CRYSTAL SIZE DISTRIBUTION ANALYSIS OF NEW NAKHLITES AND LOS ANGELES: HOW DO THEY COMPARE WITH SNCs

Introduction: Crystal size distribution (CSD) analysis is a useful technique for quantifying textural information and linking petrographic texture to original crystallization conditions [1]. In previous studies we have conducted CSD analyses of multiple thin sections of nakhlite meteorites [2], and several basaltic and mixed shergottites [3,4]. Based on results of these analyses, we derived crystallization scenarios for some of these meteorites. With the addition of Los Angeles to the basaltic shergottite group and identification of two new nakhlites (Yamato 000593/000749, and NWA 817), new CSD analyses seem in order to test the validity of previous interpretations and perhaps to lend new insight into the crystallization histories and relationships within these meteorite groups. Crystal Size Distribution: CSD analysis involves quantifying petrographic textures by calculating grain population densities in different size bins [1]. Theory predicts that steady-state growth would generate a negative linear correlation between grain size and the log of grain number density, where the slope of such a line is inversely related to the growth rate and crystal-lization time (m =-1/Gt) [1]. Deviations from linear-ity suggest the involvement of such geologic processes as multistage growth, grain accumulation, or solid-state ripening (annealing). The primary crystallizing phase in these martian meteorites is pyroxene, so all measurements represent pyroxene grain sizes. (Plagio-clase actually co-crystallized with pyroxene in the ba-saltic shergottites, but since it has been shocked to glass, grain boundaries are obscured.) Since pyroxene is an elongate crystal, we have chosen to define grain size by width, to minimize any geometric bias due to the two-dimensional view of the thin section [2]. The Los Angeles meteorite is coarse-grained, so we analyzed three non-serial thin sections to ensure that a statistically significant number of grains were measured (313 in all). One complication of determining the boundaries of the pyroxene grains in Los Angeles is the presence, within some otherwise subhedral pyrox-ene boundaries, of an intergrowth of fayalite, heden-bergite and silica [5]. This has been interpreted to be a late-stage breakdown product after a pyroxenoid [6] or after Fe-rich pigeonite and Ti-magnetite [6]. Since the origin of these intergrowth patches remains unclear, we considered only unaltered pyroxene material in determining the grain boundaries. In most cases, however , this issue was not a factor.