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資料番号: AA0063350025

Y980459, a new Martian meteorite from Antarctica, is composed of coarse porphyritic olivine grains (up to 2mm) set in the groundmass of olivine and pyroxene with abundant glassy mesostasis containing dendritic olivine and pyroxene. The overall petrography of Y980459 is similar to those of olivine-phyric shergottites, but the absence of plagioclase and Ca phosphates makes Y980459 unique. Because of the absence of maskelynite, Y980459 is not a shergottite if we employ the classic definition of shergottite. Both olivine and pyroxenes are extensively zoned. The most magnesian olivine composition is Fo86 and the olivine compositions are related to three different occurrence types of olivine (large phenocrysts, groundmass, and mesostasis). Pyroxenes have orthopyroxene cores (En81Fs17Wo2) mantled by pigeonite with the rims of augite. The mineralogy of Y980459 suggests that rapid crystallization of the parent magma caused significant undercooling and plagioclase did not nucleate. Probably, rapid transport of the Y980459 parent magma from the depth to the Martian surface crystallized olivine and pyroxene at first and eruption onto the surface quenched the magma producing the glassy mesostasis. Because olivine and pyroxene compositions of Y980459 are the most magnesian among Martian meteorites, Y980459 would represent one of the most primitive Martian magmas and derive from a highly reduced mantle. It seems that Y980459 contains no cumulus component, suggesting that Y980459 is a melt. In this sense, Y980459 is similar to QUE94201. The similarity in mineralogy and chemistry between Y980459 and olivine-phyric shergottites suggests derivation from a similar highly reduced mantle. However, Y980459 was the only sample that directly erupted onto the Martian surface without any accumulation processes.

The cooling rate (and burial depth) was recalculated by using different diffusion coefficients from those used in the previous studies on the basis of the Fe-Mg and CaO chemical zoning profiles observed for pallasite (Esquel) olivine. The cooling rate obtained from the Fe-Mg zoning profile is broadly consistent with that from the CaO profile and is 2 to 5 C/yr (300 to 200 m in depth) for cooling from 1,100 C. The results of recalculation for the cooling rate also show a relatively fast cooling at the high-temperature range. Therefore, a shallow origin for pallasites during high temperature should be further explored., 資料番号: AA0047898057

資料番号: AA0047898055

Pallasites, which consist mainly of olivine and Fe-Ni metal, are generally thought to have formed at the core-mantle boundaries of igneously differentiated parent bodies. Pallasite olivine, however, shows chemical zoning for Fe-Mg, CaO, Al2O3, Cr2O3 and MnO within a few hundreds of micrometer from the rim. The cooling rate of pallasite on the basis of Fe-Mg chemical zoning of olivine was calculated by assuming that zoning is controlled by Fe-Mg interdiffusion. A calculated cooling rate of 2 C/year from 1,100 C to 600 C gives the best fit to the observed Fe-Mg profile by using the diffusion coefficient reported by Misener (1973) with oxygen-fugacity variation. This result suggests a shallow origin of pallasite, although this cooling rate is one order of magnitude slower than that previously reported by using the Fe-Mg diffusion coefficient of Buening and Buseck (1973)., 資料番号: AA0046650031

資料番号: AA0046650030