser 1972; Lipman1984;Drenth andKeller 2004;Drenth et al. 2012; Lipman and763

Bachmann2015).764

765


766

767

Figure7: Comparison between the SRMVFbatholithmodel (LipmanandBachmann, 2015; A768

andBareschematizedcross-sectionsbasedoneruptedproductsandgravitydata)andajoint769

ambientnoise-receiverfunctioninversionS-velocitymodelfortheAltiplanoregionoftheAndes770

(C;Wardetal.2014).771

Evidencefrompresenceofcumulatelithologiesinvolcanicandplutonicunits:772

773

Thepresenceofcrystal-rich,cumulateblockseruptedinvolcanicunits(Wager1962;774

Arculus andWills 1980;Heliker 1995;Ducea and Saleeby1998), aswell as largeplutonic775

massesincrustalsectionswithclearcrystalaccumulationzones(Voshageetal.1990;Greene776

etal.2006; JagoutzandSchmidt2012;Otamendietal.2012;Cointetal.2013)suggestthe777

presenceofmushzoneswithinthecrustinactivemagmaprovinces.Cumulatesignaturesare778

not only seen in deep crustal zones, but also in shallow plutonic bodies, both using bulk779

chemistry(McCarthyandGroves1979;Bachletal.2001;MillerandMiller2002;Gelmanet780


al.2014;LeeandMorton2015)and/ortexturalfeatures(BeaneandWiebe2012;Graeteret781

al.inpress).782

783

784

785

Figure 8: Schematic diagram of the polybaric mush model (modified from Lipman, 1984,786

Hildreth,2004,andBachmannandBergantz2008c).(1)Pre-existingcrust,(2)Uppermantle,787

(3)Feedingzoneofprimitivemagmas fromthemantle(“basalts.l.”), (4)Lowercrustalmush788

zone, with internal variability in melt content, (5) Upper crustal mush zone, (6) Melt-rich789


pockets in upper crust, (7) Melt-rich pockets in the lower crust, (8) Caldera structure, (9),790

Stratovolcano(e.g.,MountSt.Helens,WA,USA).791

Overthenextsections,wewilltrytorevisitthequestionsaskedintheintroduction,792

and see how mush zones (or the “Mush model”) can help accounting for the main793

observationsthatwehavelaidout.794

795

4.1 Presence of crystal-poor rhyolites, and zoned ignimbrites 796

Firstofall,weobservemanysiliciccrystal-poordepositsat thesurfaceof theEarth797

(SeeHildreth1981;Lindsayetal.2001;Masonetal.2004;Huberetal.2012a).Hence, the798

extraction of high-SiO2, cold (<800 ºC), viscous melt from crystalline residue in large799

quantities (several 100s of km3) clearly happens in many cases. In some locations, these800

crystal-poorpocketsof eruptedmagmasare ratherhomogenous (Dunbaret al. 1989;Ellis801

and Wolff 2012; Ellis et al. 2014), in others they grade into crystal-rich, typically hotter802

intermediatemagmasattheendoftheeruption(upperpartsofthedeposits;Lipman1967;803

Hildreth1981;BaconandDruitt1988a;Wolffetal.1990;Deeringetal.2011b;Huberetal.804

2012a;LipmanandBachmann2015;Figure9).805

806

Generating these eruptible silicic pockets is challenging, as extracting such viscous807

melt (up to 105-106 Pa s, evenwith severalwt% dissolved volatiles, Scaillet et al. 1998b)808

fromanetworkofmm-sizedcrystals is, atbest, sluggish (McKenzie1985;Wickham1987)809

andtheprocesscanbeeasilydisrupted.Forexample,convectioncurrentsshouldnotoccur,810

asthiswillsignificantlystirthewholemagmareservoir,andleadtore-homogenizingofthe811

crystal-meltmixture(i.e.,impedingcrystal-meltseparation).Hence,apossibility,particularly812


in long-lived, incrementallybuilt, sill-likemagmabodies, is thatmelt is slowly extracted by813

gravitationalprocesses(hinderedsettling,microsettling,compaction)asthemagmareaches814

the rheological lock-up (~50 vol%; Brophy 1991; Thompson et al. 2001; Bachmann and815

Bergantz 2004; Hildreth 2004; Solano et al. 2012), an extraction process potentially816

enhancedbygasfilter-pressing(SissonandBacon1999;BachmannandBergantz2006;Ellis817

etal.2014;Pistoneetal.2015).Thismeltextractionfollowinglock-upinlargemushzones818

has the advantage of (1) preventing stirring and mixing of the crystal-melt mixture by819

convectivecurrents, and (2)keeping thecrystal-poormeltpocket inawarmenvironment,820

leading to significantly reduced crystallization rate and more time for crystal-liquid821

separation.Moreover,asill-likegeometry(seeFigure8)optimizesextractionas it reduces822

the average vertical distance traveled by the viscousmelt. The interstitialmelt extraction823

frommushesseemsnottoberestrictedtoevolvedmagmas,butmayalsooccurredatmore824

mafic,lessviscouscompositions,suchasbasaltsandandesites(DufekandBachmann2010).825

826


827

828

Figure9: (a)Variations inSiO2andcrystal contents for ignimbrites inwesternUnitedStates829

(LC—LavaCreekTuff;T—TshigereMemberofBandelierTuff;BT—BishopTuff;WP—Wason830

ParkTuff; CR—CarpenterRidgeTuff;RC—RatCreekTuff;NM—NelsonMountainTuff;AT—831

Ammonia Tanks Tuff; FC—Fish Canyon Tuff; BC—Blue Creek Tuff, SM—SnowshoeMountain832

Tuf;MP—MasonicParkTuff).ModifiedfromHildreth1981andHuberetal.2012a;datafrom833

Hildreth 1981; Lipman2000; Lipman2006. (b) REEpatterns from crystal-poor pumices and834

crystal-rich clasts from the Carpenter Ridge Tuff,with, for reference, patterns for lamproïtic835

magmas,indicatingthatmixingwithsuchhigh-K,incompatible-element-enrichedliquidsisnot836

an option to generate the high Ba-Zr composition of the late-erupted crystal-rich clasts837

(modifiedformBachmannetal.,2014).838

839

Petrological observations in many large, zoned ignimbrites support such a magma840

reservoir model, with a cap of crystal-poor material immediately underlain by a more841


crystal-rich zone. Some well-exposed deposits such as the 7700 BP Crater Lake, or 1912842

Katmaieruptionsshowabruptchangesincrystallinity, fromnearlycrystal-freeearlyinthe843

eruption, to40-50vol%crystals late inthesequence,withaverysimilarmeltcomposition844

throughout the stratigraphy (Bacon andDruitt 1988a;Hildreth and Fierstein 2000; Bacon845

andLanphere2006).Otherlargeignimbritespresentmoregradualvariations,asexemplified846

byfamousBishopandBandelierTuffs(Hildreth1979;HildrethandWilson2007;Wolffand847

Ramos2014)amongmanyothers(seeHildreth1981andBachmannandBergantz2008afor848

listsofthesedeposits).Suchzonedignimbritesarebestexplainedbyeruptingamelt-richcap849

followed by partial remobilization and entrainment of a silicic cumulate from the mushy850

roots of the system, chocking the eruption (see Smith 1979; Worner and Wright 1984;851

Deeringetal.2011b;Bachmannetal.2014;Sliwinskietal.2015;Wolffetal.2015;Evanset852

al.2016).Thispartialcumulateremobilizationislikelyaconsequenceofhotterrechargeat853

thebaseofthesiliciccap,assuggestedbytextural,mineralogicalandgeochemical features854

indicativeofmixingandreheating(Andersonetal.2000;Warketal.2007;Bachmannetal.855

2014;WolffandRamos2014;Fornietal.2015).856

857

Otherprocesses, suchas incomplete/partialmixingbetweendifferentupper crustal858

magmapockets and recharge (e.g., Dorais et al. 1991;Mills et al. 1997; Eichelberger et al.859

2000; Bindeman and Valley 2003) or co-erupting physically-separated and chemically860

differentmelt-rich lenses (GualdaandGhiorso2013a)havealsobeensuggested toexplain861

the ubiquitous chemical zoning in ignimbrites. Although we do not question that such862

processesarelikelyplayingaroleingeneratingchemicalcomplexitiesintheerupteddeposit,863

these hypotheses fail to account for the presence of co-magmatic crystal-rich cumulate864


blocksthateruptconcurrentlywiththecrystal-poorpartsofthereservoirs(seealsoElliset865

al.2014foracaseinahot-dryrhyoliticsystem,andsection4.3below).Hence,wearguethat866

magma recharge reaching the base ofone or several crystal-poor caps above a cumulate867

zone(asdepictedinFigure8)isthemostlikelyreservoirgeometrytoexplainsuchdeposits.868

869

4.2 The “Monotonous intermediates” - remobilized mushes 870

A second conspicuous group of ignimbrites are the “Monotonous Intermediates”,871

consisting of homogeneous crystal-rich, typically dacitic units, without any evidence for872

significant crystal accumulation (Hildreth 1981; Lindsay et al. 2001;Maughan et al. 2002;873

Folkesetal.2011c;Huberetal.2012a).Suchunitstypicallydisplaycorrosiontexturesonthe874

low-temperatureminerals(quartzandfeldspars;Figure10)andpervasivereheatingpriorto875

eruption(BachmannandDungan2002;Bachmannetal.2002).Smallereruptionsofcrystal-876

richmagmas(e.g.,Montserrat,Murphyetal.2000;KosPlateauTuff,Keller1969;Bachmann877

2010a;TaupoVolcanicZone,Molloyetal.2008,Cascadearc;CooperandKent2014;Klemetti878

and Clynne 2014) also show a very similar reheating and partialmelting signal following879

rechargefromdeeper,hottermagmas(althoughmeltingevidencecanbesubtleorevennon-880

existentinsomecases).Thephysicalprocessesactinguponthisremobilizationprocesshave881

been explored by a number of papers (Bacon and Druitt 1988b; Druitt and Bacon 1989;882

Couch et al. 2001; Huber et al. 2010a; Burgisser and Bergantz 2011; Huber et al. 2011;883

Karlstrom et al. 2012). The reheating signal and the partialmelting signature inminerals884

favor amodelwhereby the reactivation results from the combination ofmelting andpore885

pressure increase in the mush in response to melting and the slow and progressive886

disaggregation of the weakened mush, a process coined as thermo-mechanical887


remobilization by Huber et al. 2010a. This model is also consistent with several striking888

characteristics (whole-rock homogeneity, high crystallinity, partial corrosion of minerals)889

thatsuchdepositsdisplay(Huberetal.2012a;CooperandKent2014;Parmigianietal.2014).890

891

Thermo-mechanicalremobilizationoflockedareasfollowingrechargeislikelytobea892

very common process in incrementally-built magma reservoirs. In most cases, such893

remobilizationeventswouldnotleadtoaneruption,butcouldpotentiallybeinferredfrom894

surface deformation, change in gas outputs, and/or seismic signals related to the895

emplacement of the recharge. When partial melting of crystal-rich, mostly anhydrous896

material,occurs,thenewmeltreleasedisdry,andwillimpacttherheologicalresponseand897

phase diagram of the newly producedmixture (Evans and Bachmann 2013; Caricchi and898

Blundy 2015; Wolff et al. 2015). As drier melt increases the temperature of mineral899

precipitation (e.g., JohannesandHoltz1996), theamountofmeltingwill tend tobe rather900

limited inmost cases (Huber et al. 2010a;Wotzlaw et al. 2013). Hence, the condition for901

eruptionduring remobilizationmight onlybe reachedwhensignificant heat andvolatiles902

(mainlyH2O) are injected by the incoming recharge. Exactly howmuch heat and volatiles903

need to be injected strongly depends on a number of factors (including relative size of904

rechargeandmush,geometry,compositionsofmagmaticendmembers,averagecrystallinity905

ofthemush,…),whichwillvaryfromcasetocaseandaredifficulttopredict.906

907


908

Figure10:Phasemapsofcrystal-richignimbrites,bothremobilizedcrystalmushesatdifferent909

stagesofrejuvenation;theMasonicParkTuffnolongerhasanysanidineandshowsonlytiny910

quartz microcrysts, whereas the Fish Canyon Tuff still shows large, albeit highly resorbed911

quartzandsanidinephenocrysts(Bachmannetal.2002).Inbothcases,plagioclaseandmafic912

crystals(hornblendeintheFCT,andpyroxeneintheMPT)showreversezoning(Lipmanetal.913

1996;BachmannandDungan2002).914

915


4.3 The unzoned, crystal-poor to intermediate deposits 916

There is a third type of ignimbrite that commonly appears in volcanic sequences,917

particularlyinareaswheremagmatismisrelativelydry(hotspotorriftenvironmentssuch918

as Yellowstone – SnakeRiver Plain or TaupoVolcanic Zone). In such cases, homogeneous919

ignimbriteswithcrystalcontentsontheorderof10-20%arecommonplace(Dunbaretal.920

1989;Nashetal.2006;EllisandWolff2012;Ellisetal.2013).Ubiquitousinsuchunitsare921

mm to cm-sized glomerocrysts of pyroxene-plagioclase-oxides (Figure 11). These922

glomerocrystsaremuchmoremafic (whenanalyzed inbulk) than thebulk rocks theyare923

found in,andentirely lacksanidineandquartz,while theyare foundasphenocrysts in the924

hostunits.However, thechemicalcompositionsof theirpyroxenesandplagioclasecrystals925

areidenticaltotheindividualphenocrystsaroundthem.Theseglomerocrysticpyroxeneand926

plagioclasealsoshowveryevolvedREEpattern,suggestinggrowthfromrhyoliticmelts(Ellis927

etal.2014).928

929


930

Figure 11: Glomerocrystswith cumulate characteristics found in ignimbrites from the Snake931

RiverPlain(modifiedfromEllisetal.2014)932

Within the framework of the mush model, such deposits are best understood as933

pockets of melt-dominated regions in relatively hot/refractory mushes built in such dry934

magmaticenvironments.Uponrechargefrombelow,thesedrymushesarelikelynottomelt935

aseasilyastheonesthatcontainsanidineandquartz(asfoundinwetter,coldersubduction936

zoneenvironments).Hence, thezoningproducingbypartialmeltingofcumulatesdoesnot937

happen, and thedeposits remainhomogenous.Asgas sparging canalsoplayan important938


role in mush defrosting (Bachmann and Bergantz 2006; Huber et al. 2010b), mushes in939

magmaticallydryenvironmentsaremoredifficult to remobilizebecauseexsolvedvolatiles940

fail to reach the critical volume fraction required for efficient heat transfer by advective941

transport(Huberetal.2010b),andproducemostlythisremarkabletypeofunzoned,crystal-942

poortointermediatedeposits(Ellisetal.2014;Wolffetal.2015).943

944

4.4 Chemical fractionation in mushy reservoirs: the development of 945compositional gaps in volcanic sequences 946

Compositionalgaps,orthepaucityoferuptedproductswithinarangeofcompositions,947

arecommonplaceinvolcanicsequencesandcalledtheBunsen-Dalygaps,inhomagetoearly948

workfromBunsen(in1851)andDaly(in1925)onIcelandandAscensionIsland.Suchgaps949

canbeseenindifferentelements(majorandtrace), inallseries, includingtholeiitictrends950

(e.g., Thompson 1972) and subduction zones (Brophy 1991; see Figure 11). Possible951

hypothesestoexplainthesegapsdominantlyrevolvearoundthreemainlinesofreasoning;952

(1)twomainmagmatypes(onemafic,onefelsic/silicic)aregeneratedintheupperpartsof953

our planet, mostly bymeltingmantle and crustal components (e.g., Bunsen 1851; Chayes954

1963;ReubiandBlundy2009),(2)twomagmatypesaregeneratedby liquid immiscibility955

(e.g., Charlier et al. 2011) and (3)magma evolution is dominated by crystal fractionation956

from mafic parents (generating a continuum of melt composition) but due to some957

mechanical trapping (potentially enhanced by non-linear temperature-crystallinity958

relationships),magmareachingthesurfacearedominantlyclusteredatsomecompositions959

(Marsh 1981; Grove and Donnelly-Nolan 1986; Brophy 1991; Bonnefoi et al. 1995;960


Francalanci et al. 1995; Grove et al. 1997; Freundt-Malecha et al. 2001; Thompson et al.961

2001;DufekandBachmann2010;Melekhovaetal.2013).962

963

Althoughthesehypothesescanallplayarole indifferentmagmaticsystemsaround964

theworld, themechanical trappingofmelt, as seenwithin the contextof themushmodel,965

shouldbedominantinlargemagmaticprovinceswheresuchgapsareobvious(oceanislands,966

subduction zones in thin oceanic crust or thin continental crust such as the Aegean and967

Taupovolcaniczones)andcrustalmelting/liquidimmiscibilityarelikelynotefficient.The968

keymechanicalreasoningfortrappingmeltisthefollowing:atlowmeltfractions(0to~50969

vol% crystals),melt and crystals are not easily separable, due to (1) the stirring effect of970

magma convection and (2) the short time that these magmas spent at low crystallinities971

(Marsh1981;KoyaguchiandKaneko1999;Huberetal.2009;DufekandBachmann2010).972

Hence, melts are dominantly extracted from the crystal cargo only after significant973

crystallization, showing then a composition that is very different from the mixture974

composition(Deeringetal.2011a).Asanexampleofageologicalsettingwheretheamount975

of crustal contamination shouldbevery limited (Sliwinski et al. 2015), the compositionof976

erupted magmas in Tenerife displays 3 principal modes in SiO2 (mafic, intermediate and977

felsicmagmas) likely indicatingmelt compositions released from(1) themantle (themost978

primitive), (2) the lower crustal differentiation zone (intermediate magmas) and (3) the979

upper crustal differentiation zone (the most felsic magmas). Although it was claimed by980

Melekhovaet al. (2013) that themagnitudeof thegaps shoulddiminishwith time (on the981

basisofpurelythermalcalculations),thisdoesnotappeartobetrueinmagmaticprovinces982

aroundtheworld(e.g.,Tenerife;Sliwinskietal.,2015).983


984

985

986

Figure 12: (a) Compositional groups observed in the Kos-Nisyros volcanic center (Eastern987

Aegean; see Pe-Piper and Moulton 2008 and Francalanci et al. 1995). (b) Tri-modality in988

compositionsofvolcanicrocksfromTenerife(ModifiedfromSliwinskietal.2015).989

4.5 The volcanic-plutonic connection 990

How compositionally and temporally-related (i.e., same age range, see section on991

timescales below) volcanic and plutonic units fit onto a common framework has been992

actively debated for nearly 2 centuries (discussion started by James Hutton at the 18th993

century, andpushed further in classicalbooksbyLyell1838;Daly1914;Bowen1928and994

papersormemoirssuchasRead1948;Read1957;Buddington1959;HamiltonandMyers995

1967; Pitcher 1987). Several recent review papers summarize the main issues at play996

(Bachmann et al. 2007b; de Silva and Gosnold 2007a; Lipman 2007; Glazner et al. 2015;997


Keller et al. 2015; Lipman and Bachmann 2015). Focusing on felsic/silicicmagmas (mafic998

plutons are clearlymostly considered cumulates for decades; e.g.,Wager et al. 1960), the999

main hypotheses revolve around whether intermediate to silicic plutons are mostly the1000

expression of crystallized melts (“failed eruptions”, i.e., have not undergone significant1001

crystal-liquid separation), or cumulate leftovers (“crystal graveyards”) from volcanic1002

eruptions (those two endmember hypotheses not being mutually exclusive in plutonic1003

complexes containing multiple facies). Textural analysis of plutonic lithologies and trace1004

element geochemistry (in both bulk rock and minerals) should be the two main lines of1005

arguments to differentiate between these competing hypotheses. If crystal accumulations1006

occursinareasofthecrust,1007

• Compatible trace elements should show significant increase in their1008

concentrations, while incompatible elements should remain low (e.g.,1009

Mohamed1998;Bachletal.2001;Wiebeetal.2002;Kamiyama2007;Deering1010

andBachmann2010;Turnbulletal.2010)1011

• Plutonic rocks should show some mineral orientations or preferential1012

alignment/foliation (e.g.,Wager et al. 1960; Arculus andWills 1980; Shirley1013

1986; Seaman 2000) related tomagmatic processes (e.g., hindered settling1014

and/orcompaction),whichwilltendtoorientanisotropically-shapedcrystals1015

astheyaccumulate.1016

1017


1018

Figure13:Exampleofacross-sectionthroughawell-exposedpluton(Searchlightpluton,NV),1019

showing areas of rich in crystallizedmelts and areaswith cumulate characteristics, but still1020

containingtrappedmelt(modifiedfromBachletal.2001;Gelmanetal.2014).1021

1022

Despite several attempts to look in details at this issue, this topic remains1023

controversial(seedeSilvaandGregg2014;Glazneretal.2015;LipmanandBachmann2015).1024


Therootofthecontroversymaylargelystemfromthefactthattheamountofcrystal/melt1025

segregation is less important in evolved (silicic) magmas compared to than more mafic1026

compositions (i.e., more trapped melt component in the intermediate to silicic crystal1027

cumulates)due largely to viscositydifference;Bachmannet al. 2007b).Hence, silicicunits1028

havemoresubtlegeochemicalor textural signaturesofcrystalaccumulation(ormelt loss)1029

than their mafic counterparts, and can easily be overlooked (see Gelman et al. 2014).1030

However, although recent publications involving compilations of geochemical data from1031

large databases (e.g., Glazner et al. 2015; Keller et al. 2015) cannot clearly differentiate1032

volcanic from plutonic compositions at the felsic/silicic end of the spectrum, other1033

compilations do see significant variabilitieswith volcanic units being on average richer in1034

SiO2 (Lipman 1984; Halliday et al. 1991; Lipman 2007; Gelman et al. 2014; Deering et al.1035

2016).Recentstudiesonfocusingonspecificfieldexamplesshowthatseveralintermediate1036

tosilicicplutoniclithologieshavehighcompatible/lowincompatibleelementconcentrations1037

inbulkrock(althoughsuchunitsclearlystillhavesignificanttrappedmeltcomponents;e.g.,1038

Bachl et al.2001;Barneset al.2001;DeeringandBachmann2010;LeeandMorton2015;1039

Figure13).Additionally,detailedtexturaldata,usingElectronBackscatteredElectron(EBSD)1040

techniques, on non-metamorphosed granitoïds (i.e., observedmineral orientationmust be1041

magmatic) indicate crystal alignment compatiblewith compaction/hindered settling, even1042

forveryviscousmagmacompositions(BeaneandWiebe2012;Graeteretal.2015).1043

1044

4.6 The high eruptability of rhyolitic melt pockets 1045

Whilehigh-SiO2 rhyolitesarenot rare in thevolcanic record, including severalvery1046

largeunits(>100km3;Lipman2000;Christiansen2001;Masonetal.2004),granitessensu1047


stricto appear much less commonly. The upper crustal plutonic record is dominantly1048

granodioritic/tonalitic in composition (Taylor andMcClennan1981;Rudnick1995).When1049

compiling data from large geochemical database, such as the NAVDAT1050

(http://www.navdat.org),onenoticesthatlowSrrocksare,relativelyspeaking,muchmore1051

abundant in the volcanic realm than in theplutonic record (Parmigiani et al. Inpress), an1052

observationalreadydiscusseda¼ofacenturyagoonamuchsmallerdatabase(Hallidayet1053

al.1991;CashmanandGiordano2014).1054

1055

A possible explanation for the unusually high volcanic/plutonic ratio of evolved1056

magmas is the fact that rhyolites are generated and stored in the upper crust (Tuttle and1057

Bowen1958;Gualda andGhiorso2013b;Ward et al. 2014; LipmanandBachmann2015),1058

wherevolatiles can exsolved inquantity. If exsolvedmagmatic volatiles can accumulate in1059

suchmeltpools, itwill render themhighlyeruptible(increasing thegravitationalpotential1060

energyof themagma)andwill tend topromotemorevoluminouseruptions (Huppertand1061

Woods, 2002). As a consequence itwill also lead to a deficit in the plutonic record.Using1062

multiphase numerical simulations of exsolved volatile transport in magma reservoir,1063

Parmigiani et al. (in press) have shown that bubbles tend to accumulate in crystal-poor1064

environments. In crystal-rich environments, the volume available for the exsolved volatile1065

phase is significantly reduced (porosity) by crystal confinement and promotes bubble1066

coalescence and the formation of buoyant fingering pathways. Once the pathways are1067

established,theveryviscousmeltdoesnotlimitthemigrationefficiencyofthevaporphase1068

andtransportisefficient.Incontrast,incrystal-poorenvironment,fingeringchannelsarenot1069

stableandbreakintobubblesundertheactionofcapillaryforces.Discretebubblescanonly1070


riseasfastastheviscousmeltcanbemovedoutoftheirway,whichsignificantlyreducesthe1071

migrationefficiencyofexsolvedvolatilesincrystal-poormagmas.Asaresult,bubblestendto1072

accumulateintheconvectingrhyoliticcap,providingadditionalpotentialenergytodriveor1073

sustainfutureeruptions.1074

1075

The accumulation of a buoyant volatile phase in these high SiO2melt pools favors1076

largeeruption,butitisunlikelytoprevententirelytheformationplutoniccounterpartsasit1077

onlybringsthesemagmasclosertoacriticalstate.Infact,zonesofevolvedhigh-SiO2granites1078

doseldomoccur inplutonicseries.Theyappeartobemostlypresent intheupperpartsof1079

plutons, near the roof. Typical examples of such evolved pockets have been described in1080

plutonsfromNevada(Bachletal.2001;Barnesetal.2001;MillerandMiller2002),Klamath1081

Mountain, CA (Coint et al. 2013), Sierra Nevada batholith, CA (Putirka et al. 2014) and1082

PeninsularRangeBatholith,Mexico(LeeandMorton2015).1083

1084

4.7 Pluton degassing and volatile cycling 1085

The chemical composition of the atmosphere is tied to some extent to magmatic1086

degassing (e.g., CO2 and H2O cycles). As magmas are dominantly forming plutons1087

(plutonic/volcanicratiosaretypicallyatleast5:1to10:1,andpossiblyevengreaterinsome1088

areas;Crisp1984;Whiteetal.2006;Wardetal.2014),thecycleofdegassingandoutgassing1089

inplutonsexertsan importantcontrolonthechemistryof thesurfacereservoirsonEarth.1090

Theprotractedperiodsofstorageandslowcrystallizationathighcrystallinitypredictedby1091

the mush model are prone to lead to particularly efficient magma degassing at shallow1092

depths.Insuchslowlycooledcrystal-richenvironments,exsolvedmagmaticvolatilebuildup1093


in the mush by second boiling (exsolution produced by the crystallization of dominantly1094

anhydrousminerals),andprogressively fillmoreof theconstrictedporespaceleadingtoa1095

more efficient outgassing (Parmigiani et al. 2011; Huber et al. 2012b; Huber et al. 2013).1096

Oncetheporespace,andmorespecificallytheporethroats,aresignificantlyreducedinsize1097

(downtothescaleofafewmicrons),capillarystressescanevenbecomelargeenoughforgas1098

pathwaystodeformlocallythegranularstructureinthemushandvolatile-filled“dikes”may1099

finalize the outgassing at large crystal fractions (>80% vol; Holtzman et al. 2012;1100

Oppenheimeretal.2015),possibly leadingtothe formationof theubiquitousapliticdykes1101

seeninplutons(e.g.,JahnsandTuttle1963;Candela1997).Ineithercase,thecombinationof1102

high crystal-content and increasing exsolved volatiles volume fraction can lead to the1103

formation of efficient degassing pathways even during quiescent periods (sometimes1104

potentiallypreservedasresidualporosityingranites,e.g.,Dunbaretal.1996;Edmondsetal.1105

2003;LowensternandHurvitz2008).1106

1107

4.8 The Excess S paradox associated with the eruption of silicic arc magmas 1108

Themassbalanceofvolatilesreleasedduringvolcaniceruptionscanprovidevaluable1109

insights into the state of shallow magma reservoirs prior to the eruption. As such, the1110

mismatchinSmassbalancebetweenpetrologicalinferences(i.e.,comparingtheScontentof1111

pre-degassed melt inclusions and fully-degassed matrix glass) and remote sensing1112

spectroscopy or sulfur output estimated from ice core data can provide additional clues1113

aboutthedynamicsthatprevailinmagmachambersandmorespecificallyaboutthefactors1114

thatcontrolvolatileexsolution.ThenotionofExcessSreferstotheunderestimationofthe1115

sulfuroutputfrompetrologicalconstraints(meltinclusionrecord)duringaneruption.This1116


“ExcessS”paradoxhasbeenevidentsincetheeruptionofElChichoninMexico(Luhretal.1117

1984)anddescribedindetailsfortheeruptionofMtPinatuboin1991.AtPinatubo,remote1118

sensingmethodsestimated theSoutput tonearly20Mt(e.g.,Sodenetal.2002)while the1119

petrologicalmethod provided an estimate smaller by a factor of 10 (Gerlach et al. 1996).1120

Sincethesetwoeruptions,thereleaseofSinexcesstopetrologicestimatesseemstobemore1121

therulethantheexception,especiallyforsilicicmagmasinarcs(seeFigure14andWallace1122

2001;Shinohara2008).1123

1124

Figure 14: Excess Sulfur is based on the mismatch between the SO2 flux measured during1125

volcanic eruptions (y-axis; typically done by spectroscopic methods or ice core data) and1126

petrological estimates based on the difference in S content between melt inclusions and1127

interstitialglass,estimatingtheamountofSreleasedbytheeruptivedecompressionprocess(x-1128

axis).1129


1130

Althoughmultiplehypotheseshavebeensuggestedtoaccount forthisExcessS(see1131

Gerlach et al. 1996 for a review), the presence of a S-rich exsolved volatile phase in the1132

magmareservoir seems tobe themost commonlyacceptedpostulate (Gerlachetal.1996;1133

Scailletetal.1998a;Wallace2001;Shinohara2008).Crystallization-driven(secondboiling)1134

exsolution in a shallow magma reservoir can generate significant excess S where the S1135

trapped by vapor bubbles during crystallization is not accessible to melt trapped1136

subsequently as inclusion in crystal hosts. Although second boiling is bound to play an1137

important role on generating Excess S in crystal-rich magmas, in crystal-poor silicic1138

eruptionsthedeficitinSinmeltinclusionsrequiresanalternativeprocess.Inthelightofthe1139

previous section, accumulation of exsolved (S-rich) volatile bubbles in the erupted, low1140

crystallinity portions of magma reservoirs would provide a closure to the problematic1141

missingSincrystal-poorsilicicmagmas,Thesebubbles(poorly-constrained,butlikelyupto1142

20-30ofpercentbyvolume)canbeextremelyrichinS,duetotheveryhighaffinityofSfor1143

theexsolvedvolatilephase(Zajaczetal.2008).Hence,themushmodel,predictingefficiently1144

degassedcrystal-richzonesandgas-charged,highlyeruptiblepocketsofmagma,providesa1145

consistent framework to explain the missing source of S necessary to close the volatile1146

budgetforcrystal-poorunits.1147

1148

4.9 Caldera cycles 1149

Caldera-forming events typically happen after long periods of maturation in long-1150

lived magmatic provinces during which andesitic volcanism dominates for up to several1151

millionsofyears(Lipman,2007;deSilvaandGosnold2007b;Grunderetal.2008;Deeringet1152


al. 2011a).During this andesite-dominatedperiod, the crust initiallywarmsup to become1153

more amenable to host silicic magma reservoirs in the mid to upper crust (Jellinek and1154

DePaolo2003;deSilvaandGregg2014).Oncethismaturestageisreached,itisnotrareto1155

seeseveral,caldera-formingeventsoccurringinrelativelyshorttimespans(≈2-3Myr;multi-1156

cycliccalderasystems;Hildrethetal.1991;Grahametal.1995;Christiansen2001;Lipman1157

and McIntosh 2008; Lipman and Bachmann 2015). During such caldera cycles, magmas1158

erupting shortly (≈5-30 kyr) prior to the caldera-forming event typically appear to be1159

compositionally similar to the climactic event (e.g., Bacon and Druitt 1988b; Bacon and1160

Lanphere 2006; Bachmann et al. 2012), whereas the post-caldera magmas appear more1161

mafic,anddrier(Shaneetal.2005;Bachmannetal.2012;Gelmanetal.2013a;Barkeretal.1162

2014; Figure 14). These petrological shifts can be explained by the fact that the left-over,1163

crystal-rich (mush) portion of the magma system following the eruption will be strongly1164

affectedby thecaldera-formingevent (Hildreth2004).As thoseeruptions typically lead to1165

significantdecompressionof theunderlyingcrustalcolumn(potentiallyup to50-100MPa,1166

accountingfortheoverpressurenecessaryfortheeruptiontostartandtheredistributionof1167

massfollowingtheremovalofmuchoftheeruptiblepartsofthereservoir,seeBachmannet1168

al.2012),somerapidandsignificantdegassingandcrystallization isexpected inthemush,1169

particularly if it isnear-eutecticandvolatile-saturated(Bachmannetal.2012).Formushes1170

that were volatile-saturated prior to the decompression, the caldera eruption will likely1171

promote the rapid formationofgaschannels resulting indeepgas release.Hence, the left-1172

overmush is expected to becomemore crystalline (decompression-driven crystallization)1173

andmoredegassed than itwasbefore the calderaeruption (Degruyteret al. 2015). Itwill1174


takeanotherlongmaturationstageforthesystemtobeprimedforanewcalderaeventwith1175

ashallow,gas-charged,evolvedmagmabody.1176

1177

1178

Figure 15: A caldera cycle recorded by changes in temperature, oxygen fugacity, bulk rock1179

composition,andmineralogy in theKos-Nisyros volcanic system, easternAegean.Pre-caldera1180

units (Kefalos domes and pyroclastic units) show highly evolvedmagma compositions (high-1181

SiO2 rhyolites), low temperature, oxidized and water-rich conditions, similar to the caldera-1182

formingevent(KosPlateauTuff,KPT).FollowingtheKPT,Nisyrosvolcanobuiltup,generating1183

moretypically lessevolvedmagmas, includingtwo largerhyodaciticunits (lowerPumiceand1184


Upper Pumice), with drier, more reduced compositions, and hotter magma temperatures.1185

SimilarcycleshavebeensuggestedfortheTaupoVolcanicZone,inNewZealand(modifiedfrom1186

Bachmannetal.2012).1187

5 Timescales associated with magma reservoirs 1188

How long does a magma body stay above the solidus in the Earth’s crust (i.e.,1189

potentiallyactive)remainsoneofthekeyquestionsinmagmaticpetrologyandvolcanology.1190

The longevity, of course, impacts the extent of magmatic differentiation (including ore1191

formation)andtheeruptivevolumethatareavailableatanygiventime.Asdiscussedabove,1192

magmatic reservoirs aremostly kept as high-crystallinity regions above the soliduswhen1193

active, but some controversies remain as to how long the system remains in such amush1194

state (with estimates from a few 1’000s to more than 1’000’000 years). The crux of the1195

debate lies in the fact that different analytical techniques provide different answers, and1196

likely date different processes. Zircon dating of silicic plutons and volcanic rocks record1197

extendedcrystallizationhistories, ranging fromtensof thousands toseveralmillionsyears1198

(Reidetal.1997;BrownandFletcher1999;Lowensternetal.2000;ReidandCoath2000;1199

Charlieretal.2003;Schmittetal.2003;Colemanetal.2004;VazquezandReid2004;Bacon1200

and Lowenstern 2005; Charlier et al. 2005; Simon and Reid 2005; Bindeman et al. 2006;1201

Matzeletal.2006;Bachmannetal.2007a;Charlieretal.2007;Milleretal.2007;Walkeretal.1202

2007;Reid2008;Claiborneetal.2010;Schmittetal.2010b;Klemettietal.2011;Stormetal.1203

2012;Walkeretal.2013;Wotzlawetal.2013;Chamberlainetal.2014b;Stormetal.2014;1204

Wotzlaw et al. 2014). In contrast, crystal size distributions (CSD; Pappalardo and1205

Mastrolorenzo2012),anddiffusiontimescales(seeDruittetal.2012andthecompilationof1206


data in Cooper and Kent 2014) are typically much shorter (typically months to a few1207

hundredsofyears).1208

1209

Asmostelementsusedindiffusiontimescalemodelingmoverelativelyfast(e.g.,Fe-1210

Mg in mafic minerals, Ti in quartz, Sr and Mg in plagioclase, Li in zircon, …), it is not1211

surprising that preserved zoning patterns in such elements indicate relatively short1212

timescales. Using simple scaling, one can provide a rough estimate ofmaximum timescale1213

that diffusion re-equilibrationwouldprovide if one knows the size of the crystals and the1214

diffusioncoefficients(time~R2/D,whereRistheradiusandDthediffusivity).Elementsthat1215

are diffusing slower (REE, Ba) can be used to obtain longer timescales (e.g., Morgan and1216

Blake 2006; Turner and Costa 2007), but, in such cases, periodic resorption, concurrent1217

crystalgrowthand3-Deffectsassociatedwith the finitesizeof thehost lead tosignificant1218

complications in estimating time. For example, growth of feldspars in the Yellowstone1219

magma reservoirs (Till et al. 2015) demonstrably led to “diffusion-like” profiles that look1220

similarforelementsthatarediffusingatverydifferentrates.Similarly,CSDstudiestypically1221

assume lineargrowthrates (nopauses incrystallization,noresorptionevent),withvalues1222

obtained from experiments (hence typically faster than what actually happens in silicic1223

magmareservoirs).Hence,crystalsizedistributionsprovidealowerbound,andsometimes1224

stronglyunderestimated,timescale.1225

1226

The short timescales obtained by diffusion modeling are best interpreted as1227

reactivation/remobilization/mixingtimescales(e.g.,Martinetal.2008;Matthewsetal.2012;1228

Tilletal.2015).They,however,donotprovideatimescaleofmagmareservoirgrowthand1229


storageinthecrust.Forsuchinformation,thezirconagedistributions,orbulkU/Th/Ra/Pb1230

ages likely provide more reliable estimates (Reid et al. 1997; Cooper and Reid 2003;1231

Bachmannetal.2007a;Turneretal.2010;CooperandKent2014;Guillongetal.2014).The1232

differencebetweeneruptionage(estimatedusingtheyoungzirconpopulationortheAr/Ar1233

age when available) and the oldest co-genetic zircons (i.e, antecrysts, see Bacon and1234

Lowenstern2005;Milleretal.2007foradefinition) is typicallyat least104-105years(see1235

Costa 2008; Reid 2008; Simon et al. 2008; Bachmann 2010b for reviews on the topic).1236

Xenocrysts(foreigncrystalscomingfromunrelatedwallrocklithologies)canalsooccur,but,1237

in some cases, they can be isolated and removed from the dataset (as they can be1238

significantly older, plot off theConcordia and/orhavedifferent trace element chemistries;1239

Miller et al. 2007; Lukács et al. in press). Of course, the presence of older but co-genetic1240

zircons(antecrysts)doesnotmeanthatthesystemremainedabovethesolidusforthewhole1241

time; some parts of the system could have reached the solidus, and later recycled by1242

reactivation events (Schmitt et al. 2010a; Folkes et al. 2011a).Hence, physicalmodels are1243

neededtoaddressthisissue.1244

1245

Numericalmodelsof thethermalstateofsuchsystemsusuallyagreewithrelatively1246

long timescales of tens to hundreds of thousands of years, particularlywhen they include1247

incrementalrechargeatrelativelyhighfluxes(e.g.,Annenetal.2008;Gelmanetal.2013b).1248

Moreover, it is very costly, both in termsof volatiles andenergy, to reactivate sub-solidus1249

plutons;nearlyallvolatileelements(includingCO2andH2O;CaricchiandBlundy2015)and1250

all latentheathasbeenlosttothesurrounding.Ifthesubsolidusmaterialhastobemelted1251

again, the thermalbudget required involvesnotonly the sensible and latentheat loss, but1252


also a large fraction of energy wasted on reheating solid material that will not undergo1253

melting (Dufek and Bergantz 2005). The assimilation of these devolatilized sub-solidus1254

portionsofthebodywillalsoinduceadepletionofvolatilecontentinthereservoir.Froma1255

mechanical standpoint, entrainment/recycling of material that is not completely solid is1256

muchmore likely, as itwill disaggregatemore easily, particularly if some localmelting at1257

grainboundariesisinvolved(Beardetal.2005;Huberetal.2011)1258

1259

In summary, the periodic recharge of magma at reasonably high fluxes (>10-4-10-31260

km3/yr; Annen et al. 2008; Gelman et al. 2013a; Gelman et al. 2013b; Karakas and Dufek1261

2015)islikelytomaintainatleastpartsofsilicicmagmareservoirsintheEarth’scrustarein1262

amushstateforperiodsextendingtoatleastseveral100softhousandsofyears,eveninthe1263

upperpartofthecrust(8-10kmdepth).Shortertimescales,obtainedwithothertechniques1264

(particularlydiffusionmodeling)mainlyrefertoperiodsofreactivationfollowingsignificant1265

heatandmassadditionintothereservoirs(CooperandKent2014;Tilletal.2015).Hence,1266

crystal/liquid separation is likely to have enough time to occur, despite being sluggish, to1267

some extent in those reservoirs, driving differentiation towards more evolved, and more1268

eruptible crystal-poor magma pods (e.g., Bachmann and Bergantz 2004; Hildreth 2004;1269

Bachmann and Bergantz 2008c; Lee et al. 2015).We note that those thermalmodels are1270

lacking the mechanical part of the energy budget (overpressurization during recharge,1271

depressurizationduringeruption),whichcanhaveasignificant impactonthestabilityand1272

durationofreservoirs(DegruyterandHuber2014;Degruyteretal.2015)andthefeedbacks1273

with crustal rheology (Gregg et al. 2013; de Silva and Gregg 2014). For example the1274

exsolutionofvolatilesaffect theeffective thermalpropertiesof themagmaby reducing its1275


thermalcapacityandconductivity(Huberetal.2010b),andconsuminglatentheat.Wenote,1276

however,thataddingthismechanicalparttothemodelsisunlikelytosignificantlyshorten1277

thelifetimesofthereservoirs.1278

1279

6 Frontiers in our understanding of magma chamber evolution 1280

1281

Over the last paragraphs, we have tried to assemble many important questions in1282

volcanology/petrology into a coherent framework. The Polybaric Mush Model (Figure 8)1283

provides a context to explain several seemingly unconnected observations, such as the1284

geophysical imagingofcrystal-richzonesinmagmaticallyactivezonesatdifferentlevels in1285

the crust, the chemical differentiation of magmas dominated by fractional crystallization1286

(withsomeassimilation,explainingsomeofthevariabilityinisotopicdata)ofmaficparents,1287

the production of compositional gaps in volcanic series, the presence of cumulates in the1288

plutons records, and the volatile mass balance in shallow reservoirs. However, there are1289

manyquestions thatremaintobeanswered,andthenextsectiontries tooutlinessomeof1290

thechallengesthatourcommunityfacesintheyearstocome.1291

1292

At themost fundamental level, thegoal is tocontinuedevelopingamore integrated1293

picture between physical models, geophysical/geodetical inversions and1294

petrology/geochemistry. In particular, we should tend to obtain amulti-phase andmulti-1295

scale(timeandspace)pictureofmagmaticsystems,fromtheformationoftheprimarymelts1296

to their final resting place in the crust or at the surface (including interactions with the1297

atmosphere).Inordertoreachsuchanambitiousgoal,wewouldneed,amongmanythings:1298


• To revisit the information that can be extracted from the inversion of1299

geophysical datasets. Thebest resolution that one can obtain fromamagma1300

body at depth is much lower than most structures of interest in these1301

dynamical environments. As such, it is important to understand how the1302

implicit filteringoperatesandhowit impactsthesetofquestionsthatcanbe1303

logicallyaddressedwiththesestudies.Forexample,ifthetargetoftheanalysis1304

is to estimate melt fraction from Vp/Vs tomography, even with good1305

experimental calibration, one is left towonderwhat the estimates of amelt1306

fractionatthescaleofhundredsofmeterstoevenkilometersreallymean.Is1307

theoutcomeof the inversiontobeunderstoodasavolumetricaverage(as is1308

commonly assumed)? The effective mechanical properties of heterogeneous1309

media, which is what one measures through inversions, are generally quite1310

different from a volumetric average. For complex, but structured,1311

heterogeneousmedia,theeffectivepropertiesinferredfromelasticproperties1312

for example, result from a complex non-linear optimization process. For1313

instance, it isquitepossible that theeffectivemediumelasticproperties that1314

oneretrieves fromtomographyaredominantlycontrolledbyheterogeneities1315

thatarevolumetricallysecondary.Wecertainlyneedmoreefforttorelatethe1316

best-fit results of these inversions to the physical reality of complex1317

multiphasemagmabodies.Thisstartswithabetterunderstandingoftherole1318

ofsubgridscalestructuresontheeffectivepropertiesattheresolutionofthe1319

inversion.1320


• Tocontinuedevelopingandimprovingourinterpretationoftracersforratesof1321

processes in magmatic systems. For example, it is necessary to improve1322

precision and spatial resolution of mineral geochronology; and do more1323

diffusionmodelingofmeasuredgradients todetermine timescales, and focus1324

on what they mean. It also becomes possible to use stable and radiogenic1325

isotopes to constrain thermodynamics and kinetics, although the most1326

substantialeffortinthisdirectionsofarrelatestomaficmagmas(Richteretal.1327

2003;Watkins et al. 2009b; Huang et al. 2010; Savage et al. 2011).We also1328

need better constraints on possible growth rates of minerals and bubbles.1329

Dissolution events are transparent to the present chronometers and leaves1330

hiatus in the temporal evolution of magma bodies that are impossible to1331

quantify at the present time. Canwe find kinetic tracers thatwould provide1332

information about the duration and frequency of resorption/dissolution1333

events?1334

• Tobetter constraindisequilibrium/kinetic effects inmagma reservoirs.Most1335

of the petrology and trace element partitioning among phases in magma1336

reservoirs assumes at least a local equilibrium. The mere fact that1337

crystallization(andsometimesdissolution)takesplacerequiresafinitedegree1338

of disequilibrium, even over small (crystal sizes) scales. In order to relate1339

dynamical processes to the rock record, a kinetic framework is necessary.1340

Thereareampleinformationtominefromheterogeneitiesanddisequilibrium;1341

these clueswill becomecrucial in testingdynamicalmodels andestablishing1342

whatsetofprocessescontrolsthetemporalevolutionofthesereservoirsover1343


different spatial and temporal scales. Shall the high T geochemistry and1344

petrology community follow the low T geochemistry community and invest1345

timeindevelopingreactivetransportmodelingfortraceelementsandisotopes1346

inmagmabodies?The chemistry canbecomevery complicated, but the idea1347

eveninthecontextofoversimplifiedmodelsisworthpursuing.1348

• Tofocusoneruptiontriggeringmechanisms.Oneof the fundamentalsocietal1349

goalsinvolcanologyistounderstandthecauses(triggers)thatdrivevolcanic1350

eruptions.Thereareseveralpossible factors that can influence thestateofa1351

shallow reservoir and influence the timing of an eruption. Triggers can be1352

external (e.g. caused by a regional earthquakes, roof collapse, or magma1353

recharges from deeper; Sparks et al. 1977; Pallister et al. 1992;Watts et al.1354

1999; Eichelberger and Izbekov 2000; Gottsmann et al. 2009; Gregg et al.1355

2012) or internal (e.g. buoyancy, pressure buildup during recharge or gas1356

exsolution;e.g.,Caricchietal.2014;DegruyterandHuber2014;Malfaitetal.1357

2014).Fundamentally, thenotionof trigger,which isrelatedtoaneventora1358

process thathas a causal link to a subsequent eruption, remainsquite loose.1359

Establishing a causality between a trigger and an eruption is much more1360

challengingthanestablishingacorrelationbetweenthetwo.1361

• Tocontinueprovidingbettermodelsoflong-standingissuessuchasthe“room1362

problem”. Although the “room problem” is alleviated by having long-term1363

incrementaladditioninamainlyductilecrust,pushingmaterial(a)totheside,1364

(b) down, as dense cumulates from the lower crust founder back into the1365

mantle (Kay and Mahlburg Kay 1993; Jull and Kelemen 2001; Dufek and1366


Bergantz2005;JagoutzandSchmidt2013)and(c)upwards,asuppercrustal1367

reservoirdomeupduringresurgence(e.g.,SmithandBailey1968;Lipmanet1368

al. 1978; Marsh 1984; Hildreth 2004), the question continues to drive1369

controversies (e.g., Glazner and Bartley 2006; Clarke and Erdmann 2008;1370

GlaznerandBartley2008;Patersonetal.2008;YoshinobuandBarnes2008).1371

1372

7 Outlook 1373

At the present time, many questions remain to be answered about the migration,1374

emplacement, evolution and ultimately eruption of silicicmagmas in the upper crust. The1375

mainchallengesstemfromthefactthat:1376

(1) wehaveaccesstoonlyalimitedsetofindirectobservations,inmostcases1377

difficult to relate to the actual processes that control the chemical and1378

dynamicalevolutionofthesemagmasand1379

(2) mostmeltsgeneratedatdepthneverreachthesurface.1380

1381

Thedevelopmentofaself-consistentmodel forsilicicmagmasresiding intheupper1382

crust is complex and requires a joint effort across multiple disciplines. The mush model1383

presented in this review provides, we believe, a testable hypothesis from which better1384

models can be developed. In spite of what remains to be done to understand the fate of1385

magmasinthecrust,themushmodeloffersaself-consistentparadigmthatissupportedby1386

several independent observations from geophysics, geochemistry and petrology. For1387

instancethedevelopmentofmushzonesisconsistentwith1388


• Thermalmodelsofincrementally-growingmagmabodiesandgeophysicaldata1389

onactivesystemsrequirethatsuchreservoirsaredominantlycrystal-richon1390

long timescales (> 100 kys; Marsh 1981; Koyaguchi and Kaneko 1999;1391

BachmannandBergantz2004;CooperandKent2014).1392

• Chemical differentiation can occur internally by melt-crystal separation and1393

produce shorter-lived eruptible pockets (with melt content higher than 501394

vol%) from time to time.Melt extraction can largely occurwhen convection1395

currentswanesdown,i.e.whenmixingbystirringbecomeslessefficientthan1396

gravitational settling. Phase separation should be most efficient at1397

intermediate(40-70%)crystallinities.1398

• The combinationof incremental enthalpy additionsby recharges, latent heat1399

buffering and slower cooling rate of silicic magmas at high crystal content1400

reconciles the contrast between the longevity of magma centers/lifetime of1401

volcanicfields(asinferredfromzirconagepopulations)andthecomparatively1402

shortdurationoverwhichmagmasaremobileandpronetoerupt(Cooperand1403

Kent2014).1404

• Theanatomyofwell-exposedcross-sectionsinplutons(e.g.,Bachletal.2001;1405

Barnesetal.2001;Putirkaetal.2014)andthepresenceofplutoniclithologies1406

-cognateclastsinvolcanicunitswithcumulatesignature(Gelmanetal.2014;1407

Lee and Morton 2015; Wolff et al. 2015), although this aspect remains1408

controversial(e.g.,Glazneretal.2015;Kelleretal.2015).1409

1410


8 Acknowledgements 1411

Despite our attempt to cite the largest amount of previous work (we believe the1412

reference list is rather extensive with nearly 500 citations), we are certainly not giving1413

enoughduecredittotheamazingworkthatpeoplehavedoneovertheyearsinourtopicof1414

interest.Weapologize for this inadvance.Mostof these ideasput forwardherehavebeen1415

nucleatinginthewombofourownbiasedview.Wehavethemherefordiscussion,andtrust1416

our community that someormostof themwillbe challenged…We look forward to it.We1417

thankMikito Furuichi, NaomiMatthews,Marc-Antoine Longpré and Katy Chamberlain for1418

providing figures that we use to illustrate some recent advances in our field, and Keith1419

Putirkaformotivatingustowritethispaper.CommentsfromBenEllis,WimDegruyterand1420

PeterLipmanonapreviousversionof themanuscripthelpedbettercrystallizesomeideas1421

thatweretoountangledinamushydiscussion.WeareindebtedtoCharlieBaconandShan1422

deSilvaasformalreviewers,fortheinsightfulandconstructivecomments,aswellastothe1423

veryefficienteditorialhandlingbyFidelCosta.1424

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1426

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Silicic Magma Reservoirs in the Earth’s Crust · 1 Silicic Magma Reservoirs in the Earth’s Crust 2 3 ... 69 postulate causes for magma migration, storage, and interaction at different