Igneous Petrogenesis of Yamato Nakhlites

Introduction: We have carried out detailed petrographical-mineralogical description of three Yamato nakhlites (Y000593, Y000749 and Y000802). These three are paired. We clarified chemical compositional differences between the Yamato nakhlites and other nakhlites, and subclassified the Yamato nakhlites as an intermediate type between NWA817 and others (Nakhla, Governador Valadares and Lafayette) based on the compositional variation of pyroxenes and olivines [1]. In the present study, we summarize analytical results of the Yamato nakhlites obtained by electron probe microanalyzer, and discuss the peterogenesis of nakhlites using the bulk chemical composition of magmatic inclusions in olivine phenocrysts and mesostasis. Results: Augites: Augite phenocryst cores are homogeneous (En 38-40 Fs 24-26 Wo 40-41), and the rims consist of two thin layers, inner and outer (Fig. 1). Ferrosilite contents increase continuously from inner rim to outer rim. Though wollastonite contents are constant in the inner rim, it drastically decreases in the outer rim (Fig. 1). The aluminum content increases in the inner rim, and drastically decreases in the outer rim. Olivines: Olivines occur as phenocrysts and show normal zoning (Fa 58-80 , CaO = 0.05~1 wt%). In olivine phenocrysts, symplectic lamellae consisting of two phases (magnetite and augite; [3]) are observed, and symplectites of the same two phases commonly occur at the grain boundaries. Inclusions in olivines: Three types of large (~100 µm) inclusions occur in olivine phenocrysts; they are augite monocrystalline inclusions, angular vitrophyric inclusions (AVIs) and rounded vitrophyric inclusions (RVIs). RVIs may represent the parent magma formed nakhlites as discussed by [2]. Mesostasis: Mesostasis consists predominanltly of needle-like plagioclases. Minor phases are K-feldspar, augite, pigeonite, fayalitic olivine, titanomagnetite, pyrrhotite, apatite and tridymite. Discussion: Olivine phenocrysts and titanomagnetite phenocrysts in the Yamato nakhlites co-crystallized based on the textural evidence, and titanomagnetite inclusions occur sometimes in aguite phenocryst cores. Therefore, we suppose that