Pyroxene Polymorphs in Melt Veins of the Heavily Shocked Sixiangkou L6 Chondrite

Eur. J. Mineral. 2006, 18, 719-726

Pyroxene polymorphs in melt veins of the heavily shocked Sixiangkou L6 chondrite

AICHENG ZHANG1,2,*, WEIBIAO HSU1, RUCHENG WANG 2,3 and MINGWEI DING1

1Laboratory for Astrochemistry and Planetary Sciences, Purple Mountain Observatory, Nanjing 210008, P.R. China 2State Key Laboratory for Mineral Deposits Research, Nanjing University, Nanjing 210093, P.R. China 3Department of Earth Sciences, Nanjing University, Nanjing 210093, P.R. China

Abstract: Sixiangkou is a heavily shocked L6 chondrite that contains numerous shock-induced melt veins with varying widths. This paper focuses on pyroxene polymorphs in shock-induced melt veins in the meteorite and discusses their implication to the high pressure and temperature conditions during impact metamorphism. The major high-pressure polymorphs are majorite and majorite-pyrope solid solution. Diopside, akimotoite and jadeite were also observed in melt veins of the Sixiangkou chondrite. Diopside coexists with ringwoodite (+ akimotoite) in coarse-grained fragments of the melt veins. Two mechanisms were proposed for the lack of phase transformation of diopside: a) low local temperature and b) a very sluggish transformation rate. Majorite and majorite-pyrope occur as coarse-grained polycrystalline and fine-grained solid solution. They were formed through solid state transformation and crystallized from a melt under pressures, respectively. Akimotoite coexists with low-Ca pyroxene in coarse- grained fragments of melt veins. They have nearly identical chemical compositions to the host low-Ca pyroxene, implying a solid state transformation mechanism. Jadeite appeared as fine-grained and was originated through retrograde transformation of lingunite under moderate post-shock pressure and high temperature conditions. The coexistence of different high-pressure pyroxene polymorphs reflects an inhomogeneous distribution of temperature in the shock-induced melt veins and a kinetic effect of phase transformations of minerals during shock metamorphism.

Key-words: Sixiangkou L6 chondrite, heavily shocked, melt veins, akimotoite, phase transformation.

1. Introduction al., 1997; Irifune et al., 2000; Asahara et al., 2005). In nature samples, majorite, akimotoite, and vitrified Pyroxene is one of the most important components in perovskite were observed in the Tenham L6 chondrite stony meteorites and the upper mantle of the Earth. During (Tomioka & Fujino, 1997, 1999; Xie et al., 2006), and asteroidal impact events, pyroxene would transform into majorite and vitrified perovskite were reported in the various high-pressure polymorphs (e.g., majorite, akimo- Suizhou L6 chondrite (Xie et al., 2001; Chen et al., 2004b). toite, perovskite) which have been observed in meteorites Akimotoite and vitrified MgSiO3 perovskite have also been (Smith & Mason, 1970; Sharp et al., 1997; Tomioka & reported in the Acfer 040 L5-6 chondrite. Recently, Fujino, 1997; Tomioka & Kimura, 2003). These high-pres- Tomioka & Kimura (2003) reported the existence of both sure phases potentially provide information about tempera- Ca-rich (~ 12 wt% CaO) and Ca-poor majorite in the ture and pressure conditions prevailing in the region of Yamato 751000 H6 chondrite. Ca-rich majorite was Earth’s mantle. Static high-pressure experiments (Liu, thought to be a breakdown product of diopside. These 1976; Presnall, 1995) demonstrated that low-Ca pyroxene chondrites belong to a high-degree stage (S5-6) of shock could transform to akimotoite (17.5-27.5 GPa and 600- classification. However, in another heavily shocked L6 2050°C), majorite (16-22.5 GPa and 1600-2500°C), and chondrite, the Sixiangkou chondrite, only garnet-structured perovskite (> 23 GPa and ~ 2000°C). Under high pressures, pyroxene was reported (Chen et al., 1996). Here, we revis- high-Ca pyroxene would decompose into assemblages of ited Sixiangkou with an electron microprobe and a micro- high-pressure phases, such as MgSiO3-garnet (majorite) + Raman spectroscopy and observed akimotoite, diopside CaSiO3-perovskite; MgSiO3-ilmenite (akimotoite) + and jadeite in shock-induced melt veins. This paper aims to CaSiO3-perovskite; MgSiO3-perovskite + CaSiO3- discuss their pressure and temperature stabilities and perovskite; and possible CaMgSi2O6-perovskite (Oguri et significance to shock metamorphism.

*E-mail: [email protected] 0935-1221/06/0018-0719 $ 3.60 DOI: 10.1127/0935-1221/2006/0018-0719 © 2006 E. Schweizerbart’sche Verlagsbuchhandlung. D-70176 Stuttgart 720 A. Zhang, W. Hsu, R. Wang, M. Ding

Fig. 2. A back-scattered electron image of a shock-induced melt vein containing a diopside + ringwoodite fragment. Ol: olivine; Ri: ringwoodite; Di: diopside; Pyx: pyroxene; Ak: akimotoite; Jd: Fig. 1. Optical (Reflectance) microscopy photograph of the jadeite; Chr: chromite; Mj: majorite; Mj-Py: majorite-pyrope; Msk: Sixiangkou chondrite thin section (PMO-1016). In this thin section, maskelynite; Lng: lingunite. two set veins with an angle of about 45° could be observed. The white part in the thin section is FeNi metal or troilite. 3. Results

3.1. Petrography of the Sixiangkou chondrite 2. Sample and analytical methods Petrographic textures and distribution of shock-induced The Sixiangkou meteorite fell on August 15, 1989, in melt veins are very similar in the three thin sections studied Sixiangkou Town of Taizhou city, Jiangsu Province of (Fig. 1). The shock-induced melt veins range in width from China. In total, 375 g sample was collected and the main several μm to 4 mm. These melt veins align in two direc- mass was preserved in the Purple Mountain Observatory, tions and cross with an angle of about 45°. Some melt Chinese Academy of Sciences. Three thin sections were pockets up to several hundred microns were also observed consecutively cut from a single fragment. These thin in the thin sections. The Sixiangkou chondrite was classi- sections were first observed with an optical microscope to fied as an L6 chondrite. The presence of maskelynite and locate shock-induced veins. Then they were coated with ringwoodite indicated that this chondrite was heavily carbon and were studied with a scanning electron shocked and was classified as shock stage S6 (Chen et al., microscopy (JSM-5610LV at Nanjing Normal University 1996, 2002). and Hitachi 3400N at Purple Mountain Observatory) and The host portion of the chondrite is mainly composed of an electron microprobe (JEOL JXA-8800M at Nanjing olivine, low-Ca pyroxene, diopside, plagioclase University). (completely transformed to maskelynite), FeNi metal and Mineral chemistry was determined with the JEOL JXA- troilite. Minor phases include chromite, whitlockite, and 8800M electron microprobe. Analyses were performed at a chlorapatite (Chen et al., 1995, 2002). Some relict chon- 15 kV acceleration voltage with a beam current of 20 nA, drules with barred olivine and maskelynite could be recog- except that 10 nA was used for maskelynite and jadeite. nized. Olivine, low-Ca pyroxene and diopside are euhedral The electron beam was focused to an area of less than 1 μm and display undulose extinction. Low-Ca pyroxene has low for most minerals, and a beam diameter of 10 μm was used contents of Al2O3 (< 0.05 wt%) and CaO (less than 1 wt%) for maskelynite. Natural and synthetic standards are used with a molar Fe/(Fe+Mg) ratio of 0.23, similar to that for quantitative analysis. All the data were reduced with the (0.22) of Chen et al. (1996). Diopside has a slightly high ZAF routine. The detection limits (ppm) of some minor Al2O3 content (~ 0.50 wt%) and a Fe/(Fe+Mg) ratio of elements are: Na (350), K (130), Al (135), Mn (190), Ti 0.17. Most maskelynite grains have typical flow textures (250), Cr (170), and P (100). and fit interstitially between olivine and pyroxene. Olivine Raman spectra of minerals were taken with a Renishaw and pyroxene grains surrounding maskelynite show radi- RM2000 micro-Raman spectrometer equipped with a CCD ating cracks outward from maskelynite (the right side of detector at Nanjing University. The operating conditions Fig. 2). were: excitation laser wavelength: 514 nm (Ar+ laser), laser energy: 5 mW, and spectral slit: 25 μm. The 50 × objective 3.2. Mineralogy of the shock-induced melt veins was used on a Leica DM/LM microscope. With this objective, the lateral spot-size of the laser beam was about 1 μm. The shock-induced melt veins contain two distinct Silicon (520 cm–1 Raman shift) was used as a standard. lithologies: fine-grained matrix and coarse-grained frag- Pyroxene polymorphs in the Sixiangkou L6 chondrite 721 ments. Some thick melt veins show round to dendritic occurrences of metal-troilite assemblages in the fine- grained matrix from the edge to the center (Fig. 3), similar to previous observations (e.g., Fig. 1 of Chen et al., 1996). The matrix of melt veins is composed of fine-grained majorite-pyrope (1-5 μm) and eutectic metal-troilite (from < 1 μm to 30 μm). Majorite-pyrope has two Raman bands -1 at 656 and 927 cm . It has higher contents of Al2O3 (4.10 wt%) and CaO (1.92 wt%) than low-Ca pyroxene in the host portion of Sixiangkou (Table 1). Magnesiowüstite (up to 5 μm) was reported as an important component in the fine-grained matrix (Chen et al., 1996). Some jadeite and lingunite grains look darker than the main components (majorite-pyrope) of the fine-grained matrix on backscattered electron images (Fig. 2 and 4). Jadeite has typical Raman bands at 1039, 695, 574, 524, and 375 cm-1, -1 whereas lingunite at 977, 771, 626, 277, and 213 cm Fig. 3. A backscattered electron image of a shock-induced melt vein (Fig. 5). Both Raman spectra of jadeite and lingunite in the Sixiangkou chondrite. The melt vein was crosscut by a thin contain bands of other high-pressure phases, e.g., majorite metal-troilite vein and occurrence of metal-troilite assemblages or ringwoodite. varies from the edge (rounded) to the center (dendritic) of the Coarse-grained fragments in the shock-induced melt shock-induced melt vein. veins are commonly ovoid in shape and vary in size from several μm to 150 μm. They are mainly composed of olivine, low-Ca pyroxene, ringwoodite, majorite, diopside, chlorapatite, and chromite. Within the shock-induced melt contain closely contacted low-pressure and high-pressure veins, we also found some polymineralic fragments that phases. Here we report three typical coarse-grained

Table 1. EMPA analyses of minerals (wt %) in the Sixiangkou chondrite. Ol Pyx Di-h Msk Ri Mj Mj-Py Di-v Ak Aka Akb Jd

Na2O––00.11 08.56 – 00.04 00.26 000.12 00.11 000.67 00.42 1.63 TiO2 – 00.04 00.23 – – 00.11 00.10 000.24 00.09 000.17 – – MnO 000.24 00.28 00.15 – 00.20 00.26 00.19 000.13 00.52 000.28 – – MgO 037.94 28.43 16.01 00.01 37.58 28.48 26.47 016.28 27.17 028.39 035.34 0.11 K2O–––01.07 – – 00.04 000.02 – – – 1.08 FeO 024.13 13.85 05.97 00.17 24.58 15.59 12.24 0005.891 03.391 003.54 003.72 0.68 Al2O3 – 00.13 00.49 20.91 00.03 00.13 04.10 000.50 00.04 000.07 004.442 1.67 CaO 000.07 00.89 23.05 02.15 – 00.85 01.922 003.11 00.61 000.38 – 2.13 Cr2O3 – 00.08 00.32 – 00.03 00.03 0.15 000.35 00.08 000.16 000.50 0.07 SiO2 037.29 55.24 53.65 67.00 37.61 54.17 52.96 053.89 56.90 056.35 055.54 69.04 P2O5 000.60 – – 00.02 – – 00.05 000.02 – – – 0.10 Total 100.30 98.95 99.98 99.90 100.06 99.66 98.48 100.55 98.91 100.00 100.00 96.51 Number of cations per formula unit Na – – 0.008 0.727 – 0.003 0.018 0.009 0.016 0.09 0.06 0.139 Ti – 0.001 0.006 – – 0.003 0.003 0.007 0.004 0.01 – – Mn 0.005 0.009 0.005 – 0.004 0.008 0.006 0.004 0.032 0.02 – – Mg 1.483 1.539 0.885 0.001 1.480 1.547 1.435 0.894 2.924 3.02 3.60 0.007 K – – – 0.060 – – 0.002 0.001 – – – 0.061 Fe 0.524 0.417 0.183 0.006 0.538 0.471 0.369 0.180 0.800 0.81 0.21 0.025 Al – 0.006 0.021 1.079 0.001 0.006 0.174 0.022 0.004 0.01 0.36 1.121 Ca 0.002 0.034 0.910 0.101 – 0.033 0.074 0.907 0.048 0.03 – 0.100 Cr – 0.002 0.009 – 0.001 0.001 0.004 0.010 0.004 0.01 0.03 0.002 Si 0.971 1.994 1.977 2.939 0.987 1.962 1.914 1.974 4.084 4.02 3.79 3.036 P 0.017 – – 0.001 – – 0.002 0.001 – – – 0.004 Fe# 0.260 0.210 0.170 0.100 0.270 0.230 0.200 0.170 0.210 0.21 0.06 0.23 Oxygens 4000 6000 6000 8000 4000 6000 6000 6000 120000 1200 1200 8 Fe#=Fe/(Fe+Mg) in atom. Ol: olivine; Pyx: low-Ca pyroxene; Di: diopside; h: in the host portion of the chondrite; Ri: ringwoodite; Msk: maskelynite; Mj: majorite; Py: pyrope; v: in melt veins; Ak: akimotoite; Jd: jadeite. a and b are energy-dispersive spectroscopy data from Tomioka & Fujino (1999) and Sharp et al. (1997), respectively. –: below detection limit. 722 A. Zhang, W. Hsu, R. Wang, M. Ding

Fig. 4. A back-scattered electron image of a shock-induced melt Fig. 6. Optical (Reflectance) microscopic photograph of the frag- vein containing a polymineralic fragment. The fragment is ment akimotoite + low-Ca pyroxene. Ak: akimotoite; Pyx: composed of diopside, pyroxene, akimotoite, and ringwoodite, pyroxene. which are in triple-junction contact. Ol: olivine; Ri: ringwoodite; Di: diopside; Pyx: pyroxene; Ak: akimotoite; Jd: jadeite; Mj: majorite; Msk: maskelynite; Lng: lingunite.

Fig. 5. Raman spectra of some minerals in shocked Sixiangkou chondrite. Pyx: pyroxene; Mj: majorite; Ri: ringwoodite. Pyroxene polymorphs in the Sixiangkou L6 chondrite 723 polymineralic fragments found in melt veins of bands at 797, 617, and 479 cm-1 (Fig. 5), which is in good Sixiangkou. agreement with the result of Ohtani et al. (2004). A broad One thin lensoid fragment (Fig. 2) of diopside and ring- band was observed in the range of 820-1200 cm-1, which woodite was observed at the edge of a shock-induced melt indicates the presence of amorphous substance. vein. The melt vein is about 50 μm wide, and was mainly composed of fine-grained majorite-pyrope, re-crystallized FeNi metal and troilite, and possible magnesiowüstite. A 4. Discussion subhedral chromite grain (~ 7 μm) was also observed. The diopside + ringwoodite fragment was arbitrary defined as a 4.1. Stability of diopside during shock metamorphism polymineralic fragment due to the existence of high-pressure phase. The fragment is about 20 μm long and 5 μm The pressure and temperature of the shock-induced wide. In the fragment, diopside has the same chemical melt veins of the Sixiangkou chondrite could be composition as that in the host portion. It has sharp Raman constrained by high-pressure mineral assemblages (Chen et bands at 1013, 666, 394 and 323 cm-1 (Chopelas & al., 1996, 2002). By comparing the assemblage of majorite- Serghiou, 2002), indicating that no phase transformation pyrope + magnesiowüstite with static high-pressure exper- occurred in diopside. Ringwoodite has a similar composi- imental data of Zhang & Herzberg (1994) and Agee et al. tion to olivine in the host portion. Ringwoodite exhibits two (1995), Chen et al. (1996) concluded that the shock- Raman bands at 799 and 845 cm-1, and olivine at 821 and induced melt veins should have crystallized at pressures of 852 cm-1. 20-24 GPa and temperatures of above 2000°C. Based on A fragment of diopside + ringwoodite + the occurrence of metal-troilite-magnetite assemblage in akimotoite/low-Ca pyroxene was found at the center of a the shock-induced melt veins and ringwoodite lamellae in shock-induced melt vein (Fig. 4). The vein varies in width olivine, a long duration of high pressure up to several from 55 to 115 μm. The matrix of the melt vein is minutes was suggested (Chen et al., 2002, 2004a, and composed of fine-grained majorite-pyrope, FeNi metal and 2006). sulfide, and possible magnesiowüstite (Chen et al., 1996). Diopside has not been observed from melt veins and the Mineral fragments of ringwoodite and majorite (10 to host portion of the Sixiangkou chondrite in previous inves- 35 μm in size) also occur in this melt vein. The diopside + tigations (e.g., Chen et al., 1996), but was reported in other ringwoodite + akimotoite/low-Ca pyroxene fragment chondritic meteorites, such as the Suizhou L6 chondrite appears ovoid in shape, and is about 50 μm long and 20 μm (Chen et al., 2004b), the Yamato 791384 L6 chondrite wide. Diopside, ringwoodite, akimotoite/low-Ca pyroxene (Ohtani et al., 2004), and the Yamato 751000 H6 chondrite are in triple-junction contact. Akimotoite coexists with (Tomioka & Kimura, 2003). In this study, diopside was low-Ca pyroxene. Diopside has similar Raman spectra and observed not only in some fragments of melt veins but also chemical composition (En45Fs9Wo 46) to that in the host in the host portion of the Sixiangkou chondrite. It indicates portion. Ringwoodite has a composition of Fo75Fa25, that diopside was a relict mineral during the shock meta- similar to olivine in the host portion. Ringwoodite is char- morphism of the chondrite rather than crystallized from the acterized with two sharp Raman bands at 795 and 843 cm-1. shock-induced melt veins described in other heavily Akimotoite is identified with two typical Raman bands at shocked chondrites, e.g., Umbarger L6 chondrite (Xie & 798 and 476 cm-1 that could not be assigned to other high- Sharp, 2004). pressure polymorphs of low-Ca pyroxene. Akimotoite In the shock-induced melt veins of the Sixiangkou (En78Fs21Wo 1) and coexisting low-Ca pyroxene chondrite, diopside was closely associated with ring- (En77Fs21Wo 1) have the same chemical composition, iden- woodite (+ akimotoite) within the same fragments. The tical to the result of Chen et al. (1996). The chemical occurrence of diopside in the veins suggests that either the composition of akimotoite is similar to akimotoite reported local pressure-temperature conditions did not exceed the in Tenham (En78Fs21Wo 1: Tomioka & Fujino, 1997) and stability field of diopside or the rate of phase transforma- Yamato 791384 (En78Fs21Wo 1: Ohtani et al., 2004), but tion of diopside is very low and the duration is too short to differs significantly from those in Acfer 040 (En94Fs6: induce the transformation. Sharp et al., 1997), Umbarger (En88Fs11Wo 1: Xie & Sharp, High-pressure experiments have been performed to 2004), and Tanhem (En88Fs12: Xie et al., 2006). understand the stability and phase transition of diopside, A fragment of akimotoite and low-Ca pyroxene was and most of them were carried out at high temperatures also observed in a shock-induced melt vein (Fig. 6). This (> 1000°C) (e.g., Canil, 1994; Gasparik, 1996 and refer- melt vein is about 50 μm in width. The matrix is also ences therein; Oguri et al., 1997; Irifune et al., 2000; composed of fine-grained majorite-pyrope, dendritic or Asahara et al., 2005). These experimental data show that round metal-troilite, and possible magnesiowüstite. No diopside would become unstable at pressures higher than large mineral fragments were observed in this melt vein. 18-19 GPa and temperatures > 1000°C, and would disso- The fragment is lensoid and about 15 × 25 μm in size. ciate into high-pressure phases of CaSiO3 and MgSiO3 (or Under reflectance optical microscope, akimotoite and low- Mg2SiO4 + SiO2) (Irifune et al., 1989; Canil, 1994; Oguri Ca pyroxene exhibit different reflectance (Fig. 6). et al., 1997). Recently, Tomioka & Kimura (2003) reported Akimotoite is mainly set at the center of the fragment. in a shocked H chondrite the breakdown of diopside to Ca- Akimotoite and low-Ca pyroxene have very similar chem- rich majorite and glass, and inferred a breakdown pressure ical compositions. Akimotoite was characterized with three about 18-24 GPa and temperature at 1100-1900°C. 724 A. Zhang, W. Hsu, R. Wang, M. Ding

Chopelas & Serghiou (2002) measured Raman spectra of 4.3. Transformation from pyroxene to akimotoite diopside with increasing pressures at room temperature, and found that the Raman spectrum of diopside at 20.2 GPa Akimotoite has been previously found in a few of L6 was nearly identical to that at 1 atm, suggesting no phase shocked chondrites (Acfer 040, Umbarger, Yamato transformation. These investigations suggest that tempera- 791384, Tenham) (Sharp et al., 1997; Tomioka & Fujino, ture is an important factor affecting the phase transition of 1997, 1999; Ohtani et al., 2004; Xie & Sharp, 2004; Xie diopside. At a given high pressure, diopside is more stable et al., 2006). Two models have been proposed for its at low temperature than high temperature. The presence of formation: solid-state transformation (Tomioka & Fujino, ringwoodite rather than wadsleyite in the coarse-grained 1997, 1999) and crystallization from a high-density melt fragments would suggest a pressure greater than 20 GPa under high pressures (Sharp et al., 1997; Xie & Sharp, (Chen et al., 1996). At such a high pressure, a low temper- 2004; Xie et al., 2006). Tomioka & Fujino (1997, 1999) ature could be inferred in order to preserve diopside during observed chemical similarities among clinoenstatite, the heavily shock metamorphism. akimotoite, and perovskite in Tenham and suggested their The above explanation did not exclude the kinetic effect formation by solid-state transformation at high pressures. of phase transformation of diopside during shock meta- Our results in Sixiangkou are consistent with those of morphism. At a given high pressure and temperature, the Tomioka & Fujino (1997, 1999). On the other hand, phase transformation of diopside is controlled by the trans- akimotoite grains in Acfer 040 and Umbarger have formation rate of diopside and the duration of high pressure different chemical compositions from their respective and high temperature. Recent observations showed that the host pyroxenes. In particular, akimotoite in Acfer 040 and duration of high pressure could be up to several seconds, Umbarger is rich in Al2O3, indicating crystallization from even several minutes in shock-induced melt veins (Chen et the shock-induced melt (Sharp et al., 1997; Xie & Sharp, al., 2002, 2004a; Ohtani et al., 2004; Chen et al., 2006; Xie 2004; Xie et al., 2006). et al., 2006). A long duration of high pressure up to several Akimotoite in the Sixiangkou chondrite has a similar seconds was also suggested in the Sixiangkou chondrite mineral chemistry as that in Tenham (Tomioka & Fujino, (Chen et al., 2002, 2004a, and 2006). With such a long 1997, 1999) and Y791384 (Ohtani et al., 2004), but differs duration many minerals would have been partly or significantly in chemical composition from that in Acfer completely transformed into their high-pressure poly- 040 (Sharp et al., 1997) and in Umbarger (Xie & Sharp, morphs. The survival of diopside suggests that phase trans- 2004). This discrepancy may indicate that akimotoite has a formation rate of diopside would be very sluggish. A rather large compositional stability field. However, up to possible interpretation is that the crystal structure of diop- date, no compositional effect on stability of akimotoite has side contains the large radium cation Ca2+ (0.99 Å), which been known, and it remains an open question for further is more difficult to modify its coordination under high investigation on the effects of Fe/(Fe+Mg) ratio, Fe3+, and pressures than Mg2+ (0.66 Å) and Fe2+ (0.74 Å). However, Al3+ on the high-pressure stability of akimotoite. In addi- the kinetics of phase transformation of diopside remains tion, the fact that low-Ca pyroxene was partly transformed unclear. into akimotoite may suggest that the phase transformation of low-Ca pyroxene to akimotoite was more sluggish than 4.2. Transformation from pyroxene to majorite that of olivine to ringwoodite (Hogrefe et al., 1994; Sharp & DeCarli, 2006 and references therein). As reported in previous investigations (e.g., Chen et al., 1996), majorite is the main high-pressure phase with 4.4. Formation of jadeite in the shock-induced melt veins a pyroxene composition in the shock-induced melt veins of the Sixiangkou chondrite. Two mechanisms have been In this study, jadeite was only observed in the shock- proposed to explain the formation of coarse-grained poly- induced melt veins, similar to those described in other crystalline majorite and fine-grained majorite-pyrope shocked chondrites (Kimura et al., 2000, 2001; Tomioka & (Chen et al., 1996; Ohtani et al., 2004; Sharp & DeCarli, Kimura, 2003; Ohtani et al., 2004). In these chondrites, 2006). The coarse-grained polycrystalline majorite was jadeite had chemical compositions similar to plagioclase believed to have formed through solid state transforma- (Ohtani et al., 2004), suggesting that jadeite has originated tion from low-Ca pyroxene. The main reason was that the from plagioclase or its high-pressure polymorphs. Liu coarse-grained polycrystalline majorite grains lack an (1978) reported that with increasing pressures albite would idiomorphic polycrystalline appearance and have the first transform to jadeite and SiO2 (quartz, coesite, and same chemical composition as low-Ca pyroxene in the stishovite) and then to hollandite-structured. A possibility host portion (Chen et al., 1996, 2004b; Ohtani et al., has been proposed by Kimura et al. (2000) that jadeite 2004). Sharp & DeCarli (2006) suggested that this solid might have formed through retrograde transformation of state transformation mechanism was homogenous lingunite under moderate post-shock pressure and high intracrystalline nucleation followed by interface- temperature conditions. This explanation may also be suit- controlled growth. The fine-grained majorite-pyrope was able to the jadeite in this study. The undetected SiO2 may thought to have crystallized from melts under pressure be due to the overlap of the Raman bands at ~ 520 cm-1 because the solid solution contained higher contents of Ca with jadeite. More detailed studies are needed to determine and Al than low-Ca pyroxene in the host portion (Chen et the origin and formation mechanism of jadeite in the al., 1996). future. Pyroxene polymorphs in the Sixiangkou L6 chondrite 725

4.5. Implications for shock metamorphism into ringwoodite and majorite. The incomplete transforma- The coexisting of a variety of pyroxene phases in shock- tion of low-Ca pyroxene to akimotoite observed in some induced melt veins has an important implication for the P- fragments of melt veins must reflect a low transformation T history of melt veins and phase transformation processes rate. This conclusion is consistent with previous observa- of minerals. Chen et al. (1996) reported that the pressure tions that the transformation of enstatite to akimotoite is and temperature in the shock-induced melt veins of the more sluggish than that of olivine to ringwoodite (Sharp & Sixiangkou meteorite could be up to 20-24 GPa and DeCarli, 2006). ~ 2000°C, respectively. This pressure and temperature were suitable for the formation of majorite in melt veins, and the Acknowledgements: We thank Prof. Christian Koeberl and high temperature is too high for the preservation of akimo- an anonymous reviewer for critical and helpful reviews, toite. Akimotoite and majorite have a similar pressure and Prof. Ekkehart Tillmanns for editorial handling. This stability range (Presnell, 1995). If the temperature of the work was supported by the National Natural Science whole melt veins was up to 2000°C, no akimotoite was Foundation of China for Distinguished Young Scholars expected to be observed. The presence of akimotoite in (Grant No. 40325009), the Minor Planet Foundation of melt veins must reflect a local low temperature that has China, the Post-doctor Foundation of Jiangsu Province induced the transformation of low-Ca pyroxene to akimo- (Grant No. 0502014C) and the Open Foundation of State toite and could not induce the transformation of akimotoite Key Laboratory for Mineral Deposits Research, Nanjing to majorite. Chen et al. (2004b) explained that the inhomo- University (Grant No. 12-06-14). geneity of temperature in shock-induced melt veins was related to the widths of melt veins. Relatively large amounts of shock-induced melt were produced in thick References shock veins (up to several mm in widths), and they must contain larger amounts of heat and could heat all material Agee, C.B., Li, J., Shannon, M.C., Circone, S. (1995): Pressure- inside the veins to a homogeneous high-temperature (Chen temperature phase diagram for the Allende meteorite. J. et al., 2004b). However, thin shock veins produced less Geophys. Res., 100, 17725-17740. melt and the relatively small amounts of heat could not heat Asahara, Y., Ohtani, E., Kondo, T., Kubo, T., Miyajima, N., all materials to a homogeneous high-temperature in a short Nagase, T., Fujino, K., Yagi, T., Kikegawa, T. (2005): time (Chen et al., 2004b). In this study, our observations Formation of metastable cubic-perovskite in high-pressure are generally consistent with this interpretation by Chen et phase transformation of Ca(Mg,Fe,Al)Si2O6. Am. Mineral., 90, 457-462. al. (2004b) with akimotoite only occurring in some thin μ Canil, D. (1994): Stability of clinopyroxene at pressure-temperature melt veins (not thicker than 150 m). The fact that diopside conditions of the transition region. Phys. Earth Planet. Inter., remained intact and coexists with ringwoodite (+akimo- 86, 25-34. toite) might be another evidence of local low temperature Chen, M., Wopenka, B., Xie, X.D., El Goresy, A. (1995): A new in melt veins. The low temperature failed to overcome the high-pressure polymorph of chlorapatite in the Shocked large activation energies of reconstructing the crystal struc- Sixiangkou (L6) Chondrite. Lunar Planet. Sci., 26, 237-238. ture of diopside (DeCarli et al., 2002; Sharp & DeCarli, Chen, M., Sharp, T.G., El Goresy, A., Wopenka, B., Xie, X.D. 2006). (1996): The majorite-pyrope + magnesiowüstite assemblage: Phase transition of minerals during shock metamor- Constraints on the history of shock veins in chondrites. Science, phism is a rather complex kinetic process, and different 271, 1570-1573. from static high-pressure experiments. For instance, in Chen, M., Xie, X.D., Wang, D.D., Wang, S.C. (2002): Metal- static high-pressure experiments, at a high pressure of 18- troilite-magnetite assemblage in shock veins of Sixiangkou 20 GPa and a temperature of above 900°C, olivine would meteorite. Geochim. Cosmochim. Acta, 66, 3143-3149. transform into ringwoodite on an observable timescale Chen, M., El Goresy, A., Gillet, P. (2004a): Ringwoodite lamellae (Sharp & DeCarli, 2006 and references therein). However, in olivine: Clues to olivine-ringwoodite phase transition mech- ringwoodite has never been recovered in shock recovery anisms in shocked meteorites and subducting slabs. Proc. Natl. experiments (Stöffler et al., 1991). Thus, it was suggested Acad. Sci. USA, 101, 15033-15037. Chen, M., Xie, X.D., El Goresy, A. (2004b): A shock-produced that phase transition of minerals during shock metamor- (Mg,Fe)SiO3 glass in the Suizhou meteorite. Meteorit. Planet. phism is both temperature and time dependent (Sharp & Sci., 39, 1797-1808. DeCarli, 2006). As discussed above, the inhomogeneous Chen, M., Li, H., El Goresy, A., Liu, J., Xie, X.D. (2006): Fracture- temperature in the shock-induced melt veins has lead to the related intracrystalline transformation of olivine to ringwoodite phase transition of some minerals (olivine to ringwoodite, in the shocked Sixiangkou meteorite. Meteorit. Planet. Sci., 41, low-Ca pyroxene to akimotoite and majorite) and the 731-737. preservation of some minerals (e.g., diopside). Chen et al. Chopelas, A. & Serghiou, G. (2002): Spectroscopic evidence for (2002, 2004a, and 2006) have concluded that the duration pressure-induced phase transitions in diopside. Phys. Chem. of high-pressures in melt veins was up to several seconds Minerals, 29, 403-408. and even several minutes. This duration is long enough for DeCarli, P., Bowden, E., Jones, A.P., Price, G.D. 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