Yamato nakhlites: Petrography and mineralogy

We carried out the petrographical and mineralogical study of new Yamato nakhlites, Yamato 000593 (Y000593), Y000749 and Y000802, with electron probe microanalyser and Fourier transform infrared microspectrometer. Euhedral pyroxenes (En_<20-40>Fs_<22-40>Wo_<38-40>) are the predominant phase with a modal proportion of 75-80 vol% and occur as elongated grains (～1mm x 0.5mm) with thin Fe-rich rims. The chemical composition of the augite cores comprising most of the volume of these pyroxene crystals is homogeneous and nearly identical with those in other nakhlites. Fe-rich rims are present in contact with the mesostasis. Anhedral ferroan olivines (Fa_<62-78>; 8-18 vol%, ～0.5 mm) and subhedral titanomagnetites (< 1 vol%, smaller than ～0.1mm) are minor phenocrysts. The remainder of the meteoritic material is mesostasis (8-15 vol%), which predominantly consists of plagioclase (An_<26-32>Ab_<63-65>Or_<4-8>). Minor phases in the mesostasis are pyrrhotite (Fe_<0.86-0.88>S; smaller than 20μm), apatite (smaller than 20μm), titanomagnetite, Ca-poor pyroxene, fayalitic olivine (Fa_<～85>), tridymite and iddingsite. We distinguish these Yamato nakhlites from other nakhlites based on the chemical compositional ranges of the cores and rims of olivine and pyroxene phenocrysts. We suggest that the chemical variations of these minerals for the Yamato nakhlites are intermediate between those of NWA817 and others (Nakhla, Governador Valadares, and Lafayette). The study by Fourier transform infrared microspectrometer of altered phases both on rims and fractures in olivine phenocrysts and in mesostasis revealed the existence of OH-bearing minerals, which might be mixtures of montmorillonite (70%) and goethite (30%). The existence of bubbles in an OH-bearing phase in olivine grains in contact with the fusion crust suggests that the alteration occurred before atmospheric entry (i.e. Martian origin).