Mineralogical Comparison of Y000593 with Other Nakhlites: Implications for Relative Burial Depths

Introduction: Nakhlites are martian clinopyrox-enites with minor amounts of olivine and mesostasis. All the known nakhlites show similar mineralogy to each other as well as common crystallization and ejection ages and are believed to have formed in the same igneous body on Mars. The discovery of new nakhlites could offer substantial information to better understand the formation condition and geological setting of these unique martian meteorites. In this abstract, we present mineralogy and petrology of Y000593, a new nakhlite from Antarctica, and compare with other nakhlites to understand relative burial depths of each nakhlite. Petrology & mineralogy of Y000593: Y000593 shows an unbrecciated cumulate texture (~80 % augite, 10% olivine, and 10 % mesostasis). Augite is euhedral to subhedral (up to 1.5 mm long), having a large homogeneous core of En 39 Fs 22 Wo 39 (Fig. 1). The rim adjacent to the mesostasis shows Fe enrichment with almost uniform Wo content (En 39 Fs 22 Wo 39 ~ En 20 Fs 40 Wo 40) except for the 10-20 µm overgrown edge (En 20 Fs 40 Wo 40 ~ En 15 Fs 70 Wo 15) (Fig. 1). There is a sharp compositional boundary between the homogeneous cores and the Fe-enriched rims. Fine exsolution lamellae (<0.5 µm wide) are present at the overgrown edges. Olivines are mostly interstitial to augite and show extensive chemical zoning (Fa 58-85) (Fig. 1). Symplectic inclusions composed of augite and magnet-ite are commonly observed in large olivine grains. Olivines are pervasively altered along fractures and brown rusty products are present. The mesostasis is mainly composed of radiating spray of plagioclase (An 35 Ab 62 Or 3 ~ An 20 Ab 74 Or 6) sometimes intergrown with silica and K-feldspar. The mesostasis also shows extensive secondary alteration as is seen in olivine. Crystallization history of Y000593: Fig. 2 illustrates crystallization history of Y000593. At first augite (Aug) crystallized (a) and then olivine (Ol) followed (b). Both augite and olivine show chemical zon-ing in Y000593. Augite has a large homogeneous core and the chemical zoning is present only in the rim area. In contrast, olivine shows much more extensive chemical zoning. We explain this due to faster atomic diffusion of olivine than pyroxene. It is obvious that the chemical zoning was produced by interaction with the Fe-rich intercumulus melt. We consider that both augite and olivine would be homogenized before accumulation (plus symplectite (symp) formation) (c). Faster atomic …