Paragenesis of magma chamber internal wall discovered in ...Terra Nova, 00: 000–000, 2015 Introduction Ophiolites in Oman originated at a fast-spreading ridge, and their crustal

Paragenesis of magma chamber internal wall discovered inOman ophiolite gabbros

Adolphe Nicolas, Franc�oise Boudier and David MainpriceGeosciences, Universit�e Montpellier II, Montpellier 34095, France

ABSTRACT

A detailed study in Wadi Farah reveals a singular point in

Jebel Dihm (southern Oman ophiolite). Here, we investi-

gated some primitive features of accretion at a fast-spread-

ing ridge that has exceptionally been exempt from hydrous

alteration while crossing the TBL (thermal boundary layer).

The conjunction of two factors has made this possible in

the Farah area: its location at the point of sharp foliation

rotation from the upper to the transitional gabbros and a

locally twisted TBL. These factors may have favoured the

intrusion within the TBL of hydrous wedges transverse to

the ridge. Deeper into the TBL, near the active ridge, the

wedges merge and are covered by veneers of totally fresh

gabbros bearing glittering acicular clinopyroxene. These

gabbros are interpreted as relics of the crystallizing internal

wall of the magma chamber that have interacted with

hydrous fluids. This internal TBL has anisotropic thermal

properties and acts as a thermal blanket, maintaining high

magma-chamber temperatures.

Terra Nova, 00: 000–000, 2015

Introduction

Ophiolites in Oman originated at afast-spreading ridge, and their crustalsection retains evidence of its driftthrough the magma-chamber wall.The cooling rate was initially veryfast and then progressively slowed.In the Oman ophiolite, the laterphases at the lower temperatures(LT, at ~500 °C and below; green-schist facies conditions) have beenthe most studied (Nehlig et al.,1994). These phases tend to erase evi-dence of the hydrothermal eventsthat occurred at higher temperatures(HT, 700–825 °C) (Manning et al.,2000; Kawahata et al., 2001; Cooganet al., 2002), and at very high tem-peratures (VHT ~900–1000 °C)(Nicolas et al., 2003; Bosch et al.,2004; France et al., 2013). Here, wetentatively try to understand thedynamics of the initial cooling withinthe internal wall of the magmachamber, and, in particular, thegeometry of the hydrothermal flowchannels recorded in selected out-crops of the Oman ophiolite alongthe Wadi Farah, in the Jebel Dihm.

Magma-chamber cooling: thermalsink and thermal boundary layer(TBL)

The conventional model of the initialcooling of oceanic ridges by domi-nantly conductive cooling (Parsonsand Sclater, 1977) has largely beenabandoned in favour of the uncon-ventional ‘thermal sink’ model,which explains the thermal heat defi-cit near the ridge (Stein and Stein,1994). This model has been sup-ported by the Galapagos ridge exper-iment (Fehn et al., 1983), by physicalmodelling (Cathles, 1993; McCollomand Shock, 1998), by numerical mod-elling (Cherkaoui et al., 2003; Spi-nelli and Harris, 2011; Hasencleveret al., 2014) and by some field evi-dence in Oman (Cathles and Nicolas,2009). The thermal sink model pre-dicts that cold water penetratesthrough cracks within the oceaniccrust to near the Moho, as proposedby Manning et al. (2000), Nicolaset al. (2003) and Bosch et al. (2004).Hydrous fluids heat up when the sea-water penetrates close to the magmachamber within an internal thermalboundary layer (TBL) that totallyenvelops the magma chamber. Theheated water flushes back throughthe external TBL and LT channels toblack smokers on the seafloor.Assuming that the TBL wrapping

the magma chamber has a constantthickness, its ~100 m thickness is bestestimated from the vertical distance

between the roof of the melt lens andthe root zone of the sheeted dike com-plex (Nicolas et al., 2008). In theinternal part of the TBL, only a fewtiny hydrous cracks of VHT paragen-esis have been described from VHTconditions (Nicolas et al., 2003). Thissuggests that at VHT, cooling oper-ates mainly by conduction (Cooganet al., 2002), in contrast to the exter-nal TBL, which is a fluid-rich domainmarked by hornblende-bearing recrys-tallized gabbros (Manning et al.,2000). Locally, this grades into adense network of green hornblendeveins, which are replaced at LT(

in areas of warmer hydrous alter-ation, is transverse to the sheeteddikes (Nicolas et al., 2000b). Thisconforms to Hasenclever et al.’s

(2014) hybrid modelling. Field datain Wadi Farah supporting this geom-etry of the hydrothermal flow chan-nels initiated this study.

Farah area

We focus here on a local spot(800 m 9 300 m) along the Wadi

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Fig. 1 Structural map (planar structures) of eastern Jebel Dihm (see insert) and location of the Farah site. Farah is east of oneof the major N–S faults limiting the Farah-Him segment. The crust of this segment (marked in pale yellow) is much thinnerthan in the western domain. This is partly ascribed to the fact that the mantle foliations and the Moho are steeper here thanthey are westward. Blue dotted lines are inferred limits inside the gabbro section. Two limits between the upper gabbros andtransitional gabbros are marked in the Farah-Him segment: the southern limit corresponds to layering vs. foliation measure-ments; the northern limit corresponds to foliation dips of over 70° in the upper gabbros.

2 © 2015 John Wiley & Sons Ltd

Magma chamber internal wall in Oman ophiolite • A. Nicolas et al. Terra Nova, Vol 0, No. 0, 1–10.............................................................................................................................................................

Farah, in Jebel Dihm (Fig. 1), a 30-km-long monocline, which exposesthe entire ophiolite crustal section in aremarkably simple and homogeneousway (Pallister and Hopson, 1981;Nicolas et al., 2000a; Nicolas andBoudier, 2015b). Wadi Farah (Fig. 1)is located 7 km east of the inferredridge at Kazinah, which is suggested tohave died in situ (Nicolas and Boudier,2015a). Age determinations (Riouxet al., 2012) suggest that at the latitudeof Farah the spreading rate was fast.The Farah site exposes a critical junc-tion where the foliation in the uppergabbros rotates sharply to the transi-tional gabbros (Fig. 2A and E). Thepresent study was also motivated bythe discovery, in totally fresh gabbros,of acicular cm-long glittering-clinopyr-oxene laths (Fig. 3). A few similaroccurrences of spectacular pyroxene ingabbros have been found nearby.

In conclusion, the Farah area dis-plays, in excellent outcrop condi-tions, several features that raisequestions: is there a causalitybetween the presence of glitteringclinopyroxene and the occurrence oftransverse alteration wedges, as theyare illustrated in Fig. 3? Or do theyrelate to where the foliation in theupper gabbros rotates to the transi-tional gabbros? How sharp andirregular is this rotation, and howdoes this exposure compare to otherOman gabbros? This exceptionalarea may cast light on the, so far,enigmatic magma chamber wall.

Structural change from the uppergabbros to the transitional gabbros

The upper gabbros display strong S1foliations and plunging L1 lineationsdue to subsidence (Nicolas et al.,

2009). They rotate into the moder-ately dipping transitional gabbros,where the S0 layering is more con-spicuous than the S1 foliation. Atthe Farah site (Fig. 2), this rotationoccurs at a depth of 1 km below thelid (basalts and sheeted dikes). Thetransitional gabbros are folded onvarious scales and record differentmagmatic flow conditions (Nicolasand Boudier, 2015b).

The glittering-clinopyroxene-bearinggabbro in the field

The presence of glittering acicularclinopyroxene in totally fresh gabbrosis best observed in the fine-grainedupper gabbros from the top of theFarah hill (Figs 2 and 3), but theclinopyroxene is also found in the transi-tional gabbros. Otherwise, the distribu-tion of glittering-clinopyroxene-bearing

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Fig. 2 (A) Structural map of the Farah hill showing traces of layering in the transitional gabbros (solid lines) and foliation inthe upper gabbros (dashed lines); UTM coordinates. (B) This ridge along the Wadi Farah exposes the contact between (C) thetransitional layered gabbros, which dip moderately southeast, and (D) the upper foliated gabbros, which trend N–S and aresteeply dipping. The glittering acicular pyroxene is mostly developed in the upper foliated gabbros. (E) Cross-section NNW–SSE. The green band is the level of glittering pyroxene exposed.

© 2015 John Wiley & Sons Ltd 3

Terra Nova, Vol 0, No. 0, 1–10 A. Nicolas et al. • Magma chamber internal wall in Oman ophiolite............................................................................................................................................................

gabbros in the field is limited to smalloutcrops mixed with the normal gab-bros. The thickness of these forma-tions has been evaluated on GoogleEarth maps at five field stations. Thesurprising result is that their thicknessis negligible, in the meters range.Where glittering-clinopyroxene-bear-ing gabbros are present in the steeplyfoliated upper gabbros, they areclearly discordant with the gabbrofoliation (Fig. 2E). Indeed, every-where they appear to be a flat veneerunrelated to the gabbro foliation.

Petro-structure of the glittering-clinopyroxene-bearing gabbro

A thin-section sample described inFigs 4 and 5 is representative of the

gabbro covering the Farah hill(Fig 3). The elongated clinopyroxene(shape ratio: 1 9 3 mm to 1 98 mm) dominates the gabbro fabric;plagioclase is equigranular (

ene. The TiO2 contents of the clinopy-roxene can be split into two groups,with low values characterizing thelarge elongated clinopyroxenes. TheAn content of plagioclase is veryhomogeneous at 80 � 1.

Pole figures of crystal preferredorientation (CPO) measured by elec-tron back-scattering diffraction(EBSD) are shown in Fig. 6 in thesame reference frame as the views inFigs 4 and 5; the traces of the

foliation and lineation are N–S. Pla-gioclase and clinopyroxene CPOshow planar fabrics dominated bythe steep foliation plane. Theextreme elongation of the clinopyrox-ene crystals leads to a linear trend.

The wedge-shaped and SW-openingVHT–HT alteration bands

In gabbros on the eastern slope ofthe hill, traces of typical HT alter-ation grade into veneers of the glit-tering-clinopyroxene gabbros nearthe top of the hill (Fig. 3). The HThydrothermal alteration (~800–600 °C) is marked by green horn-blende, zoisite and magnetite parage-nesis in recrystallized gabbros(Fig. 3D). The detailed map inFig. 3A highlights the three wedge-shaped exposures of glittering-clino-pyroxene gabbro grading eastwardsinto HT gabbros.The western slope of the ridge is

fairly homogeneous in terms oftopography and presents a uniformHT alteration. On this slope, thegabbros shift from upper gabbros totransitional gabbros with irregularcontours (Fig. 2).

Discussion

The Farah area was selected for itsexceptional outcrops located 1 kmbelow the sheeted-dike–basalt lid,where the fabric in the upper gab-bros rotates sharply in response tonew magma-chamber flow conditionsin the transitional gabbros (Nicolasand Boudier, 2015b). A key has beenthe discovery of spectacular glitteringclinopyroxene in totally fresh gab-bros. They crop out as a metre-thickveneer, covering the Farah hill(Fig. 3). The veneer defining a nearlyflat thermal horizon is totally inde-pendent of the steep foliation of theupper gabbros (Fig. 2E).

Hydrothermal growth of theglittering-clinopyroxene marker

At the Farah site, glittering clinopy-roxenes incorporated tiny plagioclaselaths (hundreds of microns in size) intheir cores, before growing very fastin static conditions, and finally beingcorroded by continuously growingplagioclase (Fig. 4). This crystalliza-tion path records transient conditions

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Fig. 4 (A) Optical microscope image (polarized transmitted light) of glittering-clin-opyroxene-bearing gabbro 11OA168; section perpendicular to the foliation andparallel to the loose lineation (vertical in the image). Elongated clinopyroxenes(cpx) (av. 1 9 6 mm), all twinned along the (100) plane, mark the foliation. (B andD) Plagioclase (pl) is polygonal and equigranular (

of clinopyroxene growth, whichcould be triggered by a suddenhydration of the crystallizing magma.Experiments on the crystallization ofhydrated gabbroic magmas in thetemperature range 940–1220 °C doc-ument an extension of the stability ofclinopyroxene accompanied by alowering of the solidus (by up to250 °C) and a drastic increase in themelt fraction, depending on theamount of hydration (Feig et al.,2006). The observed growth oforthopyroxene after clinopyroxene isalso predicted by these experiments,because, below 1100 °C, the hydra-tion favours the stability of orthopy-roxene. The clinopyroxene developswithin the foliation of the enclosinggabbro. However, the orientation ofits noteworthy elongation is randomwithin the foliation plane. Theabsence of a lineation indicates thatclinopyroxene growth was not

controlled by magmatic flow as itwas in all the upper-level foliatedgabbros. This suggests static growthcontrolled by a strong local thermalgradient. Such conditions could besimilar to those of experimental gemgrowth, where strongly anisotropicfast-growing crystals are producedunder an imposed strong thermalgradient (i.e. Horvat et al., 2012).Consequently, the glittering clinopy-roxene is the fast and staticcrystallization product of a hydratedmelt-rich domain at the inner limit ofthe magma chamber. According toFeig et al.’s (2006) experiments, jointcrystallization of plagioclase andclinopyroxene would occur for 3%water in the melt at 200 MPa pressure.Considering that the thermal diffu-

sivity of diopside is highly anisotropic(Hofmeister and Pertermann, 2008),we calculated the anisotropy of ther-mal diffusivity of the glittering gabbro

(Fig. 7) on the basis of the CPOs. Thecombined diopside–bytownite modelgabbro has an anisotropy of 14.4%with high values in the foliation planeand low values normal to the folia-tion. Hence, the TBL is an anisotropicenvelope with high thermal diffusivityparallel to the foliation and low ther-mal diffusivity in the transverse direc-tion. The anisotropy should help tomaintain the high temperatures in themagma chamber and favour thermaldiffusion within the TBL like a ther-mal conduit.

The inner wall of the chamber andthe thermal structure of the TBL

The occurrence of glittering clinopy-roxene (i) thermally constrains thechamber wall at VHT conditions of1180–1080 °C, the upper limits of sta-bility of clinopyroxene and orthopy-roxene respectively, (ii) occurs over a

Fig. 5 Electron Back-Scattered Diffraction (EBSD) image of section 11OA168 (same area as in Fig. 4). Inserts correspond tothe enlarged views in Fig. 4. The map provides the modal compositions of the indexed phases (see Fig. 6). The primary phasesclinopyroxene and plagioclase are remarkably devoid of alteration. The large elongated clinopyroxene crystals contain pris-matic plagioclase crystals a few hundred microns in size. The non-indexed phases include oxides, poorly represented as seen inthe enlarged views in Fig. 4, and dominantly low-T hydrous alteration products (talc, chlorite, serpentine). Orthopyroxenepatches develop locally at the expense of clinopyroxene, along alignments that mark the locations of former cracks. Pargasiticamphibole is rare and is dominantly associated with orthopyroxene. Mid-amphibolite facies alteration (low-grade amphiboleand zoisite) is totally absent in this section. EBSD analyses were obtained with a CamScan X500FE CrystalProbe equippedwith an EBSD system at Geosciences Montpellier (France).



distance of only metres and (iii) is dis-cordant with the structures of theother gabbros of the Farah hill. Anunexpected result of this discovery isthat hydrous fluids have been intro-duced into the margin of the magmachamber. From a different perspec-tive, this has been proposed toaccount for the ‘black gabbros’ com-monly observed at the Moho level(Mainprice et al., 2014).At decreasing temperatures, under

HT conditions (~800–600 °C), gab-bros alter to green hornblende–clino-zoisite; this zone extends in the fieldover 150 m with a thickness of~20 m, and represents the externalTBL. Thus, the general descriptionsof the TBL apply well on the easternside of the hill, where only a fewpatches of the glittering clinopyroxene

are dispersed within the more homo-geneous pattern of alteration inhornblende and clinozoisite, defininga wedge-shaped pattern (Fig. 3A).

Ridge-normal hydrothermalcirculation

The wedge pattern separating theVHT/HT hydrothermal domainsfrom the LT alteration implies a ther-mal gradient along the ridge axis, thusfitting the hybrid hydrothermal circu-lation model of Hasenclever et al.(2014). In addition, Hasencleveret al.’s model locates the transversehydrous flow at the very level, 2 kmbelow the seafloor, documented at theFarah site.Farah lies east of the palaeo-ridge

located in the Kazinah area (Fig. 1),

so the hydrous fluids in the thermal-sink domain next to the active ridgehave crossed the TBL. Thus, thehydrous wedges should point to theSW and converge towards the west-ern ridge (Fig. 8). At this depth, thewedges progress by opening and pen-etrating deeper within the TBL, asevidenced by the HT to VHT alter-ations, and ultimately join whilethinning to a veneer in the VHT gab-bros near the top of the hill. Veneersthus represent remnants of the inter-nal wall of the chamber.

Irregular surface of the TBL betweenthe upper and transitional gabbros

We have documented above thatmapping at small scale, as in Farah,reveals that the critical limit betweenthe subsiding upper gabbros and thetransitional gabbros might be irregu-lar or corrugated and not curvipla-nar (Fig. 1). This may be a responseto distinct flow regimes in themagma chamber (Nicolas and Bou-dier, 2015b). Such an irregular sur-face is traceable at the scale ofmapping in Farah and is suggestedat the scale of the Farah-Himsegment (Fig 1), but is smoothed atthe 30-km scale of the Jebel Dihmmap.

Possible model of transverse fluidtransport through the TBL

The hydrothermal wedges mappedtransverse to the ridge can penetratethroughout the TBL and deliverhydrous fluids that cool the magmachamber, and they are now frozeninto its inner wall (Fig. 8). Coupledwith a corrugated surface in the TBLat the base of the upper gabbros, thesharp rotation to transitional gab-bros may provide a way to explainhow the internal wall of the magmachamber was preserved in the Faraharea, protected by the transitionalgabbros’ TBL. In other words, thismay be how the internal wall of thechamber was contaminated by thefluids carried by an advancing wedge.Thereafter, it was preserved fromfurther alteration. We also speculatethat the irregular discordancebetween the upper and transitionalgabbros, creating grooves and ridges,may have guided the transversewedges.

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BYTOWNITE 46,42 %

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PARGASITE 4,10 %

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Fig. 6 EBSD measurements at high resolution (step-size= 0.5 lm) of crystal pre-ferred orientations (CPO) of the three indexed phases, plagioclase bytownite,clinopyroxene diopside and amphibole pargasite, and modal compositions in percent of the indexed phases (see also Table 1). Lower hemisphere pole figure; traceof foliation is vertical (as in Figs 4 and 5); loose lineation is N–S; n is the numberof measured grains, and J indexes (Mainprice and Silver, 1993) measure thestrength of the CPO (partly dependent on n). Bytownite shows a weak planar fab-ric of plane (010) parallel to foliation S and a slight orientation of the pole of (001)parallel to lineation L. Diopside exhibits a more marked fabric, with the (100)plane parallel to the foliation, and a random distribution of the elongation axis(pole of (001)) in the foliation plane, with a slight tendency to orient parallel to thelineation. Pargasite CPO follows that of diopside.



Table 1 Major elements analysis

CPX/42,16%

DataSet/point 1/1. 2/1. 3/1. 8/1. 11/1. 14/1. 15/1. 21/1.

Mg # 86 87 83 87 86 88 85 86

%En, Wo, Fs 47, 43, 10 47, 43, 10 46, 44, 10 47, 43, 10 46, 43, 10 48, 41, 11 45, 44, 11 48, 42, 10

CPX (continued)

DataSet/point 27/1. 32/1. 36/1. 37/1. 43/1. 44/1. 47/1. 23/1.

Mg # 85 87 86 86 87 88 86 84

%En, Wo, Fs 46, 43, 11 46, 45, 9 47, 44, 9 46, 44, 10 47, 44, 9 47, 44, 9 45, 44, 11 45, 44, 11

OPX/1, 35%

DataSet/point 25/1. 22/1. 45/1. 46/1. 48/1.

Mg # 79 80 79 78 78

%En, Wo, Fs 72, 7, 21 72, 7, 21 75, 3, 22 75, 4, 21 75, 3, 22

PLAG/46, 42%

DataSet/point 5/1. 6/1. 7/1. 9/1. 10/1. 12/1. 13/1. 16/1.

An % 80 79 80 79 81 80 80 80

PLAG (continued)

DataSet/point 18/1. 19/1. 29/1 incl 30/1. 34/1 incl 35/1. 39/1 incl 17/1.

An % 79 78 80 80 81 79 79 81

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Fig. 7 Calculated thermal diffusivity pole figure in mm2 s�1 for glittering-clinopyroxene-bearing gabbro with 47.8% diopside and52.2% bytownite. Note the correspondence between the (001) pole figure in Fig. 6 and the gabbro model thermal diffusivity polefigure. The diopside and bytownite CPOs have been used to calculate the thermal diffusivity anisotropy of the TBL. A two-phasemodel with 47.8% diopside and 52.2% bytownite was used, while other minor phases with a total of less than 5% have beenignored. The thermal diffusivity tensor of single-crystal diopside, calculated from data reported by Hofmeister and Pertermann(2008), has an anisotropy of 49.4%, with the highest value normal to (001) and the lowest value normal to (010). The thermal dif-fusivity of the diopside aggregate was calculated using the methods described by Mainprice et al. (2011) using the Hill average.The diopside aggregate has an anisotropy of 18.4% with the highest values in the foliation and the lowest values normal to the foli-ation. To evaluate the contribution of bytownite we used the data of Branlund and Hofmeister (2012); the measurements for An66from a gabbro should provide the best match to the crystal structure of our plagioclase. The measurements along [100], (010) and(001) for An66 are not enough to calculate the six independent coefficients of the triclinic thermal diffusivity tensor, so we usedthese three measurements to constrain a model single-crystal tensor; this tensor has an anisotropy of 7.7%, much lower than diop-side. The calculated bytownite aggregate is nearly isotropic with an anisotropy of 1.0%. The combined diopside–bytownite modelgabbro has an anisotropy of 14.4% with high values in the foliation plane and low values normal to the foliation.

Fig. 8 Speculative 3D model of the TBL at the junction of the upper gabbros and transitional gabbros (see insert), integratingdata collected in Farah. The magma chamber is represented in deep yellow, with upper gabbros subsiding. The active foliation(dashed lines) is plated along the TBL wall (deep red). The transitional gabbros are flatter, folded and record a new flowregime in the magma chamber that is not controlled by the wall of the TBL (Nicolas and Boudier, 2015b). The frozen TBL is~100 m thick (Nicolas et al., 2008) and is shaded from red to blue, as the temperature rapidly falls inside the TBL. The passagefrom upper gabbros to transitional gabbros is sharp and irregular, as traced in the 5-km-long Farah-Him segment (Fig. 1). Atthis level, three hydrothermal wedges perpendicular to the ridge axis are fed by seawater ingression from the external thermal-sink domain near the ridge axis (inspired from Hasenclever et al., 2014). Fluids progress and heat up towards the magmachamber, where they merge. When they reach the magma chamber, at VHT conditions generating hydrous melting, they createits inner wall. Instantly, the inner wall is remelted at VHT conditions and can now be identified as the glittering-clinopyrox-ene-bearing gabbros in veneers a few meters thick. In such a dynamic environment, we speculate that these melt pockets couldbe sheared and injected within the HT part of the TBL.



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{TBL

Spreading

km 0

1

2

3

Transitional gabbros

Uppergabbros

Lid

Magma chamber

with traces

of gabbro-mush

flow

Flow channel

Flow chan

nel



Acknowledgements

We acknowledge the seminal visit a fewyears ago of Larry Cathles and, as usual,the friendly cooperation in Oman withthe Directorate of Minerals of the MCI,as well as the support of GeosciencesMontpellier, within our group. We alsothank Christophe Nevado and DorianeDelmas for high-quality polished thin-sec-tions, Fabrice Barou for the EBSD acqui-sition and processing, and Bernard Boyerfor microprobe technical assistance. Twocareful reviews helped to clarify someaspects of the manuscript; they are grate-fully acknowledged.

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Received 30 July 2015; revised version

accepted 18 November 2015


http://dx.doi.org/10.1029/2001GC000215http://dx.doi.org/10.1029/2001GC000215http://dx.doi.org/10.1007/s00410-006-0123-2http://dx.doi.org/10.1002/ggge.20137http://dx.doi.org/10.1002/ggge.20137http://dx.doi.org/10.1038/nature13174http://dx.doi.org/10.1029/2002JB002094http://dx.doi.org/10.1029/2007GC001918http://dx.doi.org/10.1029/2012JB009273http://dx.doi.org/10.1029/2011JB008248http://dx.doi.org/10.1029/2011JB008248

Documents

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