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). synchrotron X-ray beam. Only Mw (Fe# ¼ ∼64) was identified The coarse-grained granular parts include fragments (dimen- from the X-ray diffraction (XRD) patterns. XRD patterns corre- sion less than approximately 0.5 μm) having lamellar intergrowth sponding to ðMg;FeÞSiO3 Pv could not be distinguished because (Fig. 2 B–D). Many vitrified ðMg;FeÞSiO3 Pv and Mw grains most of them are superimposed with XRD reflections of the exist in the interstices of the fragments although their dimensions original olivine. Slices of the olivine dissociation assemblage were (10–20 nm) are much smaller than those described above prepared by an FIB system and studied by both a transmission (Fig. 2A). In addition, most Mw grains have cylindrical or sphe- electron microscope (TEM) and scanning transmission electron rical shapes. The bulk compositions of the fragments are similar microscope (STEM) equipped with energy dispersive spectro- to those of original olivine. Most lamellae are not straight but meters (EDS) (Fig. 1). Bright-field (BF) TEM images show that curved, jagged, crossing, and terminated. The lamellae become the fine-grained granular parts consist of equigranular Mw and loose around the edges of the fragments. Two different layers poorly crystallized (or amorphous) material (Fig. 2A). Both have are alternatively stacked in the lamella. We obtained only semi- dimensions of 50–100 nm. The chemical composition of the quantitative compositions of each layer because they were too poorly crystallized material is identical to ðMg;FeÞSiO3. Although thin for the electron beam. Selected area electron diffraction clinopyroxene, majorite, and akimotoite are candidates for such (SAED) patterns and the semiquantitative analyses indicate that a composition, they occur in shocked meteorites usually in the the lamellae are alternating Mw and poorly crystallized material crystalline state (4, 14, 22). Hence, the glass with the with a composition of ðMg;FeÞSiO3. We assume that the latter is ðMg;FeÞSiO3 composition is unlikely any of the three mentioned vitrified ðMg;FeÞSiO3 Pv. The thicknesses of Mw and vitrified above. The volume ratio between Mw and poorly crystallized ma- ðMg;FeÞSiO3 Pv layers are approximately 10 nm and approxi-

Fig. 1. Back-scattered electron (BSE) images of a dissociated olivine grain in DaG 735. (A) An olivine grain adjacent to a shock melt vein is partly dissociated into ðMg;FeÞSiO3 Pv þ Mw. (B) High-magnification image of the box in A. Two different dissociation textures (fine-grained granular and coarse-grained gran- ular) are encountered and spatially arranged. TEM slices prepared with a FIB system are shown numbered in A and B. Ol, olivine; Cpx, clinopyroxene.

6000 ∣ www.pnas.org/cgi/doi/10.1073/pnas.1016921108 Miyahara et al. Downloaded by guest on October 1, 2021 Fig. 2. TEM images of dissociated olivine. (A) Equigranular ðMg;FeÞSiO3 Pv (pv) and Mw (mw). (B) A fragment consisting of alternating ðMg;FeÞSiO3 Pv and Mw laminae between equigranular ðMg;FeÞSiO3 Pv and Mw assemblage. SAED patterns correspond to granular Mw and ðMg;FeÞSiO3 Pv (b ¼ 4.95ð5Þ Å, a and c axes could not be determined). (C) A fragment consisting of alternating Mw and olivine-like mineral [b ¼ 9.92ð14Þ and c ¼ 4.75ð4Þ Å; a axis could not be deter- h100i ∥ð001Þ ð ; Þ mined] with pyroxene composition. SAED patterns show Mw Ol-like. Granular Mw and Mg Fe SiO3 Pv surround the fragment. (D) High-resolution TEM image of the box in C depicting alternating Mw and olivine-like phase.

mately 20 nm, respectively. Most Mw appears to be elongated stacked with Mw instead of the expected vitrified ðMg;FeÞSiO3 along h100i . Some spherical Mw grains are stacked in rhyth- Pv layer. The semiquantitative analyses of the olivine-like layers Mw ð ; Þ mically intergrown Mw and ðMg;FeÞSiO3 Pv glass layers. In some revealed Mg Fe SiO3 composition. There is a crystallographic cases, we encounter a crystallographically olivine-like layer relationship between Mw and olivine-like mineral: (Ol-like); h100i ∥ð001Þ ð ; Þ Mw Ol-like. Although olivine with a Mg Fe SiO3 com- position was previously reported from a shock melt vein of Table 1. Chemical compositions obtained by STEM-EDS Tenham L6 chondrite (31), its crystal structure details are still Lamellar unclear. Alternating akimotoite and Mw lamellae or ringwoodite GEOLOGY Equigranular texture texture were previously reported as metastable phases during dissocia- tion of ringwoodite or olivine to Mw þðMg;FeÞSiO3 Pv (32). Oxide Pv σ Mw σ Pv Mw* —— — The olivine with ðMg;FeÞSiO3 composition may be a metastable SiO2 54.5 1.7 54.1 – FeO 17.2 1.6 79.8 3.6 23.8 57.6 phase as well. Polycrystalline olivine (50 100 nm in one dimen- MgO 28.2 1.3 20.2 3.6 22.1 42.4 sion) assemblages and the abundance of olivine fragments are Total (wt %) 100 100 100 100 increasing with increasing distance from the shock melt vein. ∕ð þ Þ FeO MgO FeO 0.38 0.03 0.80 0.04 0.52 0.58 Their compositions (Fa39-43) are identical to the original olivine. O ¼ 61 61Very fine-grained (less than approximately 10 nm) granular Mw Si 1.98 0.04 ——2.02 — and ðMg;FeÞSiO3 Pv were also observed around the polycrystal- Fe2þ 0.52 0.05 0.69 0.05 0.74 0.43 Mg 1.53 0.07 0.31 0.05 1.23 0.57 line olivine assemblages. Total 4.02 0.04 1.00 0.00 3.98 1.00 The dissociation mechanisms of olivine or ringwoodite to n ¼ 32 10 1 1 Mw þðMg;FeÞSiO3 Pv were previously studied using high-pres- sure generating devices (26–29, 33–35). Microtextural evolution All iron is assumed ferrous. Pv, ðMg;FeÞSiO3 perovskite; Mw, magesiowüstite. from lamellar into equigranular, similar to the textural sequence —, not determined. Analyses are normalized to 100%. encountered here, was previously observed with increasing tem- *Computed by subtracting Pv composition from comprehensive lamella perature but at the same pressure conditions. The microtextural composition. evolution observed in DaG 735 probably resulted from the de-

Miyahara et al. PNAS ∣ April 12, 2011 ∣ vol. 108 ∣ no. 15 ∣ 6001 Downloaded by guest on October 1, 2021 creasing thermal gradient into the olivine grain during a dynamic take place below ∼700 °C even at higher overstepped pressure. event; i.e., the portion adjacent to the shock melt vein was Accordingly, pressure condition recorded in the shock melt vein the hottest. It is likely that the polycrystalline olivine breakdown and melt pockets of DaG 735 would be slightly beyond approxi- assemblages kinetically favor grain-boundary nucleation and mately 25 GPa, and temperature condition should be >700 °C at growth mechanisms, leading to form the granular texture least. The discovery of natural dissociation of olivine at high pres- (Fig. 2 A and B). Conversely, olivine fragments favor the intra- sure and high temperature should balance the pressure-tempera- crystalline mechanism, which could be initiated with defects ture estimates during shock events on Mars and bring better such as stacking faults and micropipes, leading to form lamellar understandings about the dynamics in the Earth’s interior. In texture (Fig. 2 B–D). When temperature is high enough, only addition, it will be a clue to clarify not only the dynamic events A the equigranular texture would evolve (Fig. 2 ). Their grain sizes on Mars but also the Martian interior because recent studies become coarser closer to the shock melt vein, suggesting that suggest that abundant ðMg;FeÞSiO3 Pv may exist in the lower- their growth rates are probably temperature dependent (36). most Martian mantle (e.g., refs. 42 and 43). Olivine studied here is much more Fe-rich than expected for the Earth’s interior. Nonetheless, we could apply our findings for Materials and Methods the Earth because the dissociation mechanism of olivine does A polished DaG 735 chip sample was prepared for this study. The mineralogy not depend on the Fe content of olivine. was determined using a laser micro-Raman and a JASCO NRS-2000 spectro- KPv∕Mw ¼½ ∕ ∕½ ∕ meter with a liquid nitrogen-cooled CCD detector. A microscope was used to The partitioning coefficient, FeO MgO Pv FeO MgO , between ðMg;FeÞSiO3 Pv and Mw in equigranular and focus the excitation laser beam (the 487.99-nm lines of a Princeton Instru- Mw þ lamellar textures is approximately 0.15 and approximately 0.78, ments Ar laser). The laser power on the sample was kept at 12–14 mW respectively. Although the partitioning coefficient of the lamellar to hinder material damage. We used a FE-SEM, JEOL JSM-71010 for detailed texture is an approximate value, the difference between the equi- fine textural observations. An accelerating voltage of 15 kV was employed. granular and lamellar textures is evident. The partitioning coeffi- Chemical compositions were determined using the wavelength-dispersive procedure of an EMPA, JEOL JXA-8800M. Analyses were carried out using cient increases with increasing temperature at the same pressure accelerating voltage of 15 kV, a beam current of 10 nA, and a defocused condition (33, 35). The partitioning coefficient between constitu- beam of 1–10 μm. Forsterite (Si and Mg), rutile (Ti), corundum (Al), fayalite ents of the lamellar texture is extremely high although the lamel- (Fe), Mn-olivine (Mn), Cr2O3 (Cr), wollastonite (Ca), jadeite (Na), adularia (K), lar texture analyzed here is spatially very close to the equigranular NiO (Ni), and pyrrhotite (S) were used as standards. Matrix, absorption, setting (distance less than approximately 2.0 μm). Equilibrium stopping power, and fluorescence corrections were conducted by the ZAF was not attained in the lamellar texture even if phase transforma- procedure. tion was completed, as a result of the short duration of high- A part of the sample was excavated with a FIB system, JEOL JEM-9320FIB, pressure and high-temperature events. Atomic diffusion at such and the extracted slice was placed on a culet of single diamond. The dissociation reactions, especially in the case of the lamellar tex- excavated samples on the diamond were scanned at the BL10XU beam ture would be delayed compared to the phase transformation. line (SPring-8). A monochromatic incident X-ray beam with a wavelength Alternatively, KPv∕Mw might strongly depend on temperature at of 0.41456(5) Å was collimated to a diameter of 15 μm (44). X-ray diffraction the same pressure condition. spectra were collected on an imaging plate (IP) using an exposure time The equilibrium shock pressure of the dynamic events re- of 5 min. The two-dimensional X-ray diffraction image was integrated as a function of 2θ to give a conventional one-dimensional diffraction profile. corded in Martian meteorites has been controversial over two The X-ray diffraction spectrum of cerium dioxide (CeO2) was used to deter- decades (8, 9, 11, 13, 37). A shock pressure recoded in DaG mine the wavelength and the distance between the sample and the IP. 476, which is paired with DaG 735, was suggested to be in the Slices for TEM observations were prepared by a FIB system. A gallium range of 40–50 GPa based on the shock metamorphic parameters ion beam was accelerated to 30 kV during the sputtering of the slices. of olivine in comparison to shock recovery experiments (38). This The slices were approximately 100 nm in thickness. Detailed FIB procedures estimation is too high. Raman spectra of grains studied in DaG are described in ref. 45. We also employed an argon (Ar) ion milling device 735 show that pyroxene glass exists in pigeonite, augite, or ortho- equipped with a liquid nitrogen cooling system to conduct additional Ar ion pyroxene around the melt pockets. A part of these pyroxene milling with some slices. A JEOL JEM-2010 transmission electron microscope would dissociate at such high pressures into orthorhombic or operating at 200 kV was employed for conventional TEM and SAED. We cubic ðMg;Fe;CaÞðSi;AlÞO3 Pv during a dynamic event. Some used also a STEM, JEOL JEM-3000F field emission transmission electron microscope operating at 300 kV with a JEOL EDS detector system. The ðMg;Fe;CaÞðSi;AlÞO3 Pv might directly crystallize at such high pressures from silicate melts induced by the dynamic event chemical compositions of individual minerals were obtained by EDS under because pyroxene glass was also indentified from the matrix of STEM mode. The compositions were corrected using experimentally deter- mined k factors [San Carlos olivine and synthetic (Mg,Fe)O]. shock vein. Based on static high-pressure and high-temperature experiments, the pressure condition during the dynamic event ACKNOWLEDGMENTS. We are grateful to two anonymous reviewers who should have been slightly above approximately 23 GPa because helped in considerably improving the manuscript. We appreciate the help majorite and akimotoite did not accompany the pyroxene glass of Y. Ito and Y. Kawanobe for EMPA analysis. Part of this work was (39–41). The dissociation of olivine into ðMg;FeÞSiO3 Pvþ supported by the “Nanotechnology Support Project” of the Ministry of Mw indicates that the pressure might have overstepped approxi- Education, Culture, Sports, Science and Technology (MEXT), Japan. This mately 25 GPa (1). Postshock temperature in Martian meteorites study was also supported by grant-in-aids for the Scientific Research – (18104009 and 22000002) of MEXT (to E.O.) and grant-in-aids for the was once estimated to be around 200 300 °C (8). This estimate Scientific Research (19540500 and 22540488) of MEXT (to M.K.). This work appears unlikely because a kinetic barrier exists around 700 °C for was conducted as a part of Tohoku University Global COE program “Global the dissociation of olivine (34); i.e., the dissociation does not Education and Research Center for Earth and Planetary Dynamics.”

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