LAMELLAR OLIVINE IN THE DIVNOE ACHONDRITE: EVIDENCE FOR HIGH- PRESSURE EXSOLUTION? M.I.Petaev, Harvard-Smithsonian Center for Astrophysics, Cambridge, MA, USA

The olivine-rich Divnoe achondrite contains numerous large olivine grains which have a lamellar or banded appearance in backscattered electron images, caused by minor compositional differences. One such grain, viewed in transmitted light, displays a system of lamellae with the same orientation and scale as the compositional banding. The only process known to produce such stmcture and chemical variability in olivine grains is high-pressure transformations between a-, and y-olivines, but in other meteorites and in experimental products the structure is 100 times finer than the Divnoe lamella. INTRODUC~ON The meteorite Divnoe is an olivine-rich achondrite with subchondritic chemistry and mineralogy 11-41, It has a coarsegrained granoblastic olivine groundmass with relatively large poikilitic patches of pyroxene and, rarely, plagioclase. The groundmass also contains areas of troiliteandlor metal-rich d-px fine-grained lithology, having reactionary boundaries with the groundmass and different mineral chemistry [5]. Numerous pmto mm-thick veins of troilite and, rarely, metal cross all the lithologies found in the meteorite. The major silicate minerals display evidence of week to moderate shock metamorphism: wavy to blocky extinction in olivine, pyroxenes, and to a lesser degree, plagioclase; irregular, and, rarely, planar fractures in olivine and low-Ca pyroxene, often filled by troilite; strong planar fractures in high-Ca pyroxene. Some plagioclase grains display welldeveloped twinning, sometimes with diffuse boundaries between twins. No evidence of shock has been found in metal and troilite. Olivine and pyroxenes display minor intergrain variations in composition [5] and complex zoning patterns. OUWE STRUCTUREAND cnwsmr The most unexpected and intriguing result of this study was the discovery of fine p -sca le chemical variability in olivine grains in the rock whose textural and mineralogical characteristics suggest extensive recrystallization and slow cooling in the temperature range from 1000'C to 500'C and lower. In spite of this, in back-scattered electron images many if not all of olivine grains show a lamellar appearance, which seems to be crystallographically controlled (Fig. la), caused by minor chemical variations (Fig. lb). The points on the graph are microprobe analyses made at the dark rounded spots seen in Fig. l a . In some cases these fine-scale variations are superimposed on large-scale compositional zoning in olivine grains where they are in contact with pyroxene (Fig. 2). In all grains studied so far, the BSE variability is caused by differences in FeIMg ratio between lighter (Fe-rich) and darker (Fe-poor) lamellae. The range of variations in most of the microprobe scans done is less than 3 mol.% Fa; however, these points were equally spaced in the scans, not placed according to tones in the BSE images. To define more precisely compositional variability, the lightest, darkest, and some intermediate lamellae were analyzed in three olivine grains. Differences of 2.7, 3.0 and 3.2 mol.% Fa were found (Fig. 3), as well as a strong positive correlation between Fe and Mn, which was only detectable (>0.05 wt.%) minor element. Among several lamellar olivine grains studied by transmitted light, one was found to have 5 12 km-thick lamellae (Fig. 4a) with the same orientation and thickness as the lamellae seen in back-scattered electron images (Fig. 4b). Several other grains with similar structure have been found in thin sections. D I S C U S S I ~ The scale of structural and compositional variations found in Divnoe olivines has not been observed in terrestrial and other extraterrestrial divines. Ferromagnesian olivines are generally understood to form an almost ideal solid solution between end members, and there is no reason to expect any exsolution to occur. Detailed structural studies of natural olivines has shown some preferred occupation of M I sites by Fe at high temperatyres [6 and references therein], but this would not promote exsolution. Coexisting minerals with olivine stoichiometry and slightly different Fe/Mg ratios have been found in several highly shocked ordinary chondrites (7-1 11. These grains are composed of isotropic polyaystalline aggregates of high-pressure olivine polymorphs wadsleyite (P) and ringwoodite (y) and are characterized by compositional differences up to 8 mol.% Fa between coexisting phases. Some grains display ultrafine (up to 0.1 pm) lamellar structure (8.g. Fig. 2c in [ I I]), qualitatively similar to the structure of Divnoe olivine grains. Experimental [12 and references therein] and thermochemical 1131 studies of the high-pressure transformations in olivine have shown that transformations at pressures of 100 160 kbar and temperatures of 800 1600 K result in substantial difference in Fe/Mg ratios between coexisting polymorphs. Lamellar structure was not found by [12] in their experimentally produced olivine polymorphs, but experimental studies of transformations between a, $ and y polymorphs of Mg2Si04 at 150 kbars 1141 did produce ultrafine (up to 0.01 p ) striated microstructure in $ and y phases similar to the structure observed in Divnoe olivine by transmitted light. This structure was interpreted by [14] to be a system of stacking faults that arises during the phase transformation. 1141 also found lamellar intergrowths of $ and y phase, but the scale of the lamellae is even finer (0.001 pm). Thus, only process known to produce exsolution in olivine is high-pressure transformation between its polymorphs. The structural and compositional similarity between Divnoe olivine grains and the occurrences discussed above indicates that this could explain the Divnoe olivines. But what process was responsible for the transformations? Such high static pressures seem to be unrealistic for meteorites, and the presence of plagioclase in Divnoe limits the static pressure to less than 30 kbars. The shock features recorded in Divnoe silicates are characteristic of shock pressure of X ) O 300 kbars, enough to produce transformations between a,