When Epitaxy Controls Garnet Growthkk

7/30/2019 When Epitaxy Controls Garnet Growthkk

1/12

When epitaxy controls garnet growthR. SPIESS,1 C. GROPPO 2 AND R. COMPAGNONI 21 Department of Geoscience, University of Padova, Via Giotto 1, I-35137 Padova, Italy ([email protected]) 2 Department of Mineralogical and Petrological Sciences, via Valperga Caluso 35, I-10125, Torino, Italy

ABSTRACT Within a mica schist from the coesite-bearing Brossasco-Isasca Unit (Western Alps), microstructuralanalysis shows that Alpine garnet grains are aligned with the crenulated foliation. Garnet crystallo-graphic orientation was analysed with electron backscatter diffraction (EBSD): the obtainedcrystallographic dispersion patterns and distribution patterns of misorientation axes suggest a strongparallelism of {110} garnet planes with a 56 W-dipping foliation. The data are interpreted as evidencefor an epitaxial growth of garnet upon (001) biotite planes, sometime during and/or after dispersion of the biotite/garnet crystals from their initially foliation-parallel orientation by rotation about the Alpinecrenulation axis. This interpretation is based on the comparison of the measured EBSD data with: (i)theoretical dispersion trajectories of garnet crystallographic data, (ii) numerically modelled pole gures,and (iii) numerically modelled misorientation axis distribution patterns. Our data suggest that epitaxial

growth of garnet upon biotite is allowed by distortion of the pseudohexagonal basal oxygen ringstructure on (001) biotite surfaces, and that distortion is driven by introduction of missing ions. Our datafurther suggest that the spatial distribution of precursor phases inuences the distribution patterns of garnet within mica schists.

Key words: biotite; electron backscatter diffraction; epitaxial growth; garnet; misorientation analysis.

INTRODUCTION

Nucleation and growth processes of metamorphicphases can be described by physical laws involvingmeasurable entities/parameters such as the spatialdistribution and compositional zoning of grains, theradius of depletion zones surrounding them and theirgrain size. Systematic variations of these entitiesconstrain the limits within which one nucleation andgrowth process prevails over another. Kretz (1969,1974) with his pioneering work has attempted tounderstand garnet nucleation and growth mecha-nisms in such a context. Carlson (1989, 1991) setlandmarks in the numerical analysis and compre-hension of garnet nucleation and growth processesusing the novel approach of computed X-raytomography combined with appropriate statisticalmethods (Denison et al. , 1997; Hirsch et al. , 2000;Ketcham et al. , 2005) to determine garnet crystal sizeand spatial distribution.

In the nucleation and growth model of Carlson(1989), the spatial distribution of garnet crystals isordered: the order is linked to intergranular diffusionas the rate limiting process in crystal growth. Twoquantitatively perceptible microstructural features willcharacterize rocks where garnet growth is controlledby this process: growing porphyroblasts will be sur-rounded by depletion zones wherein further nucleationis depressed, and clustered grains will be small com-pared with isolated grains, because they have to com-pete for nutrients.

A critical assumption in the model of Carlson (1989,1991) and Denison et al. (1997) is that the spatialdistribution of crystals is not inuenced by factorsother than intergranular diffusion, excluding defor-mation or preferential growth from precursor phasesas potential controls. It is therefore essential to assess

whether factors other than intergranular diffusionhave inuenced the spatial distribution of garnet. Apowerful approach to reveal this is the analysis byelectron backscatter diffraction (EBSD), because it hasthe potential to analyse at a statistically signicantlevel whether the crystallographic orientation of garnetgrains relative to potential precursor phases is ordered,allowing the assumption that nucleation and growth of garnet was controlled by these precursor phases. Thisstudy aims to test this through the analysis of garnetgrains nucleated within an ultrahigh pressure (UHP)mica schist from the Western Alps. The natural dataand the numerical simulations suggest that epitaxialgrowth upon biotite has controlled the spatial distri-bution and crystallographic orientation of garnet inthe studied sample.

GEOLOGICAL SETTING

The studied garnet mica schist was collected within theNW part of the coesite-bearing Brossasco-Isasca Unit(BIU) (Fig. 1), which is a coherent slice of Penniniccontinental crust (southern Dora-Maira Massif),tectonically sandwiched between two units whichexperienced a quartz-eclogite facies recrystallization

J. metamorphic Geol. , 2007, 25, 439450 doi:10.1111/j.1525-1314.2007.00704.x

2007 Blackwell Publishing Ltd 4 3 9


2/12

( 550 C and 1.5 GPa, Chopin et al. , 1991; Schertlet al. , 1991; Sharp et al. , 1993; Compagnoni et al. ,1995; Matsumoto & Hirajima, 2000; Compagnoni &Rolfo, 2003; Hermann, 2003). The BIU, originallycomposed of a Variscan amphibolite facies metamor-phic basement (Compagnoni et al. , 1995), was intru-ded at 275 Ma by porphyritic granitoids (Gebaueret al. , 1997), as shown by local preservation of relictpre-Alpine igneous and metamorphic structures(Compagnoni et al. , 1995; Compagnoni & Rolfo,2003). During the Alpine orogeny, the BIU experi-enced an early Alpine coesite-eclogite- (Gebauer et al. ,1997; Rubatto & Hermann, 2001), and a late Alpinegreenschist facies recrystallization so that the Variscanamphibolite facies basement plus the late-Variscangranitoids were converted to the present Polymeta-morphic Complex and Monometamorphic Complex ,respectively (Compagnoni et al. , 1995).

The Polymetamorphic Complex, from which thesample was collected, mainly consists of paragneissand paraschist with marble and eclogite intercalations(Chopin et al. , 1991; Compagnoni et al. , 1995; Com-pagnoni & Rolfo, 2003). The paraschists contain rare

microstructural and/or mineralogical relics of the pre-Alpine amphibolite facies regional metamorphism,such as garnet, red-brown biotite partially replacedby phengite rutile, K-feldspar partly replaced byphengite-quartz intergrowths and prismatic sillimanitepseudomorphically replaced by kyanite aggregates(Compagnoni et al. , 1995; Compagnoni & Rolfo,2003).

Ultrahigh pressure metamorphic peak conditions forthe BIU were 750 30 C and 3.3 0.3 GPa, i.e.below the diamond stability eld (Chopin, 1984, 1987;Chopin et al. , 1991; Kienast et al. , 1991; Schertl et al. ,1991; Sharp et al. , 1993; Compagnoni et al. , 1995).However, higher peak-pressure conditions have beenobtained by the most recent thermodynamic modellingby Groppo et al. (2006b), which corroborate thehypothesis of Hermann (2003) based on experimentalresults that peak metamorphic conditions were wellwithin the diamond stability eld at 730 C and4.3 GPa.

The retrogressive metamorphic evolution of the BIUis characterized by a substantial decompression cou-pled with a continuous cooling (Chopin et al. , 1991;

(a)

(b)

Fig. 1. (a) Simplied tectonic sketch-map of the Western Alps. Helvetic-Dauphinois Domain: Mont Blanc-Aiguilles-Rouges (MB);Penninic Domain: Grand St Bernard Zone (SB), and Monte Rosa (MR), Gran Paradiso (GP), Dora-Maira (DM) and Valosio (V)Internal Crystalline Massifs; Piemonte Zone of Calcschists (light grey) with meta-Ophiolites (dark grey); Austroalpine Domain: DentBlanche Nappe (DB), Monte Emilius Klippe (ME), and Sesia-Lanzo Zone (SZ); Southern Alps (SA); Embrunais-Ubaye Flysch Nappe(EU); Penninic Thrust Front (PF); Sestri-Voltaggio Line (SVL). (b) Enlargement of the southern Dora-Maira Massif. Tectonic units of the Massif are listed from the structurally lowermost Pinerolo Unit to the structurally highest Dronero-Sampeyre Unit.

4 4 0 R . S P I E S SE T A L .

2007 Blackwell Publishing Ltd


3/12

Schertl et al. , 1991; Compagnoni et al. , 1995; Com-pagnoni & Rolfo, 2003; Groppo et al. , 2006, 2007). Atrelatively low pressures ( 0.5 GPa), this cooling pathis followed by a moderate heating up to the boundarybetween upper greenschist- and amphibolite facies(Compagnoni et al. , 1995; Rubatto & Hermann, 2001).Overall, the Alpine metamorphic evolution of the BIUis dened by a clockwise PT trajectory, similar to thePT path of other tectonic units from the InnerWestern Alps.

MICROSTRUCTURE

The studied metapelite is a UHP garnet + kya-nite + quartz/(coesite) + (jadeite) + high-Si pheng-ite mica schist with accessory rutile and apatite (formeroccurrence of minerals inside brackets is inferred frommicrostructural and mineralogical evidence). In thediscussed thin section, the microstructure is dominatedby an EW trending, closely spaced Alpine crenulation

cleavage that wraps around a large pre-Alpine garnetporphyroblast (Grt 1 ) (Fig. 2a,c).

Adjacent to the garnet porphyroblast, strain shad-ows with a left hand asymmetry contain tails of blockyquartz that are parallelized with the enveloping Alpinecrenulation cleavage (S A in Fig. 2c). In the remainingparts of the strain shadows a relict, NS orientedfoliation (S V in Fig. 2c) is preserved that rotates pro-gressively towards the enveloping Alpine foliation inthe strain shadow to the east, whereas it is deformed toa widely spaced differentiated crenulation cleavage(Bell & Rubenach, 1983) in the strain shadow to thewest. Along the central left margin of the porphyro-blast, a corona of small Alpine garnet grains (Grt 2 )overgrows a microstructure that resembles a strain capof biotite (Passchier & Trouw, 1998, p. 148). It isimportant to stress that no biotite crystals are left inthe whole analysed mica schist. However, many Alpinegarnet grains are crowded with rutile needles thatoccasionally mimic the outlines of pseudo-hexagonal

(a) (b)

(c)

Fig. 2. (a) Photomicrograph (crossed nicols) of studied thin section showing a relict foliation preserved within the strain shadows of the large Variscan garnet porphyroblast, overprinted to an EW oriented Alpine differentiated crenulation cleavage outside. Opencrenulations are preserved within the western strain shadow, whereas in the eastern strain shadow the foliation curves smoothly. Tailsof blocky quartz have a left-hand asymmetry. Small Alpine garnet grains are aligned with the oblique to crenulated foliation in thestrain shadows, whereas outside they may be enveloped by the differentiated crenulation cleavage. (b) Rectangle in (a) locatesenlargement. Enlargement under plane light shows small Alpine garnet grains overgrowing the crenulations preserved within the strainshadow. The axial plane of the open crenulations is WNWSSE. (c) Sketch showing the main microstructural features of the mica

schist. Crenulated relict foliation is S V . Alpine foliation is S A . Quartz is light grey, pre-Alpine garnet is grey, Alpine garnet overgrowingthe large porphyroblast in a corona adjacent to the western strain shadow is dark grey. Black full dots show the analysed small Alpinegarnet crystals (1 indexed EBSD pattern per grain). Matrix of the mica schist (white) mainly consists of quartz and phengite.

W H E N E P I TA X Y C O N T R O L S G A R N E T G R O W T H4 4 1



4/12

(Fig. 3b,c) or tabular-shaped crystal forms (Fig. 3d).As Ti-solubility in garnet at 4.3 GPa and 730 C is low(Zhang et al. , 2003), we interpret these rutile needleinclusions as evidence for Grt 2 formation at theexpense of pre-Alpine Ti-rich biotite broken downduring HP to UHP metamorphism.

The differentiated Alpine foliation is made up bydiscontinuous layers of high-celadonitic phengite(partially replaced by ner grained, less-celadoniticphengite) alternating with medium-grained lens-likequartz-rich domains. Fine-grained staurolite andgreenish-blue chloritoid idioblasts statically overgrowthis main foliation. A peculiar feature of the micaschist is the nucleation and growth of hundreds of small Alpine garnet grains (Grt 2 ) ranging in apparent(2D) diameter from some tens of microns up to somehundreds of microns within the matrix. In the westernstrain shadow many of these garnet grains arearranged according to the open crenulations (Fig. 2b),whereas in the matrix outside the strain shadow, garnet

grains may be enveloped by ne seamed foliation do-mains or overgrow them.

Microstructural data suggest that Grt 2 nucleatedduring the high pressure Alpine metamorphism pref-erentially after the mica domains of the crenulatedrelict foliation (Fig. 2a,b). Because of the crenulationdevelopment, mica akes and garnet grains nucleatedupon them would have been rotated away from theiroriginal orientation. The microstructural analysis isunable to dene how much garnet has been rotatedafter overgrowing mica akes and how much micaakes have been rotated before garnet nucleation. Themicrostructural analysis cannot unequivocally revealthe timing of garnet growth upon mica akes relativeto the Alpine crenulation cleavage development. Theopen crenulations within the strain shadow could havebeen overgrown by garnet when in the matrix astrongly differentiated crenulation cleavage alreadyhad formed. In fact, in the matrix, garnet becomesenveloped by, and overgrows, the differentiated

(a) (b)

(d)(c)

Fig. 3. (a) Central left margin of the pre-Alpine garnet porphyroblast (Grt 1 ), 2 cm in diameter, with a corona of small Alpine garnetcrystals (Grt 2 ). (bd) Details of Alpine garnet crystals (Grt 2 ), the cores of which are crowded with rutile needles, suggesting theirgrowth at the expense of a former pre-Alpine Ti-bearing biotite. Rutile needles dene the outline of pseudo-hexagonal forms (b, c) andtabular forms (d) of earlier biotite crystals. Mineral abbreviations according to Bucher & Frey (2002).

4 4 2 R . S P I E S SE T A L .



5/12

crenulation cleavage, suggesting that nucleation andgrowth of garnet progressed through time.

EXPERIMENTAL PROCEDURES

For every Alpine garnet grain one EBSD pattern wascollected and indexed. The total number of analyseswas 666. Of these, 391 were from garnet grains of thewestern strain shadow, whereas 275 were from garnetcrystals within the foliation domains outside the strainshadow. Fifty analyses of 391 came from the over-grown strain cap. Because the orientation of these last50 garnet crystals may have been controlled by thecrystallographic orientation of the large garnet por-phyroblast (both types have similar orientations), theyare omitted then from the pole gures. Therefore, theoverall number of processed EBSD patterns is 616.

The thin section was polished using Syton uid toremove the mechanical damage generated during pre-vious mechanical polishing (Flynn & Powell, 1979;

Prior et al. , 1996). After polishing, the sample wascoated with a thin carbon lm to prevent electricalcharging problems. EBSD analysis was performedusing a CamScan MX2500 SEM equipped with atungsten lament at the Department of Mineralogyand Petrology, the University of Padova (Italy). Anacceleration voltage of 25 kV, a lament emissioncurrent of 150 l A, and a working distance of 25 mmwere used. EBSD patterns were indexed using CHANNELCHANNEL5.0 software from HKL-Technology. Indexing wasaccepted when at least ve detected Kikuchi bandscorresponded with those contained in the standardreector le for garnet. Collected data were processedusing the Mambo component of the CHANNELCHANNEL 5.0software.

For the numerical simulation of garnet dispersion,data les were constructed and imported to the ProjectManager component of CHANNELCHANNEL 5.0 for plotting withMambo. Forty degree concentric dispersion wassimulated numerically, whereas 20 lateral dispersionwithin the contoured pole gures was simulated bymeans of the half width angle during contouring. Forthe positioning of the rotation axis the CS0 > CS1facility of the Virtual Chamber component of CHANNELCHANNEL5.0 was used.

DESCRIPTION OF {100}, {111} AND {110} POLE

FIGURESThe {100} poles (Fig. 4a) show a weak but clearlyperceptible preferred orientation, that denes a con-centric girdle with a pole clustering density >1 mud(multiples of uniform density) centred on a minimumwith a clustering density of 0.55 mud. Point maxima of up to 1.45 mud exist within the girdle, whereas outsidethe girdle the density is fairly constant at 0.80.9 mud.The two most signicant point maxima are positionedat 090/80 and 010/56, whereas the minimum, on whichthe girdle is centred, is placed at 280/56.

The {111} pole gure (Fig. 4b) is dened by twomain characteristics: (i) a maximum of {111} polescoincides with the minimum of {100} poles, and (ii) agirdle of {111} poles extends from NNE towards SSW

(within the stereonet frame of reference) and is closelycentred on the {100} minimum. Outside these areas theclustering density of {111} poles varies constantlybetween 0.7 and 0.9 mud.

Compared with the {100} and {111} pole gures, the{110} pole gure (Fig. 4c) has only very subtle densitydifferences. The reason for this is the high number of equivalent faces belonging to this family (six facesplotting in the same hemisphere), so that the generaltendency is a constant density of 0.9 mud throughoutthe pole gure. Nevertheless, a small density increaseof {110} poles is observed in correspondence with the{100} minimum and within an irregularly shaped girdlecentred slightly off the {100} minimum. Inside thisgirdle, a maximum with the highest density of 1.33 mud is positioned at 065/30.

INTERPRETATION OF POLE FIGURES

The concentric small girdle dispersion of {100} polesabout a {100} pole-minimum, the coincidence of thisminimum with a {111} pole-maximum, plus the girdle-like distribution of {111} poles (and less clearly of the{110} poles) suggest that these crystallographic distri-bution patterns have not formed coincidentally, butreect the interaction of specic processes that haveoperated during garnet nucleation and growth.

It can be inferred that the {100} minimum/{111}maximum coincides with a rotation axis, and that thearrangement of {100} poles around this minimuminside a broad small girdle with an opening angle of 70110 reects the dispersion of {100} poles thatinitially had a crystallographic preferred orientation. If this preferred orientation were a unique {100} orien-tation, because of the 90 interrelationship between(100), (010) and (001) poles, the opening angle of thesmall girdle would be 90 . Therefore, more than just asingle {100} orientation is required to explain theactual {100} pole arrangement. These orientations

(a) (b) (c)

Fig. 4. Distribution patterns of poles to the {100} (a), {111} (b)and {110} (c) crystallographic planes of Alpine garnet crystals.The pole gures are spatially oriented relative to the thin sectionshown in Fig. 2, with the relict foliation running NS and theAlpine crenulation cleavage running EW.




6/12

must have been related to each other in such a way thatpart of the {111} poles were allowed to align closelywith the rotation axis, whereas the rest fall in distinctpositions within the broad great girdle closely centredon the {100}-minimum.

In order to understand the evolution of this disper-sion pattern it is important to assess the signicance of

the rotation axis. As the crenulation axis (Fig. 5) of theanalysed mica schist is spatially correlated with thisrotation axis, it is most reasonable that it is a kinematicaxis.

The observation that in some cases garnet preservesrutile inclusions tracking the outlines of pseudohex-agonal biotite planes (Fig. 3bd) could suggest that thepreferred crystallographic orientations of garnet beforedispersion have been controlled by the nucleation site,i.e. the crystallography of biotite (and possibly whitemica) contained within the differentiated crenulationcleavage. Considering the spatial coincidence of thecrenulation and rotation axes, we hypothesize that theobserved dispersion pattern of crystallographic orien-tations results from rotation of foliation-parallel micaplanes upon which garnet was nucleating.

Frondel (1940) observed that garnet commonlygrows with one {110} face parallel to the (001) of whitemica, forming angles of 0 , 30 , 60 or 90 between the{001} garnet traces and the (100) muscovite trace.Analogous observations have been reported by Powell(1966). He studied inclusion bands and trails of micasin Moinian garnet crystals that were approximatelyparallel to a dodecahedral garnet plane. Based on hisown and Frondels observations, Powell (1966) con-

cluded that garnet may grow epitaxially upon mica. Heattributed this to a close relationship between theoxygen atom arrangement adjacent to the K-interlayerin mica (considering the conguration of apical oxy-gen) with that of garnet on {110} planes, so that Alatoms could possibly be accommodated in the potas-sium positions in a pattern very similar to that shownby Al on the {110} of garnet.

We have compared the oxygen distributions in bio-tite adjacent to the K-interlayer with that of garnetadjacent to {110} planes using CRYSTALMAKERCRYSTALMAKER 1.3.5Demonstration version plus the crystal structure datales of Brigatti & Davoli (1990) and Quartieri et al.(1995) available in the American Mineralogist CrystalStructure Database (Downs & Hall-Wallace, 2003).From this comparison shown in Fig. 6 we deduce that:1 the biotite oxygen ring structure on the (001) nucle-ation surface needs rotation and distortion to t theoxygen structure on the {110} garnet plane;2 a systematic variation of the [001] garnet directionsby multiples of 30 relative to the (100) mica tracerequires nucleation to occur with a similar probabilityin all six-fold directions on the pseudohexagonal basaland the pseudohexagonal apical oxygen rings, as theyare rotated relative to each other by 30 (Fig. 6c,d).

Fig. 5. Drawing of the garnet-bearing mica schist, showing thespatial arrangement of the relict foliation (NS), Alpine crenu-lation cleavage (EW) and trend of crenulation axis. Arrowpoints to the N, crenulation axis plunges 280/56. Rectangleshows studied thin section.

(a) (c)

(b) (d)

Fig. 6. Comparison of the oxygen conguration of garnetadjacent to {110} planes (a, b), and of biotite on (001) planes (c,d). (a, b) Oxygen is red and Al is green, (c) open green circles arebasal oxygen atoms, (d) open red circles are apical oxygen atoms.Blue full dots in panels c and d are K + ions. The [001] directionof garnet is NS in panel a and EW in panel b. The (100) traceof mica is NS in panel c and d. Arrows in the central pseudo-hexagonal oxygen rings in panels c and d show in-plane sixfoldsymmetry directions. After rotation and distortion, hexagons inpanel c formed by O 2

)and K + ions could match the oxygen

structure of garnet in panel a (supposed that introduction of O 2)

in positions of K + is possible), whereas hexagons designed byapical oxygen in panel d could potentially t the oxygen struc-ture of garnet in panel b.

4 4 4 R . S P I E S SE T A L .



7/12

If garnet nuclei can arise by distortion of the biotitelattice in this way, then it is possible to predict thatgarnet would show preferred crystallographic orienta-tions relative to the (100) mica trace, with the [001]garnet direction forming angles of 0 , 30 , 60 , or 90 .

The above hypothesis forms the background for thegeometrical and the numerical simulations we presentin this paper to interpret the formation of the garnetcrystallographic dispersion pattern observed in theanalysed mica schist. For the scope of simulation it isassumed that (001) mica planes with (100) traces run-ning NS are aligned parallel to the 56 W dippingrelict foliation (Fig. 7bg). During the Alpine defor-mation, this foliation becomes crenulated about a 280/56 plunging axis, and the mica akes together with thegrown garnet crystals become rotated up to 40 in a

clockwise and/or an anticlockwise sense about thecrenulation axis. If the crenulations have a cylindricalgeometry, then the crystallographic orientations of thegarnet crystals will describe concentric dispersionpaths about the rotation axis. The opening angles of the dispersion paths (Fig. 8) will vary as a function of the crystallographic directions taken into considera-tion, i.e. {001}, {111}, {110}, and as a function of theoriginal garnet crystal orientations, i.e. 0 , 30 , 60 , or90 relative to the (100) trace.

GEOMETRICAL SIMULATION OF DISPERSIONTRAJECTORIES

Let us consider that garnet grew with a {110} faceupon the (001) face of biotite (parallel to the papersheet in Fig. 7a). Rotation of garnet by 30 or multi-ples of it about the horizontal [110] axis (normal to thepaper sheet in Fig. 7a) results in angles of 0 , 30 , 60 ,90 between the [001] directions of garnet (dashed lines

in Fig. 7a) and the (100) traces of biotite (vertical inFig. 7a). Accordingly, plotting the poles to the {001},{111} and the {110} planes of garnet in equal areaprojections give the pole-gures shown in Fig. 7bd,respectively [primitive great circle in equal area plots is(100) of biotite, NS great circle is (001) of biotite,garnet [110] rotation axis is EW].

Tilting the {001} biotite plane by 56 towards W willcause this plane to intersect with the Alpine foliation(dipping 80 towards N) along the crenulationaxis preserved within the analysed mica schist (seeFig. 7eg and simulation in Fig. 8). After tilting, if weconsider only garnet positions of 0 , 30 and +60 ,then no {100} poles will coincide with the {100}-mini-mum, whereas all other {100} poles occupy distinctpositions within the broad small girdle (Fig. 8a) alongwhich natural {100}-data disperse (Fig. 8a). Addition-ally, part of the {111} poles overlap with, or fall close to,the crenulation axis (Fig. 8b), while the rest occupiesdistinct positions within the large great girdle in Fig. 8balong which natural {111}-data disperse. Poles to {110}planes (Fig. 8c) are arranged along a small girdle and agreat girdle centredon the crenulation axis. A maximumof {110} poles coincides obviously with the tilted [110]rotation axis that plunges 34/090 (Fig. 8c).

In the geometrical simulations of Fig. 8 we show theresults of folding the foliation parallel biotite akes(upon which garnet grew) around the observed cren-ulation axis from the postulated starting position(Fig. 7fh). If the crenulation geometry is cylindrical,then with increasing deformation, the {100}, {111} and{110} poles must disperse away from the startingposition along the concentric dispersion trajectoriescentred on the rotation/crenulation axis. Asymmetriccrenulations will result in a preferential clockwise oranticlockwise dispersion depending on the sense of shear: crenulations of coaxial domains will dispersesymmetrically, and increasing strain will result inlarger rotations.

(a)

(b) (c) (d)

(e) (f) (g)

Fig. 7. (a) Garnet growing with a {110} plane upon (001) planesof biotite. The [001] traces of garnet, shown as dashed lines,describe angles of 0 , 30 , 60 and 90 degree with the NSoriented (100) trace of biotite. The rotation axis relating thedifferent garnet growth positions to another is [110]. The {110}poles of the variously rotated garnet crystals have differentcolours. The same colours are used in the pole gures shown in

panels bg for the crystallographic orientations of garnet crystalsin different growth positions. (b) Poles to {100} planes of garnet(equal area projection in lower hemisphere) grown with a {110}plane onto a (001) of biotite. The orientation of (001) biotiteplane is vertical and NS oriented. (c) Poles to {111} planes of garnet. (d) Poles to {110} planes of garnet. (eg) Same projec-tions as panels bd but with the (001) biotite plane dipping 56 towards W (asterisk is pole to biotite plane). The trace of the(100) biotite plane onto the (001) plane is NS oriented.




8/12

Our geometrical simulation shows clockwise andanticlockwise trajectories of rotations up to 40 . Theseare probably maximum values for coaxial and rea-sonable values for non-coaxial deformation domains.A comparison of the hypothesized dispersion patternsin Fig. 8 with those observed in nature shows thatmany of the characteristics of the natural pole guredata are predicted by the geometrical simulation. Wehave therefore decided to simulate numerically polegures reecting a situation where garnet crystalsnucleate on biotite according to the orientationsdened above, and become variously dispersed fromtheir original crystallographic position by rotationabout a crenulation axis that plunges 280/56 .

NUMERICAL SIMULATION OF POLE FIGURES

The numerical simulation is based on the assumptionthat garnet nucleates with a {110} plane upon (001)of biotite, and the [001] garnet direction forms anglesof ) 30 , 0 , +30 , and +60 with the (100) trace of biotite. Garnet grains become rotated about a rotationaxis plunging 280/56 , corresponding to the observedorientation of the crenulation axis. In order to simulatesymmetrical crenulations, garnet crystals are allowedto rotate both clockwise and anticlockwise up to 40 . Itis assumed that folding was cylindrical, so that there isa constant pole density along any 040 dispersionrange. Higher densities in the simulations are conse-quently either due to a concentration of poles that wereinitially situated closer to the rotation axis (andbecame therefore less dispersed), or because dispersionhas caused poles to concentrate in certain areas. Acomparison between the natural pole gure (Fig. 9ac)and the simulated pole gure (Fig. 9df) shows thatmost of the characteristic features of both pole-gurescoincide. The concentric small girdle dispersion of {100} poles together with the concentration of poles

along the primitive circle in the SE-sector is faithfullyreproduced (Fig. 9a,d). Equally well replicated is thecharacteristic {111} pole maximum about the rotationaxis together with the broad great girdle centred onthis maximum (Fig. 9b,e).

Even the {110} pole gure shows some of the char-acteristic features, as the discontinuous great girdlecrossing the NE- and SE-sectors (Fig. 9c,f). In thenatural sample only the maximum in the NE-sector isevident (Fig. 9c), perhaps reecting the sinistral shearsense in the foliation domain outside the strain sha-dow. This would also explain the asymmetric distri-bution of poles about the rotation axis, which issymmetric in the numerical simulation. However,relying on the natural {110} pole gure will be dan-gerous, as the high number of equivalent planes (six inone hemisphere) results in small density differencesand in potentially ambiguous patterns.

Nevertheless, it is surprising how faithfully thesimulation reproduces the natural pole gures. There-fore, we believe that these data strongly support amodel that explains the actual arrangement of crys-tallographic orientations as the product of an epitaxialgrowth of garnet upon biotite followed by rotationduring a crenulation deformation event. In order toadditionally constrain this model we have simulatedthe misorientation axes distribution inherent to thismodel and have compared the data with those of thenatural sample.

DISTRIBUTION OF MISORIENTATION AXES:SIMULATION V. NATURAL DATA

Misorientation analysis (Wheeler et al. , 2001) dealswith the crystallographic orientation differencesbetween couples of grains of a crystal population (inour case garnet crystals), and expresses these differ-ences with an angle of rotation about a specic axis

(a) (b) (c)

Fig. 8. Theoretical dispersion trajectories of {100} (a), {111} (b) and {110} (c) poles about a rotation axis plunging 280/56 (equal areaprojection, lower hemisphere). Trajectories are designed for 40 clockwise and 40 anticlockwise rotation. Dispersion of poles increasesfrom hypothetical original growth positions (0 ,+30 , +60 , ) 30 , compare Fig. 7) with increasing distance from the rotation axisposition. Great circles are relict and Alpine foliations, intersection is rotation axis. Light grey colour denes areas with a pole clusteringdensity >1 mud, white colour denes areas with a pole clustering density


9/12

(angle-axis pair) that brings the two grains to a perfectcrystallographic match on each other. Misorientationdata can be plotted either within a crystal referenceframe (inverse pole gure) or within a sample referenceframe (pole gure). Plotting the data may result inpreferred orientation patterns, in which case themicrostructure of the analysed sample is the result of processes that have been controlled either kinemati-cally or crystallographically. Considering that it is as-sumed in our model that deformation has rotatedgarnet crystals about a specic kinematic axis (the 280/56 plunging crenulation axis), we should be able totest the model by the use of misorientation analysis.

The basic assumption in our model is that garnetcrystals grew epitaxially upon 56 W dipping (001)biotite planes with N-S running (100) biotite traces (i.e.the pre-Alpine foliation). Relative to these (100) traces,the [001] directions of garnet crystals formed angles of 0 , ) 30 , +30 or +60 . The crenulation deformationevent has dispersed the crystallographic data fromthese original orientations. If it is assumed that thecrenulation geometry is cylindrical, then misorienta-tion axes calculated between the actual garnet orien-tations and the hypothetical original epitaxial positionsshould plot preferentially in the position of the cren-ulation axis.

The above assumption was checked by calculatingthe misorientation axis distribution for our numericalsimulation (Fig. 10a), and the result is exactly what ispredicted. Figure 10a shows two maxima: the prom-inent one coincides with the crenulation axis position,whereas the second coincides with the tilted [110] axis.Rotation by 30 (or multiples of this value) about this

second axis brings all the garnet grains which are stillin the hypothetical original epitaxial growth positionto a perfect overmatch. If the simulated data arecompared with those established for the natural sample(Fig. 10b), then we recognize exactly the same features.Obviously, the density in the simulated sample is muchhigher, because the simulation reproduces garnetrotation that obeys our model, whereas in the naturalsample not all garnet crystals do this.

DISCUSSION

The indication obtained from all data can be synthes-ized as follows: back rotation of garnet grains by 040 about the 280/56 plunging crenulation axis results in ahigh number of garnet grains having one {110} planeparallel to 56 W dipping (001) biotite planes alignedwithin the relict foliation. Relative to a hypotheticalNS oriented, horizontal (100) biotite trace, [001]garnet directions form angles of ) 30 , 0 , +30 and+60 . It is important to note that the orientation of the (100) biotite trace is hypothetical and has not beenmeasured, rst because biotite has broken down togarnet during high pressure metamorphism, and sec-ond because EBSD analysis on mica is problematicbecause of severe polishing problems.

Because it is unlikely that such a systematic rela-tionship between the crystallographic orientation of garnet and biotite is coincidental (despite the highnumber of garnet planes belonging to the {110} fam-ily), our data support a crystallographic control of biotite on garnet nucleation. Inspired by the idea of Powell (1966), we have tested a model wherein garnet

(a) (b) (c)

(d) (e) (f)

Fig. 9. Comparison of pole gures of nat-ural data (ac) with pole gures obtained bynumerical simulation (df). The concentricsmall girdle dispersion of {100} polestogether with the concentration of polesalong the primitive circle in the SE-sector (a,d) is equally well replicated as the charac-teristic {111} pole maximum about therotation axis together with the broad greatgirdle centred on this maximum (b, e), andthe {110} discontinuous great girdle crossingthe NE- and SE-sectors (c, f).




10/12

grows epitaxially upon crystallographically isoriented(001) biotite planes, with the basal and the apicalpseudohexagonal oxygen ring structures operating asnucleation sites. In our model the pseodohexagonalsymmetry of the ring structures allows [001] garnetdirections to orient in all six-fold directions with thesame probability.

However, our data do not show [001] directionsforming angles of ) 60 and +90 , suggesting one orboth of the following two assumptions:1 traces of (100) biotite planes on 56 W dipping (001)biotite planes were not exclusively horizontal and NSdirected;2 epitaxial growth of garnet upon biotite does notoccur in all six-fold directions.

A likely explanation for the latter assumption is thatdistortion of the pseudohexagonal oxygen rings redu-ces the symmetry, so that distortion energy differencesfavour epitaxial nucleation just in one specic direc-tion.

A major issue in an epitaxial growth model is theforce that drives distortion of the pseudohexagonaloxygen structure to t that of {110} garnet planes.Distortion of the pseudohexagonal ring structure iswell known within different mica types (Ferraris &Ivaldi, 2002), and is essentially controlled by the sub-

stitution processes occurring in the octahedralcoordinated sites. This leads to an in-plane rotation of the Si(Al)-O 4 tetrahedra, eventually distorting thepseudohexagonal oxygen ring to a perfect ditrigonalring when rotation reaches the maximum value of 30 (Ferraris & Ivaldi, 2002). Such substitution processesin the octahedral layer can therefore not be invoked asthe driving force for the distortion of the oxygenstructure.

Experiments in materials science suggest that dis-tortion could be conned to the environment of brokensymmetry at the surface (Matzdorf et al. , 2000), i.e. thesurface of (001) biotite planes upon which garnetnucleates. AFM images show that on (001) mica sur-faces the basal oxygen are present, whereas K + ionsusually are not visible (Drake et al. , 1989; Tsujimichiet al. , 1997). Nucleation of epitaxial garnet shouldtherefore be controlled by distortion of the basaloxygen conguration.

Figure 6c shows that nucleation of garnet onto thebasal oxygen conguration of (001) biotite planesneeds introduction of Al and additional oxygen in thering structure. We imagine that the introduction of these ions is the driving force for distortion of thepseudohexagonal ring structure. Distortion mustnot result in a perfect t between biotite and garnet,

(a)

(b)

Fig. 10. Misorientation axis distribution. (a)The numerically simulated data set showsthat rotation of garnet grains around thecrenulation axis between 0 and 40 producesa strong maximum of misorientation axescoinciding with the crenulation axis (280/56).(b) The same maximum as in the numericalsimulation is recognized in the naturalsample.

4 4 8 R . S P I E S SE T A L .



11/12

because it is well known from materials science thatepitaxial layers relative to the substrates show miststrain (e.g. Rocher et al. , 2002).

We have suggested that epitaxial growth will prob-ably occur in one specic direction on the (001) biotiteplane, i.e. that with the lowest distortion energy. Howcan the high frequency of angles of ) 30 , 0 , +30 and+ 60 between the [001] garnet direction and thehypothetically N-S trending, horizontal (100) biotitetrace be interpreted? The explanation is straightfor-ward: it implies that a signicant number of biotitecrystals had their (001) planes parallel to the NSrunning relict foliation, whereas the (100) traces werenot horizontal, but preferentially inclined between) 30 and +60 from the horizontal.

Figures 8 and 9ac show that the systematic dis-persion trend of garnet crystallographic orientationssupporting epitaxial growth is superimposed on a dis-tribution pattern characterized by a signicant degreeof randomness. One explanation for this is that

deformation rotated several biotite/garnet crystals in amore arbitrary fashion than that assumed in our sim-plistic model.

CONCLUSIONS

Dispersion patterns of garnet crystallographic orien-tations together with misorientation axis distributionpatterns support a model for the analysed mica schistwherein Alpine garnet grew epitaxially upon biotite,and became rotated about the crenulation axis duringthe Alpine deformation. Consequently, nucleation andgrowth sites of garnet were constrained by the spatialdistribution of biotite (or mica in general) within themica schist.

The indication that nucleation of garnet may becontrolled by precursor phases is an aspect that has tobe addressed when the spatial distribution patterns of garnet crystals are contemplated in garnet nucleationand growth models (Carlson, 1989, 1991; Denisonet al. , 1997). EBSD analysis will potentially unravelsuch a control.

ACKNOWLEDGEMENTS

This research was funded by Progetto Ateneo 2003-CPDA031431 (University of Padova), by P.R.I.N.Basamenti 2005 , and by P.R.I.N. 2005-047810 . Weappreciate the reviews of D. Hirsch and D. Withneywhich helped to improve the nal version of themanuscript signicantly. Discussion on mica structuresand epitaxy problems with M. Franca Brigatti andL. Secco are greatly acknowledged.

REFERENCES

Bell, T. H. & Rubenach, M. J., 1983. Sequential porphyroblastgrowth and crenulation cleavage developement during pro-gressive deformation. Tectonophysics , 92, 171194.

Brigatti, M. F. & Davoli, P., 1990. Crystal-structure renementof 1 M plutonic biotites sample M14 from a monzonite in theValle del Cervo pluton. American Mineralogist , 75, 305313.

Bucher, M. & Frey, K., 2002. Petrogenesis of MetamorphicRocks , 7th edn. Springer-Verlag, Berlin, 341 pp.

Carlson, W. D., 1989. The signicance of intergranular diffusionto the mechanisms and kinetics of porphyroblast crystal-lization. Contributions to Mineralogy and Petrology , 103, 124.

Carlson, W. D., 1991. Competitive diffusion-controlled growthof porphyroblasts. Mineralogical Magazine , 55, 317330.Chopin, C., 1984. Coesite and pure pyrope in high-grade

blueschists of the Western Alps: a rst record and some con-sequences. Contributions to Mineralogy and Petrology , 86,107118.

Chopin, C., 1987. Very high-pressure metamorphism in theWestern Alps: new petrological and eld data. Terra Cognita ,7, 94.

Chopin, C., Henry, C. & Michard, A., 1991. Geology and pet-rology of coesite bearing terrane, Dora-Maira Massif, Wes-tern Alps. European Journal of Mineralogy , 3, 263291.

Compagnoni, R. & Rolfo, F., 2003. UHPM units in the WesternAlps. In: Ultrahigh pressure metamorphism, EMU Notes inMineralogy , Vol. 5 (eds Carwell, D.A. & Compagnoni, R.),pp. 1349. Eo tvo s University Press, Budapest.

Compagnoni, R., Hirajima, T. & Chopin, C., 1995. Ultra-high-pressure metamorphic rocks in the Western Alps. In: UltrahighPressure Metamorphism (eds Coleman, R.G. & Wang, X.),pp. 206243. Cambridge University Press, Cambridge.

Denison, C., Carlson, W. D. & Ketcham, R. A., 1997. Three-dimensional quantitative textural analysis of metamorphicrocks using high-resolution computed X-ray tomography;Part I. Methods and techniques. Journal of MetamorphicGeology , 15, 2944.

Downs, R.T. & Hall-Wallace, M., 2003. The American Miner-alogist Crystal Structure Database. American Mineralogist , 88,247250.

Drake, B., Prater, C. B., Weisenhorn, A. L. et al. 1989. Imagingcrystals, polymers, and processes in water with the atomicforce microscope. Science , 243, 4898, 15861589.

Ferraris, G. & Ivaldi, G., 2002. Structural features of mica. In:Micas: Crystal Chemistry & Metamorphic Petrology (edsMottana, A., Sassi, F.P., Thompson, J.B. & Guggenheim, S.),Reviews in Mineralogy & Geochemistry , 46, 117153.

Flynn, G. W. & Powell, W. J. A., 1979. The Cutting and Pol-ishing of Electro-optic Materials . Adams Hilger, London.

Frondel, C., 1940. Oriented inclusions of staurolite, zircon andgarnet in muscovite. Skating crystals and their signicance.American Mineralogist , 25, 6987.

Gebauer, D., Schertl, H-P., Brix, M. & Schreyer, W., 1997.35 Ma old ultra-high-pressure metamorphism and evidencefor very rapid exhumation in the Dora-Maira Massif, WesternAlps. Lithos , 41, 524.

Groppo, C., Castelli, D. & Compagnoni, R., 2006. Late chlori-toid-staurolite assemblage in a garnet-kyanite bearingmetapelite from the UHP Brossasco-Isasca Unit (Dora-MairaMassif, Western Alps): new petrological constraints for aportion of the P-T decompressional path. In: Ultrahigh-Pres-sure Metamorphism: Deep Continental Subduction , Vol. 403

(eds Hacker, B.H., Mc Clelland, W.C. & Liou, J.G.), pp. 127 138. GSA Special Paper.Groppo, C., Lombardo, B., Castelli, D. & Compagnoni, R.,

2007. Exhumation history of the UHPM Brossasco-IsascaUnit, Dora-Maira Massif, as inferred from a phengite-amphibole eclogite. International Geology Review , 49, 142168.

Hermann, J., 2003. Experimental evidence for diamond-faciesmetamorphism in the Dora-Maira massif. Lithos , 70, 163182.

Hirsch, D. M., Ketcham, R. A. & Carlson, W. D., 2000. Anevaluation of spatial correlation functions in textural analysisof metamorphic rocks. Geological Material Research , 2, 142.

Ketcham, R. A., Meth, C., Hirsch, D. M. & Carlson, W. D.,2005. Improved methods for quantitative analysis of




12/12

three-dimensional porphyroblastic textures. Geosphere , 1,4259.

Kienast, J. R., Lombardo, B., Biino, G. & Pinardon, G., 1991.Petrology of very high pressure eclogitic rocks from theBrossasco-Isasca Complex, Dora-Maira massif, Italian Wes-tern Alps. Journal of Metamorphic Geology , 9, 1934.

Kretz, R., 1969. On the spatial distribution of crystals in rocks.Lithos , 2, 3966.

Kretz, R., 1974. Some models for the rate of crystallization of garnet in metamorphic rocks. Lithos , 7, 123131.Matsumoto, N. & Hirajima, T., 2000. Garnet in pelitic schists

from a quartz-eclogite unit of the southern Dora-Mairamassif, Western Alps. Schweizerische Mineralogische Petro- graphische Mitteilungen , 80, 5362.

Matzdorf, R., Fang, Z., Zhang, I. J. et al. , 2000. Ferromagnet-ism stabilized by lattice distortion at the surface of the P -wavesuperconductor Sr 2 RuO 4 . Science , 289, 5480, 746748.

Passchier, C. W. & Trouw, R. A. J., 1998. Microtectonics .Springer-Verlag, Berlin, Heidelberg.

Powell, D., 1966. On the preferred crystallographic orientationof garnet in some metamorphic rocks. Mineralogical Maga-zine, 35, 10941109.

Prior, D. J., Trimby, P. W., Weber, U. D. & Dingley, D. J., 1996.Orientation contrast imaging of microstructures in rocks usingforescatter detectors in the scanning electron microscope.Mineralogical Magazine , 60, 859869.

Quartieri, S., Chaboy, J., Merli, M., Oberti, R. & Ungaretti, L.,1995. Local structural environment of calcium in garnets: acombined structure-renement and XANES investigation.Physics and Chemistry of Minerals , 22, 159169.

Rocher, A., Ponchet, A., Blanc, S. & Fontaine, Ch., 2002. TEMevaluation of epitaxial strain in III-V semi-conductors: evi-dence of coherent and incoherent stress relaxation. Applied Surface Science , 188, 12,

Rubatto, D. & Hermann, J., 2001. Exhumation as fast as sub-duction? Geology , 16, 577588.

Schertl, H-P., Schreyer, W. & Chopin, C. (1991) The pyrope-coesite rocks and their country rocks at Parigi, Dora-Maira

Massif, Western Alps: detailed petrography, mineral chem-istry and P-T path. Contributions to Mineralogy and Petrology ,108, 121.

Sharp, Z. D., Essene, E. J. & Hunziker, J. C., 1993. Stable iso-tope geochemistry and phase equilibria of coesite-bearingwhiteschists, Dora Maira Massif, Western Alps. Contributionsto Mineralogy and Petrology , 114, 112.

Tsujimichi, K., Tamura, H., Hirotani, A., Kubo, M., Komiy-ama, M. & Miyamoto, A., 1997. Simulation of atomic forcemicroscopy images of cleaved mica surfaces. Journal of Phy-sical Chemistry B , 101, 42604264.

Wheeler, J., Prior, D. J., Jiang, Z., Spiess, R. & Trimby, P. J.,2001. The petrological signicance of misorientations betweengrains. Contributions to Mineralogy and Petrology , 141, 109 124.

Zhang, R. Y., Zhai, S. M., Fei, Y. W. & Liou, J. G., 2003.Titanium solubility in coexisting garnet and clinopyroxene atvery high pressure: the signifcance of exsolved rutile in garnet.Earth and Planetary Science Letters , 216, 591601.

Received 31 May 2006; revision accepted 1 February 2007.

4 5 0 R . S P I E S SE T A L .


Documents

When Epitaxy Controls Garnet Growthkk