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Natural Dissociation of Olivine to (Mg,Fe)Sio3 Perovskite And

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Natural Dissociation of Olivine to (Mg,Fe)Sio3 Perovskite And Natural dissociation of olivine to ðMg,FeÞSiO3 perovskite and magnesiowüstite in a shocked Martian meteorite Masaaki Miyaharaa,1, Eiji Ohtania, Shin Ozawaa, Makoto Kimurab, Ahmed El Goresyc, Takeshi Sakaid, Toshiro Nagasee, Kenji Hiragaf, Naohisa Hiraog, and Yasuo Ohishig aInstitute of Mineralogy, Petrology and Economic Geology, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan; bFaculty of Science, Ibaraki University, Mito, 310-8512, Japan; cBayerisches Geoinstitut, Universität Bayreuth, D-95440, Bayreuth, Germany; dInternational Advanced Research and Education Organization, Tohoku University, Sendai 980-8578, Japan; eCenter for Academic Resources and Archives, Tohoku University, Sendai 980-8578, Japan; fInstitute for Materials Research, Tohoku University, Sendai 980-8577, Japan; and gJapan Synchrotron Radiation Research Institute, 1-1-1 Kouto Sayo, Hyogo 679-5198, Japan Edited by Henry J. Melosh, University of Arizona, Tucson, AZ, and approved February 8, 2011 (received for review November 10, 2010) We report evidence for the natural dissociation of olivine in a sher- induced by the dynamic events (2–7). The high-pressure and gottite at high-pressure and high-temperature conditions induced high-temperature conditions of the dynamic event recorded in by a dynamic event on Mars. Olivine (Fa34-41) adjacent to or en- Martian meteorites have been estimated by several previous trained in the shock melt vein and melt pockets of Martian meteor- works (8–15), and the estimated conditions given in some reports ite olivine-phyric shergottite Dar al Gani 735 dissociated into are considerably beyond the stability filed of ringwoodite (8, 11). þ (Mg,Fe)SiO3 perovskite (Pv) magnesiowüstite (Mw), whereby per- It is likely that olivine dissociated to ðMg;FeÞSiO3 Pv þ Mw but ovskite partially vitrified during decompression. Transmission elec- was overlooked. Accordingly, we carefully scanned olivine grains tron microscopy observations reveal that microtexture of olivine in the Martian meteorite Dar al Gani (DaG) 735 with a field dissociation products evolves from lamellar to equigranular with emission (FE) SEM and laser micro-Raman spectroscopy. Here increasing temperature at the same pressure condition. This is in we report the natural evidence for the dissociation of olivine accord with the observations of synthetic samples recovered from at high-pressure and high-temperature conditions induced by a high-pressure and high-temperature experiments. Equigranular dynamic event on Mars. – (Mg,Fe)SiO3 Pv and Mw have 50 100 nm in diameter, and lamellar (Mg,Fe)SiO3 Pv and Mw have approximately 20 and approximately Results and Discussion 10 nm in thickness, respectively. Partitioning coefficient, KPv∕Mw¼ DaG 735 is an olivine-phyric shergottite and is suggested to be ½ ∕ ∕½ ∕ FeO MgO FeO MgO Mw, between (Mg,Fe)SiO3 Pv and Mw in paired with DaG 476, 489, 670, 876, and 975 (e.g., 16, 17). These equigranular and lamellar textures are approximately 0.15 and shergottites have both basaltic and lherzolitic features. Reported approximately 0.78, respectively. The dissociation of olivine implies 147Sm-143Nd and 39Ar∕40Ar ages estimated in samples in DaG that the pressure and temperature conditions recorded in the 476 reveal ca 474 Ma as “igneous crystallization ages” (18, 19). shock melt vein and melt pockets during the dynamic event were An ejection age from Mars obtained from DaG 489 using noble approximately 25 GPa but 700 °C at least. gases reveals ca 1.08-Ma exposure age (20). The DaG 735 section studied here depicts mainly olivine phenocrysts with a dimension livine is one of the major constituent minerals of terrestrial of 1–2 mm in fine-grained matrix. A previous study (21) reported Oplanets. High-pressure and high-temperature experiments that there are two types of olivine phenocrysts in DaG 735, i.e., (i) indicate that olivine converts in solid state to its high-pressure chemically zoned and (ii) homogenous olivine. Most olivine phe- polymorphs, ringwoodite subsequent to wadsleyite, with increas- nocrysts in the present section studied are homogeneous, ing pressure and temperature and finally dissociates at higher although there are a few differences in their chemical composi- ð ; Þ ð Þþ pressures to Mg Fe SiO3 perovskite Pv magnesiowüstite tions from grain to grain (Fa34 41). The shergottite matrix consists ð Þ - Mw . It is expected that the dissociation takes place around and mainly of pigeonite, augite, orthopyroxene, plagioclase glass below the transition zone of the Earth (1). Seismic observations (maskelynite), merrillite, and chromite. Most pyroxene in the suggest that ðMg;FeÞSiO3 Pv and Mw are likely the dominant matrix is pigeonitic (En59-81Fs17-33Wo1-11) and has a composi- constituent minerals in the lower mantle. Accordingly, its disso- tional zoning as previously reported (16, 17). Raman spectra ciation mechanism is important to understand the dynamics of GEOLOGY ’ indicate that merrillite is partly replaced by its high-pressure the Earth s interior because it affects the physical and chemical polymorph, tuite. Maskelynite depicts a smooth surface and was properties such as densities and elastic velocities of mantle quenched to glass at high pressure from the shock-induced albitic materials. Since the experimental discovery of the dissociation melt induced by the impact event (13). There is a 50- to 500-μm- reaction, many scholars have been searching for evidence for thick shock melt vein in the present DaG 735 section. The shock the decomposition reactions in natural samples such as inclusions melt vein mainly contains mineral grains with a dimension less in diamonds from kimberlites. However, the dissociation of than approximately 100 nm. Although we could not identify olivine at high-pressure and high-temperature conditions has never been reported from any natural samples so far. Therefore, the nature of each grain even by a FE-SEM, several Raman spec- the dissociation mechanism of olivine in natural samples is still tra corresponding to pyroxene glass and clinopyroxene allowed unclear. the identification of their nature. Planetesimal collision phenomena are recorded in many chon- dritic, lunar, and Martian meteorites as shock melt veins or melt Author contributions: M.M. and E.O. designed research; M.M., S.O., M.K., A.E.G., T.S., T.N., pockets. Olivine in and around the shock melt veins or melt pock- K.H., N.H., and Y.O. performed research; M.M. and E.O. analyzed data; and M.M., E.O., ets was converted by the dynamic events in solid-state reactions to M.K., and A.E.G. wrote the paper. its high-pressure polymorphs or its shock-induced monomineralic The authors declare no conflict of interest. melt fractionally crystallized to Mg-rich wadsleyite and Fe-rich This article is a PNAS Direct Submission. ringwoodite at high-pressure and high-temperature conditions 1To whom correspondence should be addressed. E-mail: [email protected]. www.pnas.org/cgi/doi/10.1073/pnas.1016921108 PNAS ∣ April 12, 2011 ∣ vol. 108 ∣ no. 15 ∣ 5999–6003 Downloaded by guest on October 1, 2021 The dissociation texture of olivine was observed in olivine terial obtained using a imaging procedure application in Fig. 2A grains adjacent to the shock melt vein and olivine fragments is 30∶70, which is similar to a molar fraction ratio between Mw entrained in the shock melt vein (Fig. 1). The dissociation tex- and ðMg;FeÞSiO3 Pv as a dissociation product of olivine (23). In tures can be divided into two settings: (i) fine-grained granular rare cases, we succeeded to record weak diffraction patterns from (less than approximately 100 nm) part and (ii) coarse-grained the poorly crystallized materials. These reflections disappeared (less than approximately 1.0 μm) granular part. Fine-grained within approximately 30 s after electron beam irradiation. None- granular textures exist always adjacent to the shock melt vein theless, we could record some diffraction patterns, which are (Fig. 1B). Only fine-grained granular texture was observed in the similar to those of perovskite (Fig. 2B). We could calculate olivine fragments entrained in the shock melt vein. The coarse- from the diffraction patterns only the lattice parameter of the b grained granular texture increases in dominance with increasing a axis [4.95(5) Å], which appears close to the b-axis value of ð ; Þ distance from the shock melt vein to the interior of the olivine Mg0.75 Fe0.25 SiO3 Pv [4.9330(6) Å] (24). We anticipate that the grains adjacent to the shock melt vein (Fig. 1). Raman spectra poorly crystallized material was ðMg;FeÞSiO3 Pv prior to major corresponding to pyroxene glass (approximately 665 cm−1) amorphization based on the volume ratio, compositions, and (and minor olivine) were obtained from both dissociation set- electron diffraction patterns. ðMg;FeÞSiO3 Pv is notoriously un- tings. The bulk chemical compositions of these dissociation stable at low-pressure condition. For example, when synthetic constituents (Fa38-40) obtained by broad-beam electron microp- ðMg;FeÞSiO3 Pv was heated 150 °C at ambient pressure, it instan- robe analyzer (EMPA) are identical to those of original olivine taneously vitrified (25). It is expected that ðMg;FeÞSiO3 Pv (Fa38-42) (Fig. 1). Several melt pockets also occur in DaG 735. formed in equilibrium pressure condition during a dynamic event Pigeonite and augite around the melt pockets were converted and was probably in part vitrified during the subsequent decom- to pyroxene glass. The dissociation textures of olivine were also pression stage. Equigranular texture of Mw and vitrified encountered around the melt pockets. ðMg;FeÞSiO3 Pv is similar to the dissociation texture experimen- We focused our investigations on two olivine grains adjacent tally obtained from olivine at high pressure and high temperature to the shock melt vein or around the melt pocket. Block pieces recovered in diamond anvil cell experiments (26–30). The chemi- of the decomposed olivine (approximately 20 × 7 × 10 μm) were cal compositions obtained by STEM-EDS indicate that iron is extracted by a focused ion beam (FIB) system and scanned with a more enriched in Mw than in ðMg;FeÞSiO3 Pv (Table 1).
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