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LAMELLAR CHROMITE-DIOPSIDE SYMPLECTITES IN OLIVINE FROM THE ‘LUNA-24’ REGOLITH: TEM STUDY. Khisina N. R.1, Nazarov M. A.1, Wirth R. 21Institute of Geochemistry and Analytical Chemistry of RAS, Kosygin st. 19, Moscow 119991, Russia; e-mail: [email protected]; 2 GeoForschungsCentrum Potsdam, Telegrafenberg, Potsdam, Germany Lamellar chromite-diopside symplectites in olivine are rare to occur. They were described in olivine from some terrestrial rocks [1,2] and a Martian meteorite [3]. The origin of Chr-Di symplectites is discussed up to now.No detailed investigation of Cr,Carich lamellar symplectites in lunar olivines has ever been done. Results. Optical observartion showed [4] that individual olivine grains from the Luna-24 regolith contain Cr- and Ca-rich lamellae of 0.5-1 m thick oriented parallel to the (100) of the host olivine (Fig. 1). .Digital Formats: Line drawings AND photo Fig. 2. Dark-field TEM image of Ca,Crlamellae. Black and white platelets are diopside and chromite, correspondingly. 8 Fig. 1. Optical image of Ca,Cr-lamellae in the olivine grain from the ”Luna-24” regolith. The lamellae were investigated with EMPA, SEM and TEM. The lamellae consist of diopside - chromite intergrowths and represent a spinel-pyroxene symplectite of lamellar shape (Fig. 2). Alternating platelets of diopside and chromite 40 nm and 130 nm thick, respectively, are oriented normal to the (100) olivine/lamellae boundary, with (100)Ol // (111)Sp //(100)Cpx; [001]Ol // [011]Sp // [010]Cpx. The bulk mineral composition of the lamellae is close to FeCr2O4 + 2CaMgSi2O6. The results show that (i) the symplectitic lamellae in olivine have been formed by a solid-state reaction; (ii) subsolidus Cr2+ Cr3+ oxidation and 2Mg = Cr + Ca cation exchange reaction were related to the symplectite formation (Fig. 3). Discussion. Chromite and diopside are probably the breakdown products of some preexisting phase of Ca2FeMg2Fe(Cr3+)2Si4O16 composition inferred from the bulk chemistry of the symplectites. Ca + Cr, at.% 6 4 2 0 14 16 18 20 22 Mg, at.%Mg, at.% Fig. 3. The correlation of Mg versus (Ca + Cr). EMPA data for olivine + lamellae. The point at (Ca + Cr) =0 corresponds to the host olivine composition. A model of a deprotonation-oxidation process associated with a {Fe, 2H-} {Fe, 2Cr3+} point defect transformation is suggested to explain the origin of the pre-existing phase of the symplectites. The model seems to be a convincing explanation for the occurrence of lamellar spinel + pyroxene symplectites in terrestrial olivines, because the latter contain commonly n101 – n102 ppm of H2O. Both {Fe, 2H-} point defects and (100)-oriented lamellar precipitates of hydrous olivine [MgFeH2SiO4]*n[(Mg,Fe)2SiO4] CHROMITE-DIOPSIDE SIMPLECTITES: N. R. Khisina et al. were found in terrestrial mantle olivine [6]. A similar mechanism has been suggested to explain the origin of oxide precipitates in olivine from a terrestrial garnet peridotite [5]. How can this model be applied to lunar rocks, because the rocks are believed to be almost free of water? Recently, some arguments suggesting an H2O presence in the lunar mantle has come from a SIMS study of lunar volcanic glasses [7, 8]. Conclusion. On the basis of TEM data the formation of the symplectite due to deprotonationoxidation process is proposed. If this model of symplectite formation is correct, then the occurrence of chromite-diopside symplectites in lunar olivine of deep origin should be considered as an additional evidence for the presence of H2O in the Moon interior. We suggest that lamellar Sp-Py symplectites are probably the fingerprints of protons escaped from the olivine. The investigation was supported by grant № 0905-00444-a of Russian Fondation for Fundamental Researches and grant ОNZ-5 of theProgramm of Earth Science Department of Russian Academy of Sciences. References: [2]. Moseley D. (1984) Am. Mineral., 69, 139-153. [3]. Markl G. et al. (2001) Am. Mineral., 86, 36-46. [4]. Mikouchi T. et al. (2000) Meteoritics & Planetary Science, 35, 937 – 942. [4] Tarasov L.S. et al. (1980) In: Lunar soil from the Mare of Crisium. M., “Nauka”, 359 pp. [5]. Khisina N.R.and Wirth R. (2002). Phys. Chem. Minerals, 29, 98-111. [6]. Hwang S.-L. et al. (2008) Amer. Mineral. 93, 1051 – 1060. [7]. Saal A.E. et al. (2008) Nature, 454, 192-195. [8] Chaussidon M. (2008) Nature, 454, 170-172.