Precipitation of pyroxenes and Mg 2 SiO 4 from majoritic garnet: simulation of peridotite exhumation from great depth

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1 doi: /j x Precipitation of pyroxenes and Mg 2 SiO 4 from majoritic garnet: simulation of peridotite exhumation from great depth Larissa F. Dobrzhinetskaya, 1 Harry W. Green, II, 1,2 Alex P. Renfro, 1 Krassimir N. Bozhilov, 2 Dirk Spengler 3 and Herman L. M. van Roermund 3 1 Department of Earth Sciences, University of California at Riverside, CA 92521, USA; 2 Institute of Geophysics and Planetary Physics, University of California at Riverside, CA 92521, USA; 3 Utrecht University, Faculty of Earth Sciences, Utrecht, The Netherlands ABSTRACT Our experimental simulations of the exhumation path of mantle peridotites show that high-temperature (1400 C) decompression of lherzolite from 14 to 13 and 12 GPa results in exsolution of interstitial blebs of diopside and Mg 2 SiO 4 (wadsleyite) lamellae from majoritic garnet. At lower pressures (from 8 to 5 GPa, at T ¼ 1400 C) only enstatite exsolves as blebs at garnet boundaries. Continuous high-temperature decompression from 14 to 7 GPa produces zoned majoritic garnet containing blebs of exsolved pyroxenes inside garnet rims. No intracrystalline precipitation of pyroxene was observed in garnet, although such lamellae are found in some natural garnet peridotites. The explanation appears to be the three orders of magnitude difference in grain size between experimental and natural specimens. Our data suggest that Mg 2 SiO 4 and diopside exsolutions reflect the deepest point of the exhumation path of garnet peridotites, whereas enstatite precipitation may be restricted to garnets with less majoritic component at shallower depths. Terra Nova, 16, , 2004 Introduction Garnet peridotites occur as mantle xenoliths within kimberlites lamproites and as bodies within ultrahighpressure metamorphic (UHPM) terranes related to continental collision. Because peridotite is the principal rock type of the mantle, all garnet peridotites potentially contain geological information valuable for understanding mantle processes in general and subduction exhumation processes in particular. Many garnet peridotites from collisional terranes are associated with coesite- and or diamond-bearing crustal rocks (e.g. Sobolev and Shatsky, 1990; Xu et al., 1992; Smith, 1994; Dobrzhinetskaya et al., 1995; Wain, 1997; Nasdala and Massonne, 2000; van Roermund et al., 2002; Yang et al., 2003), suggesting a minimal pressure of 3 4 GPa and implying subduction of continental material to a depth of > km. However, some geothermobarometric calculations and microstructural observations suggest that subduction-zone garnet peridotite may originate from deeper environments, > km (Yang et al., 1993; Dobrzhinetskaya et al., Correspondence: Dr Larissa F. Dobrzhinetskaya, Department of Earth Sciences, University of California, 900 University Avenue, Riverside, CA 92521, USA. Tel.: ; fax: ; larissa@ucrac1.ucr.edu 1996; van Roermund and Drury, 1998; Ye et al., 2000). Because the early geological history of deep-seated rocks is usually camouflaged by superimposed shallow thermal and decompression events that lead to chemical re-equilibration, in many cases only microstructures survive as a record of their exhumation path. In peridotite, supersilicic (majoritic) garnet most commonly preserves a record of its history through microstructural features developed during decompression. The majoritic garnet component is stable at pressures > 5 GPa (Akaogi and Akimoto, 1977), and its chemistry may be expressed as the following complex solid solution: viii M vi 3 ðal 2 2nM n Si n ÞSi 3 O 12 ; where M ¼ (Mg 2+, Fe 2+, Ca 2+ ), 0 n 1, and superscripts indicate oxygen coordination of cations. With increasing pressure, ÔnormalÕ mantle garnet becomes progressively Al 3+ (Cr 3+ ) deficient as these ions are diluted by progressive substitution of M and Si 4+ ions in octahedral coordination, producing supersilicic garnet with silicon > 3 cations per formula unit (c.p.f.u.) (e.g. Smith and Mason, 1970; Akaogi and Akimoto, 1977; Irifune, 1987). A temperature effect was also recently recognized (Fei and Bertka, 1999). During decompression, majoritic garnet breaks down according to the reaction supersilicic garnet! exsolved pyroxenes þ less-silicic garnet: ð1þ This reaction, if quantified in terms of the relative volume percentage of exsolved pyroxene vs. ÔnormalÕ garnet, may serve as a theoretical foundation for the qualitative interpretation of the decompression path of garnet peridotites during their exhumation (van Roermund et al., 2001). Garnet peridotites containing relic majoritic garnet with abundant crystallographically orientated lamellae of pyroxenes were recently discovered within UHPM terranes, but no experimental reproduction of such microstructures has yet been reported. This paper introduces the first results of our attempts at experimental production and study of the microstructure associated with majoritic garnet during its high-temperature decompression and reannealing from to 7 GPa and from 8 to 5 GPa. Experiments were performed in a Walker style multianvil apparatus on a natural mineral mix corresponding to the bulk chemistry of garnet peridotite. The experimental results serve as a template for interpretation of the microstructure in natural garnet peridotite; this provides Ó 2004 Blackwell Publishing Ltd 325

2 Precipitation of pyroxenes and Mg 2 SiO 4 from majoritic garnet L. F. Dobrzhinetskaya et al. Terra Nova, Vol 16, No. 6, a better understanding of the depth from which such rocks may be exhumed to the Earth s surface. Microstructures recording former majoritic garnet in natural samples Garnets with intracrystalline diopside and enstatite lamellae parallel to <111> garnet have been discovered within mantle xenoliths from a kimberlitic pipe (Haggerty and Sautter, 1990; Sautter et al., 1991) and within garnet peridotites enclosed in coesite diamond-bearing UHPM rocks (e.g. van Roermund and Drury, 1998; van Roermund et al., 2000, 2001; Ye et al., 2000). In garnet peridotite from the UHPM terrane in Norway, enstatite and diopside with typical exsolution microstructures have been found in the cores of large garnets surrounded by an inclusion-free rim (Fig. 1a), whereas interstitial pyroxenes and (rare) olivine were also observed along grain boundaries and triple point junctions defined by adjacent garnet grains (Fig. 1b). The interpretation of orientated pyroxene lamellae in tectonic peridotite as a decompression product of former majoritic garnet was readily accepted by the geological community because high-pressure supersilicic garnet was already well known from experimental work and had also been reported from mantle xenoliths (Haggerty and Sautter, 1990). In many other subduction-zone garnet peridotites from UHPM terranes, e.g. in the western Alps and Central China Orogenic Belt, diopside, enstatite and or olivine are found as single grains inside and along grain boundaries within larger polycrystalline garnets and within embayments at the margins of smaller amoeboid garnets (e.g. Dobrzhinetskaya et al., 1996; Green et al., 2000), but lack intracrystalline lamellae. Although precipitation at grain boundaries, and particularly at triple junctions, is very common in any polycrystalline solid undergoing cooling or depressurization, such microstructures could also have other interpretations. Therefore, interpretation of such interstitial crystals as products of decompression of former majoritic garnet has not been compelling. As the diagnostic chemical signature of former majoritic garnet (Si > 3.0 c.p.f.u.) is lost at pressures < 5 GPa, it is clear that experimental verification of its decompression microstructures is needed to put constraints on the interpretations of natural rocks. Experimental design We have performed three types of experiments at 1400 C in which majoritic garnet was synthesized at high pressure and then decompressed to lower pressures at constant temperature. The first type was initially equilibrated at 14 GPa (MA-147) and then annealed at 13 GPa or 12 GPa (MA-144; MA-148); the second type was also synthesized at 14 GPa but then slowly decompressed to 7 GPa at constant temperature (1400 C) Fig. 1 (a) Photomicrograph of a single grain of garnet with needle-shaped precipitates of opx and cpx in the core surrounded by a precipitation-free rim. Black arrows indicate interstitial opx at triple point junctions with adjacent garnets. (b) Backscattered electron image of garnet crystals (light contrast) with interstitial opx crystals (grey contrast) precipitated at triple point junctions. The garnet texture is overprinted by late cracks. Both samples are from a nodular garnet collected in the Western Gneiss Region, Norway. without any long-term reannealing at 7 GPa (MA-213); the third type was synthesized at 8 GPa (MA149) and reannealed at 5 GPa (MA-150). Experimental conditions are given in Table 1 and Fig. 2. A mineral mix (Wards Company) of 40 vol.% San Carlos olivine, 20 vol.% each of pyropic garnet (Arizona), diopside (Yakutia) and enstatite (Norway) ground to a 5 10 lm powder was used as starting material. Oxygen fugacity was buffered by placing Ni foil at the top of the mineral charge and verified through the detection of both Ni and NiO following the experiments. Experiments were conducted in a Walker style multi-anvil apparatus with assemblies and run procedures similar to those described earlier (Dobrzhinetskaya et al., 2000). Platinum and MgO capsules containing run products were cut in half longitudinally and thick polished sections were prepared by conventional techniques for further study by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) assisted by focused-ion-beam (FIB). Results Higher pressure experiments Group 1: one-cycle experiment Majoritic garnet was successfully synthesized at T ¼ 1400 C and P ¼ 14 GPa after 17 h (MA-147). All starting enstatite (En) and about 85 90% of starting diopside (Di) were dissolved into newly recrystallized garnet (Grt), yielding run products of approximately (vol.%) 40Ol + 55Grt + 5Di. The amount of starting olivine (Ol) dissolved into garnet was small (1 2%), although it is difficult to evaluate precisely. SEM imaging revealed that the run product consists of equant 5 10 lm grains. Recrystallized diopside has a composition of (Mg 0.91 Fe 0.05 Ca 0.74 Al 0.14 Cr 0.03 Na 0.13 )Si 2 O 6. Garnet became silica-enriched, with an average composition of (Mg 2.45 Fe 0.30 Ca 0.49 )(Al 1.36 Cr 0.07 )Si 3.32 O 12 based on 15 analyses. Si cation proportions varied in the range c.p.f.u. Group 2: two-cycle experiments These experiments were designed to determine the microstructure 326 Ó 2004 Blackwell Publishing Ltd

3 Terra Nova, Vol 16, No. 6, L. F. Dobrzhinetskaya et al. Precipitation of pyroxenes and Mg 2 SiO 4 from majoritic garnet Table 1 Experimental conditions and results Run no. Stage Pressure (GPa) Temperature ( o C) Time (h) Garnet : Si (c.p.f.u.) MA MA-144a MA MA MA-148a MA MA MA-150a MA MA-213a Chemical compositions of the minerals from run products were measured by energy-dispersive X-ray spectrometry (EDS) and wavelength-dispersive spectrometry (WDS). The measurements were made on crystals situated not more than lm from the thermocouples to avoid any uncertainties that could occur due to temperature gradients. Pressure (GPa) MA-144,148 & 213 MA-148 MA-144 MA-150 MA-150 MA-213a T = 1400 o C MA-144a MA-148a MA-150a Time (h) Fig. 2 Diagram of P and time of the two-cycle experiments. Temperature (1400 C) was constant for all experiments. The decompression path at room temperature at the end of each experiment is not shown on this diagram. Fig. 3 Secondary electron image at 15 kv showing a diopside bleb among majoritic garnets from the two-cycle experiment MA-148a. This specimen contains large numbers of tiny diopside grains whereas the single-cycle specimen, MA-147, showed only rare residual diopside grains of > 5 lm diameter. Therefore, although some of these small grains are conceivably relicts, the great majority must have exsolved from the higher-pressure garnet of the first cycle. associated with the re-equilibration of majoritic garnet following decompression. Decompression of majorite was performed in experiments MA-144, MA-148 and MA-213 after initial equilibration at P ¼ 14 GPa, T ¼ 1400 C. Reannealing of MA-144 and MA-148 was performed at P ¼ 13 GPa (MA-144a) and at 12 GPa (MA-148a). In MA-213, initial equilibration was followed by slow decompression from 14 GPa to 7 GPa at 1400 C, followed by immediate quenching (MA-213a). The run products of MA-144a and MA-148a consist of 2 5 lm equant grains of olivine and garnet with rare residual diopside accompanied by large numbers of 1 2 lm interstitial grains of new diopside located preferentially at triple junctions. Figure 3 shows one of these many tiny grains. Given the small amount of starting diopside remaining in MA-147, it follows that most of these tiny interstitial grains of diopside must be products of exsolution from garnet with a higher majoritic component. These tiny interstitial diopside crystals (MA-144a) have composition (Mg 0.97 Fe 0.03 Ca 0.73 Al 0.13 Cr.0.02 Na 0.12 )Si 2 O 6 whereas diopside in sample MA-147 (equilibrated at 14 GPa) has composition (Mg 0.91 Fe 0.05 Ca 0.74 Al 0.14 Cr 0.03 Na 0.13 ) Si 2 O 6. The principal change in pyroxene composition between 14 and 13 GPa is an increase in MgO (with corresponding small decreases in other elements to maintain stoichiometry), reflecting the decrease in the majoritic component in the garnet. No (clino)enstatite was found in the run product of either MA-144a or MA-148a. After re-equilibration at 13 GPa (MA-144a), garnet has a typical composition of (Mg 2.47 Fe 0.29 Ca 0.46 )(Al 1.44 Cr 0.07 )Si 3.27 O 12 and contains less Si (3.27 c.p.f.u.) than garnet quenched directly from 14 GPa (Si ¼ 3.32 c.p.f.u.). In the run product reannealed at 12 GPa (MA-148a), the garnet contains still less silica (3.19 c.p.f.u.). SEM images revealed that some large grains of garnet re-equilibrated from 14 to 13 GPa contain micrometre-sized exsolution ÔlamellaeÕ (Fig. 4a,b). Using the FIB technique (Dobrzhinetskaya et al., 2003) combined with TEM analysis (Fig. 4.c,d), we have established that the exsolved ÔlamellaeÕ are plates of Ó 2004 Blackwell Publishing Ltd 327

4 Precipitation of pyroxenes and Mg 2 SiO 4 from majoritic garnet L. F. Dobrzhinetskaya et al. Terra Nova, Vol 16, No. 6, Fig. 4 Electron microscope digital images showing a correlation between SEM (a,b), FIB (c) and TEM (d) observations. (a) The secondary electron image at 15 kv showing the general view of the central part of the 3-mm-diameter experimental sample; tc: thermocouple. Arrowheads show places where the analytical measurements were made and where TEM foils were excavated by FIB. 1* shows the place where garnet with exsolution of platy crystals of Mg 2 SiO 4 was found. Because the areas of our analytical measurements and observations are very close to the thermocouples, such a position of the sample excludes any uncertainties in evaluation of the temperature conditions of the run product. (b) Secondary electron image at 15 kv shows numerous Mg 2 SiO 4 precipitates inside decompressed majoritic garnet. This panel corresponds to the area marked as 1* on (a). (c) Secondary electron image at 5 kv shows TEM foil preparation by a FIB of gallium ions (STRATA 235 Dual Beam system, FEI Co.). The application of FIB to the experimental samples helps to avoid uncertainties in a position of the grains prepared for TEM, allowing an unprecedented correlation with observations obtained at lower magnifications. (d) TEM image at 300 kv shows Mg 2 SiO 4 platelet on a garnet low-angle boundary from the foil in (c). Electron diffraction patterns (not shown here) indicate that Mg 2 SiO 4 platelets are represented by wadsleyite rather than olivine, which is not surprising for the given P T conditions of run MA-144. According to olivine phase diagram equilibrium (Akaogi et al., 1989), the experiment MA-144 plots in the field of the P T space where olivine and wadsleyite coexist. Details of the electron diffraction studies will be published elsewhere. Fig. 5 Secondary electron image at 15 kv of zoned majoritic garnet with exsolution blebs of enstatite (darker contrast) and diopside (light grey contrast) precipitated around a non-reacted pyropic core. The content of Si in the relict core is 2.92 c.p.f.u. The recrystallized garnet is zoned such that the content of Si decreases gradually from 3.34 (zone 1) towards 3.05 (zone 3); the highest Si content is in the zone 1 adjacent to the non-reacted core (lighter contrast). Such zoning reflects the process of slow high-temperature (1400 C) decompression from the highest majoritic component, achieved at 14 GPa, to the lowest at 7 GPa (MA-213). Numbers: 1*, zone 1; 2*, zone 2; 3*, zone 3. garnet garnet grain boundaries and within embayments at the amoeboidlike margins (Fig. 5). We have observed zoned garnet (> 15 lm in diameter), bearing non-reacted cores of stoichiometric pyropic garnet [(Mg 2.12 Fe 0.53 Ca 0.34 )(Al 1.93 Cr 0.07 )(Si 2.92 Al 0.08 )O 12 ]. The first rim adjacent to the core (Zone 1) contains nonstoichiometric majoritic garnet (Mg 2.46 Fe 0.31 Ca 0.47 )(Al 1.32 Cr 0.06 )Si 3.34 O 12, with Si ¼ 3.34 c.p.f.u., whereas the outer rim (Zone 3) also contains non-stoichiometric garnet (Mg 2.39 Fe 0.27 Ca 0.43 )(Al 1.80 Cr 0.06 )Si 3.05 O 12, but with a smaller majoritic component (Si ¼ 3.05 c.p.f.u.) than the first rim; the intermediate rim (Zone 2) is characterized by a composition of (Mg 2.51 Fe 0.27 Ca 0.41 )(Al 1.61 Cr 0.08 )Si 3.12 O 12 with Si ¼ 3.12 c.p.f.u. Such retrograde compositional zoning of garnet is in agreement with the microstructural evidence of diopside and enstatite precipitation accompanying the chemical re-equilibration during high-temperature decompression. wadsleyite that are nucleated on lowangle boundaries in the garnet (Fig. 4d). The wadsleyite lamellae composition is (Mg 1.81 Fe 0.14 Al Ca 0.01 Ni 0.01 )Si 1.00 O 4, whereas that of the host garnet is (Mg 2.47 Fe 0.29 Ca 0.45 )(Al 1.44 Cr 0.07 )Si 3.27 O 12 with Si ¼ 3.27 c.p.f.u. In contrast, TEM revealed no exsolution lamellae of pyroxene. During slow decompression from 14 GPa to 7 GPa at 1400 C (i.e. MA-213a) majoritic garnet was gradually broken down, producing interstitial precipitation of both diopside and enstatite along adjacent Lower pressure experiments Group 3: one- and two-cycle experiments We conducted another pair of experiments to observe the re-equilibration of slightly majoritic garnets at lower pressures. The chosen conditions (equilibrated at 8 GPa and reannealed at 5 GPa, both at 1400 C) are close to the pressure and temperature calculated by van Roermund and Drury (1998) for ultra-deep peridotite from Norway exhibiting enstatite exsolution lamellae and blebs at grain 328 Ó 2004 Blackwell Publishing Ltd

5 Terra Nova, Vol 16, No. 6, L. F. Dobrzhinetskaya et al. Precipitation of pyroxenes and Mg 2 SiO 4 from majoritic garnet boundaries. The one-cycle experiment (MA-149) at 1400 C and 8 GPa yielded an equant-grained microstructure consisting of (vol.%) 40Ol + 40Grt Di + 7 5En. Garnet has a composition of [(Mg 2.39 Fe 0.22 Ca 0.43 )(Al 1.85 Cr 0.06 )Si 3.05 O 12 ], with a low majoritic component (Si ¼ 3.05 c.p.f.u.). The two-cycle experiment (MA-150) was held at 8 GPa and 1400 C for 17 h, then taken to 5 GPa at constant temperature for 55 h. The run product consisted of (vol.%) 40Ol + 20Grt + 20Di + 20En. Garnet was found to have a composition of (Mg 2.28 Fe 0.28 Ca 0.44 )(Al 1.92 Cr 0.06 )Si 3.01 O 12, with minimal majoritic component (Si ¼ 3.01 c.p.f.u.). Many small crystals of enstatite appeared interstitial to garnet (Fig. 6), representing exsolution of the majoritic component. No small interstitial diopside grains similar to those present after annealing at 13 GPa were seen, suggesting that the small amount of diopside dissolved in garnet at 8 GPa had probably regrown on relict diopside crystals remaining from the first cycle. Discussion Not surprisingly, our results mirror those of Ringwood (1991), who showed that, in peridotite, pyroxenes dissolve progressively into garnet at pressures between 5 and 15 GPa. Our results clarify the microstructures to be expected if decompression of majoritic garnet occurs on a time scale in which textural re-equilibration does not keep up with chemical re-equilibration. In particular, we show that as pressure increases, enstatite dissolution into garnet becomes significant at lower pressures (5 8 GPa) but that incorporation of diopside into garnet is enhanced only at higher pressures. These systematics will be useful in interpretation of natural rocks. The co-precipitation of pyroxenes as blebs at the rim zones of the garnet during high-temperature decompression from 14 to 7 GPa shares microstructural characteristics with majorite inclusions in diamond from Sao Luis (Sautter et al., 1998). The latter is believed to originate from a depth of more than 800 km (e.g. Harte et al., 1999). Sautter et al. (1998) noted that clinopyroxene blebs are precipitated preferentially at garnet diamond interfaces rather than in the form of exsolution lamellae within decompressed majoritic garnet. One surprise is that Mg 2 SiO 4 also exsolved from majoritic garnet at very high pressures. In retrospect, the classic diagrams in Ringwood (1991) show that the percentage of Mg 2 SiO 4 declines slightly under conditions of maximum garnet stability in the mantle transition zone, implying that Fig. 6 Secondary electron image at 15 kv of run product (MA-150) showing exsolved blebs of interstitial enstatite at the garnet grain boundaries. Note that the scales of this figure and those of Fig. 1 differ by a factor of small amounts of Mg 2 SiO 4 dissolve into garnet at P > 13 GPa. A second surprise was that exsolution of pyroxenes from supersilicic garnet occurs primarily at grain boundaries, suggesting that intracrystalline nucleation is difficult. Quantitative consideration of the scale of exsolution in nature and the laboratory makes this less surprising. In the natural material of van Roermund and Drury (1998), intracrystalline precipitation was only observed in grains of > 4 mm radius (Fig. 1a); smaller grains showed only grain boundary exsolution (Fig. 1b). Furthermore, the ratio of bleb size to lamella width was greater than a factor of 50. If these dimensions scale to our experiments, intracrystalline lamellae would be expected to be 20 nm across and present only in the cores of grains in excess of 4 5 lm diameter. Thus, had we made these calculations before examination of the experimental charges, we would have predicted correctly that we would find only grain boundary exsolution in our fine-grained run products. In future studies we will examine if intracrystalline precipitation is enhanced at lower temperatures where diffusion is slower or by deformation, which will introduce high-energy internal nucleation sites. Although the starting material for our experiments has a lherzolite composition and the rocks from Norway are garnet harzburgites, we are encouraged by the similarity of microstructures (compare Figs 1b and 6) despite a difference in scale of approximately 1000 and the amount of enstatite exsolution observed in the two cases. The amount of enstatite exsolved in MA-150, decompressed from 8 GPa, is slightly more than that in the images of van Roermund and Drury (1998), consistent with their estimate of 7 GPa as the starting depth of their protolith. Our results establish an experimental framework within which to discuss microstructures of mantle peridotites that may arise as a consequence of mineral reactions induced during upwelling. Such a framework is similar to the textural microstructural work of Mercier and Nicolas (1975), which provided for the first time a classification of deformation microstructures. Their classification allows ordering of xenoliths with respect to the late-stage Ó 2004 Blackwell Publishing Ltd 329

6 Precipitation of pyroxenes and Mg 2 SiO 4 from majoritic garnet L. F. Dobrzhinetskaya et al. Terra Nova, Vol 16, No. 6, deformation they frequently undergo shortly before or during the process by which they were incorporated into kimberlite magma. There have been subsequent modifications of the details of the seminal works of Mercier & Nicolas, but their principal contributions remain unchanged. Acknowledgements We thank Matt Weschler for assistance in FIB foil preparation and Frank Forgit for technical support of our experiments. We are grateful to Gaston Godar and two anonymous reviewers for their helpful comments on the manuscript. References Akaogi, M. and Akimoto, S., Pyroxene garnet solid solution equilibria in the system Mg 4 Si 4 O 12 Mg 3 Al 2 Si 3 O 12 and Fe 4 Si 4 O 12 Fe 3 Al 2 Si 3 O 12 at high pressures and temperatures. 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