Yamato-8002: Partial melting residue on the"unique"chondriteparent body

Yamato-8002 is classified as a "unique" meteorite having a granoblastic texture with olivine and orthopyroxene and interstitial diopside, plagioclase, spinel, kamacite schreibersite, troilite, and rare graphite. Olivine, orthopyroxene, and clinopyroxene are homogeneous with respect to Mg and Fe, but Ca in pyroxenes are zoned. Plagioclase and spinel are heterogeneous within and among grains. Mineral compositions, particularly plagioclase, spinel and trace elements in all minerals along with oxygen isotopic compositions suggest a genetic link to Y-74063,Y-75274,ALH77081,Y-791493,Acapulco, and Lodran. They were derived from a common precursor through different degrees of partial melting; Y-8002 and Y-75274 have the highest Mg/(Mg+Fe) ratios of olivine, pyroxenes, and spinel, the highest An mol% of plagioclase, and the higest Cr/(Cr+Al) ratio of spinel, indicating formation through the highest degree of partial melting. They were reduced during melting, resulting in crystallization of extremely magnesian minerals. Y-74063,ALH77081,Y-791493,Lodran, and Acapulco possess nearly primary composition of the ferromagnesian minerals excepting that some have lost and some have added partial melts. The partial melting model is supported by the REE patterns; Y-74063 and ALH77081 are flat, Y-8002 and Lodran are heavy REE enriched, and Acapulco is light REE enriched. Positive or negative Eu anomalies are general in these meteorites, suggesting addition or extraction of interstitial melt rich in plagioclase component. The precursor is chondritic with higher Mg/(Mg+Fe) ratio and higher Cr and Mn contents than ordinary chondrites, and is tentatively called a "U (nique)" chondrite. Y-74357,Y-78230,and MAC88177 are probably related to the "U" chondrite. Oxygen isotopic heterogeneity of those meteorites may be due to weathering on the Antarctic ice, and the effect of weathering was excluded by assuming that all oxygen in Fe_2O_3 was from Antarctic ice. The calculated compositions are similar to each other, suggesting their derivation from a common precursor