Unusual deformation microstructures in garnet, titanite and clinozoisitefrom an eclogite of the Lower Schist Cover, Tauern Window, Austria

WOLFGANG FRIEDRICH MÜLLER1 and GERHARD FRANZ2

1 Institut für Angewandte Geowissenschaften, Technische Universität Darmstadt, Schnittspahnstr. 9,D-64287 Darmstadt, Germany

Corresponding author, e-mail: wmueller@geo.tu-darmstadt.de2 Institut für Angewandte Geowissenschaften, Technische Universität Berlin, D-1063 Berlin

Abstract: An eclogite sample from the Lower Schist Cover of the Tauern Window, only a few tens of metres distant from theintercalated Eclogite Zone, was studied by transmission electron microscopy. Garnet, titanite and clinozoisite reveal unusualdeformation microstructures not reported before from natural occurrences. Garnet shows polygonisation into subgrains of only 0.5to 3 µm in size which are separated by low angle grain boundaries. The subgrain cells show a preferred orientation parallel to (110).In titanite, most of the numerous dislocations are organized into low angle grain boundaries with Burgers vectors parallel to <011>.The Burgers vector of free dislocations is [100]. Weak fringes in the wake of dislocations are interpreted as slip traces. Clinozoisitecontains submicroscopic mechanical twin lamellae parallel to (100); the twin law is m parallel to (100). The unusual deformationfeatures are explained as the result of imbrication of the Eclogite Zone.

Key-words: garnet, titanite, clinozoisite, deformation, electron microscopy, Tauern Window.

1. Introduction

Transmission electron microscopy (TEM) of eclogite min-erals from the Tauern Window reveals a fascinating assem-bly of reaction and deformation structures in the micro-scopic and submicroscopic range (Carpenter, 1981; Zim-mermann et al., 1994; Barnert et al., 2001, 2003; Barnert,2003; Müller et al., 2004). Müller et al. (2004) described re-covery and recrystallization in omphacite evidenced by freedislocations, chain multiplicity faults, low angle grainboundaries and recrystallized grains in an eclogite samplefrom the Lower Schist Cover, and found in barroisitic am-phibole similar strong deformation. In contrast, garnet con-tained only few dislocations and rarely low angle grainboundaries.

The plastic deformation behaviour of garnet as an im-portant constituent of peridotite and eclogite has gained in-creasing attention (e.g. TEM studies by Doukhan et al.,1994; Karato et al., 1995; Cordier et al., 1996; Voegele etal., 1998a and b; Cordier, 2003; orientation contrast andelectron backscatter diffraction studies by Prior et al.,2000). Of special interest for the present work are the ex-emplary TEM studies by Voegele et al. (1998a and b) onexperimentally deformed single crystal silicate garnets andthe deformation microstructures in a variety of naturalsamples. They were able to fully characterize the glide sys-tems. In the natural samples, the dominant Burgers vectoris 1/2<111>, the most frequently found glide plane is

{110}. Polygonisation was observed in a specimen experi-mentally deformed at 1440°C; the subgrain size was about1 x 2.5 µm. Low angle grain boundaries have been ob-served in the natural samples, too, but no polygonisationcells.

Regarding the deformational behaviour of titanite andclinozoisite, the other two minerals we address here, notmuch information is available. In titanite, mechanical twinlamellae are only described from titanite exposed to shockwaves at a nuclear test site (Borg, 1970) and in a stronglyshocked anorthosite of the Manciouagan impact crater(Langenhorst & Dressler, 2003). According to Higgins &Ribbe (1976), the primitive space group P21/a is observedonly in titanite with less than 4 mole % substituents for Ti,whereas those with > 4 mole % of Ca(Al,Fe)SiO4(OH,F)have diffuse reflections k + l odd and streaks parallel to b*which become unobservable above » 20 mole % leading tospace group A2/a. Troitzsch et al. (1999) succeeded in nar-rowing down the space group transition P21/a → A2/a tocompositions between 0.09 < XAl < 0.18 [XAl = Al/(Al +Ti)] and Troitzsch & Ellis (2002) by measurement of theheat capacity to 0.15 < XAl < 0.20 at room temperature. Puresynthetic titanite, CaTiSiO5 undergoes a reversible phasetransition P21/a ↔ A2/a at about 220°C (see, e.g., Taylor &Brown, 1976). In addition, there is a temperature inducedisosymmetric (A2/a ↔ A2/a) phase transition around 552°C(Kek et al., 1997) and another P21/a ↔ A2/a phase transi-tion triggered by high pressure around 3.5 GPa (Kunz et al.,

Eur. J. Mineral.2004, 16, 939–944

DOI: 10.1127/0935-1221/2004/0016-09390935-1221/04/0016-0939 $ 2.70

ˇ 2004 E. Schweizerbart’sche Verlagsbuchhandlung, D-70176 Stuttgart

1 µm

1996). According to Kunz et al. (2000), the high pressurephase A2/a corresponds rather to the high-symmetry phaseA2/a above 552°C than to the disordered intermediate phasebetween 220°C and 552°C.

In euhedral clinozoisite crystals from veins, Ray et al.(1986) observed submicroscopic twin lamellae which theyinterpreted as growth twins. In deformed clinozoisites, theyfound abundant dislocations and stacking faults parallel to(100) but no fine scale multiple twins (see also Franz &Liebscher 2004).

Here we present unusual deformation microstructures ingarnet, titanite and clinozoisite from a pre-Alpine eclogite.Garnet shows polygonisation into subgrains of only 0.5 to 3µm in size, titanite contains numerous dislocations, whichare mostly organized into low angle grain boundaries, andclinozoisite displays deformation twin lamellae in the sub-micrometre range. To the best of our knowledge, neither po-lygonisation of this type in naturally deformed garnet, nordislocations in titanite, nor mechanical twin lamellae in cli-nozoisite have been reported before. In addition, omphacite– not the topic of the present paper – shows exsolution la-mellae parallel to (110), which has also not been reportedbefore.

2. Eclogite occurrence

Sample Fr 146 comes from the Frosnitztal, a valley in theSouth Venediger area, Tauern Window (Austria), where thetectonic units Lower Schist Cover, Eclogite Zone and UpperSchist Cover are well exposed. It was taken from a metaba-site body, which occurs in the tectonically highest rock se-ries of the Lower Schist Cover (location E4 in Zimmermann& Franz, 1988; Fr 146 in Fig. 2 of Zimmermann et al., 1994)immediately in tectonic contact with the basal series of theEclogite Zone, and thus belongs to the group of pre-Alpineeclogites (von Quadt et al., 1997). The size of the area withmetabasite bodies is about 20 x 100 m, individual bodies arem-sized. Zimmermann & Franz (1988) describe the rocks asstrongly deformed and consisting of garnet, amphibole, pla-gioclase, ± omphacite, titanite, ± rutile, ± Fe-Ti-oxides.Sample Fr 146 is a weakly foliated garnet amphibolite witha typical high pressure vein assemblage of omphacite-albite-actinolite/hornblende-calcite-apatite-titanite-quartz(Zimmermann et al., 1994).

3. Experimental

The TEM studies were carried out on a Philips CM 12 trans-mission electron microscope operated at 120 kV. Specimenssuitably thin for TEM were prepared by Ar+ ion milling.Mineral compositions were determined by energy disper-sive X-ray spectrometry (EDX) using an EDAX 9900 at-tached to the CM 12 electron microscope. The microstruc-tures were studied by conventional bright field (BF), darkfield (DF) and weak beam (WB) imaging modes, high-reso-lution transmission electron microscopy (HRTEM) and se-lected area electron diffraction (SAD).

4. TEM observations and interpretation

The following minerals were identified by the TEM meth-ods of SAD and EDX in four ion-thinned specimens takenfrom one thin section: Actinolitic and barroisitic amphibole,omphacite, garnet, titanite, clinozoisite, plagioclase, apatite,rutile, zircon. Amphiboles are the most frequent minerals.They show free dislocations and dislocation walls or net-works of low angle grain boundaries, chain multiplicityfaults parallel to (010) of single and triple chains terminatedby partial dislocations, and rarely polygonisation with a cellsize of about 1 µm. Plagioclase shows peristerite inter-growths (Zimmermann et al., 1994), apatite contains somedislocations and zircon displays defect clusters, most likelydue to radioactive radiation damage, and dislocations. Ru-tile occurs only as inclusions in titanite.

The few omphacite grains seen display roundish anti-phase domains which are 25 to 50 nm in size, chain multi-plicity faults parallel to (010) partially interacting with anti-phase domain boundaries (see Müller et al., 2004), free dis-locations, and sometimes exsolution lamellae. In one om-phacite grain the 10 nm wide exsolution lamellae are paral-lel to (110) which was hitherto never reported (see Carpen-ter, 1981).

4.1 Garnet

The most striking and surprising observation in garnet(alm48-60gro28-50prp4-12; Zimmermann & Franz, 1988) is po-lygonisation (Fig. 1) composed of 0.5 to 3 µm large sub-grains, which are separated by low angle grain boundaries.That has been found in several grains of two different TEMspecimens. The angles between the subgrains are about 2 to

Fig. 1. Garnet showing polygonisation. The long side of the subcellsis roughly parallel to (110). TEM DF micrograph with g = 004.

940 W. F. Müller, G. Franz

1 µm 0.2 µm

f

(110)

d

d'

(110)

20 nm

Fig. 2. Garnet. Dislocations connected by “weak contrast fringes”(Voegele et al., 1998b). TEM BF micrograph; g = 420 operating re-flection.

4° as measured in a few favourable orientations. The sub-grain cells have a preferred orientation parallel to (110), i.e.their long side is subparallel to (110). The ratio of the lengthand the width of the subgrains usually varies between 1 and4; the mean value from 11 subgrains is 2.1. In one case a la-mella-shaped subgrain with a width of 0.5 µm and a mini-mum length of 2.7 µm was observed.

In addition, we observed TEM contrast phenomena con-nected with dislocations similar to those found by Voegeleet al. (1998b) in natural samples (Fig. 2). They describe dis-locations in garnet which bound faults with weak fringes(“weak contrast fringes” called by them); seen edge-on, thefault planes are nearly parallel to {110}, “but it clearly devi-ated from this plane”. Perfect or nearly perfect Burger vec-tors were analyzed by Voegele et al. (1998b) as 1/2<111>and <100>. Here, we even found a fault exactly parallel to(110) extended between two dislocations (Fig. 3). HRTEMdid not reveal any deviation of the spacings of the (110) lat-tice fringes at the fault (Fig. 4).

4.2 Titanite

Titanite with an approximate average grain size of 100 x 250µm is a frequent mineral in sample Fr 146. It has about 4mole percent substituents of Al and Fe for Ti. In suitable ori-entations, e.g. zone axis [001], the electron diffraction pat-terns show streaks parallel to b*. No sharp or weak diffusereflections of the type k+l = odd were observed.

Numerous dislocations (Fig. 5) characterize the titanitecrystals. Most of them are organized into low angle grainboundaries, which consist of one, two or sometimes moresets of dislocations. The Burgers vector of free dislocationsdetermined so far is b = [100]. Dislocations of low anglegrain boundaries have a Burgers vector parallel to [011] and

Fig. 3. Garnet. “Weak contrast fringes” seen edge on. Zone axis [1-11].Note the fault parallel to (110) labelled f between the two dislocationsd – d’. In the left upper corner, additional faults of non-crystallographicdirections and parallel to (110) are seen. TEM BF micrograph.

Fig. 4. Garnet. Lattice imaging of the area with fault f and dislocationd shown in Fig. 3. Zone axis [1-11]. Lattice fringes parallel to (110),(011) and (-101).

[0-11]. It is not clear if they occur also as free dislocations.Dislocation nodes are often seen. Of special interest areweak fringes and lines in the wake of dislocations (Fig. 6).

Unusual deformation of garnet, titanite and clinozoisite 941

2 µm

1 µm

1 µm

I

II

1 µm

c*a*

ct*

Fig. 5. Titanite with numerous dislocations which are frequently or-ganized into low angle grain boundaries (three of them are arrowed).TEM DF micrograph with g = 220.

In contrast to garnet, they do not connect pairs of disloca-tions. They are apparently slip traces in the wake of disloca-tions. This contrast phenomenon is well known from metalsand alloys (e.g. Häussler et al., 1999).

In one grain we observed two twin lamellae of differentorientations, the one ending at the other (Fig. 7). The lamel-lae are about 0.5 µm wide. One twin lamella (lamella II inFig. 7) shares the trace of the habit plane and the reflections0kk with the matrix, the orientation relation of the other twinlamella was not identified.

Inclusions observed in titanite were a rutile crystal ofabout 1.8 x 3.0 µm ellipsoidal size, and an idiomorphic om-phacite, 1.4 x 0.9 µm in size, with the planes {100}, {010}and {110}.

Fig. 6. Titanite. The weak fringes in the wake of dislocations (exam-ple arrowed) are interpreted as slip traces. TEM BF micrograph.

Fig. 7. Titanite with two twin lamellae (I and II). Note the disloca-tions in the interface between twin lamellae II and matrix. TEM BFmicrograph.

4.3 Clinozoisite

Two clinozoisite crystals were observed in two differentTEM-specimens. The composition is about Ca1.8Sr0.2Al2.8-Fe0.2Si3O12(OH). Both crystals show thin twin lamellae par-allel to (100) (Fig. 8). The twin law is m parallel to (100), asdeduced from the electron diffraction pattern with the zoneaxis [010] (Fig. 8). The widths of the lamellae vary from 40nm down to 0.8 nm, the width of an elementary cell as re-vealed by HRTEM. The distances between the twin lamel-lae are irregular and vary between 25 nm and 0.5 µm. Nofree dislocations were seen, but strain fields along the lamel-lae (better seen in BF images than in Fig. 8) suggest the pres-

Fig. 8. Clinozoisite with mechanical twin lamellae parallel to (100).TEM DF micrograph taken with a twin reflection. The electron dif-fraction pattern of the corresponding zone of the crystal is inserted.

942 W. F. Müller, G. Franz

ence of dislocations. This, and their occurrence in a heavilydeformed rock lead to the conclusion that the lamellae aremechanical twins.

5. Discussion and conclusions

The dislocation microstructures of garnet and titanite arecompelling evidence for recovery processes of plasticallydeformed crystals. The polygonisation of garnet is indica-tion of an advanced state of recovery compared to the occur-rence of isolated low angle grain boundaries. In view of re-sults and conclusions of Voegele et al. (1998b) on naturalsamples, the deformation of garnet from sample Fr 146 be-longs to the high temperature deformation regime > 600°C.Dislocations experience strong lattice friction in the lowtemperature regime leading to a brittle behaviour, whereasat high temperature diffusion assists dislocation glide andclimb (Voegele et al., 1998a and b). The plastic deformationof garnet is obviously not separable from recovery, the re-covery process is syndeformational as also suggested by thepreferential orientation of the subgrains and the high shapefactor.

The unusual deformation features of garnet observedhere, an otherwise well-studied mineral, provokes the ques-tion if there is a special intensive parameter responsible inaddition to temperature. Voegele et al. (1998b) investigatedthe effect of fluids, but found no correlation between the dis-location microstructure and the hydrous component or thechemistry of the garnets. Because sample Fr 146 containshigh-pressure veins with exceptional skeletal structures inomphacite (Zimmermann et al., 1994) we cannot excludethat an unusual amount or composition of fluids is responsi-ble for the polygonisation into small subcells.

Plastic deformation of titanite by dislocation glide and re-covery shown by low angle grain boundaries has apparentlynever been observed before. The analysed Burgers vector b= [100] and those with b parallel to <011> can be madelikely by the structure and lattice of titanite, respectively:

(1) The analysed Burgers vector b = [100] is structurallyfavoured because the dominant structural motif of the min-eral are kinked chains of corner-sharing TiO6 octahedrawhich run parallel to a (e.g. Taylor & Brown, 1976); thelength of |b| is a = 7.06 Å.

(2) All facts (i.e., no reflections k + l odd observed, tem-perature around 600°C and pressure of 15 to 20 kbar pres-sure) point to a space group A2/a of titanite during deforma-tion. Therefore, the shortest Burgers vectors b are 1/2<011> and the corresponding dislocations are perfect ones.The length of this Burgers vector is about 5.4 Å. Since theelastic energy of the Burgers vector is proportional to b2,dislocations with b = 1/2<011> are preferred. In view ofFig. 2 of Kunz et al. (2000), the likely structure during de-formation at the prevailing conditions was the high pres-sure, high symmetrical A2/a phase, which enabled an espe-cially easy glide.

Mechanical twins are usually attributed to a regime of lowtemperature and/or high strain rate. This view is supported fortitanite by the observation of twins in samples affected byshock waves (Borg, 1970; Langenhorst & Dressler, 2003). It

is possible if not likely that the two twin lamellae in our titani-te were formed after the crystal was already hardened by therather immobile low angle grain boundaries, i.e. in a stagewhen the temperature was too low for creep of dislocationsbut when stress still persisted. In clinozoisite, which frequent-ly occurs in low temperature environments, mechanical twinshave not yet been observed, although growth twins parallel to(100) are a common phenomenon. We think that the unusualmechanical twinning in sample Fr 146 is a good argument foran exceptional high strain rate.

In addition to the deformation in the stability field of theA2/a phase, the strong deformation of titanite can be attrib-uted to the fact that this mineral is here not just an accessorybut a modally important rock constituent. Therefore, titanitewas not protected against deformation by other mechanical-ly weaker matrix minerals. As in the case of garnet, we as-sume that the recovery occurred syndeformationally.

Finally, when considering the unusual deformation phe-nomena especially evident and important in garnet, the geo-logical situation has to be considered and some speculativeconsiderations are offered. The intercalation of the EclogiteZone between Lower and Upper Schist Cover of the TauernWindow must have been a dramatic event. In so far it is notsurprising that rocks only decades of metres distant fromthis tectonic fault are stronger affected than those within theinterior of the Lower Schist Cover. Tectonic imbrication ofa rock slice from a depth corresponding to 2 GPa to approxi-mately 1 GPa requires high strain rates, and will producehigh temperatures in the rocks affected and neighboured.This scenario can explain the polygonisation in garnet forwhich temperatures > 600°C may be necessary.

Acknowledgements: WFM thanks Josef Kolb for the care-ful preparation of petrographic thin sections, ThomasDirsch for patient help with computer problems and StefanLauterbach for skilled preparation of the TEM foils. Sup-port by the Deutsche Forschungsgemeinschaft (grantMu358/19) is gratefully acknowledged. We thank the anon-ymous colleague for his constructive review, the associateeditor Patrick Cordier for his encouraging report and toDominique Lattard and Rainer Althaus for friendly han-dling of the manuscript.

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Received 21 April 2004Modified version received 30 July 2004Accepted 30 August 2004

944 W. F. Müller, G. Franz