The presolar grain inventory of fine-grained chondrule rims in the Mighei-type (CM) chondrites

Leitner J, Metzler K., Vollmer C, Floss C, Haenecour P, Kodolanyi J, Harries D, Hoppe P

Zusammenfassung

We investigated the inventory of presolar silicate, oxide, and silicon carbide (SiC) grains of fine-grained chondrule rims in six Mighei-type (CM) carbonaceous chondrites (Banten, Jbilet Winselwan, Maribo, Murchison, Murray and Yamato 791198), and the CM-related carbonaceous chondrite Sutter's Mill. Sixteen O-anomalous grains (nine silicates, six oxides) were detected, corresponding to a combined matrix-normalized abundance of 18 ppm, together with 21 presolar SiC grains (42 ppm). Twelve of the O-rich grains are enriched in O-17, and could originate from low-mass asymptotic giant branch stars. One grain is enriched in O-17 and significantly depleted in O-18, indicative of additional cool bottom processing or hot bottom burning in its stellar parent, and three grains are of likely core-collapse supernova origin showing enhanced O-18/O-16 ratios relative to the solar system ratio. We find a presolar silicate/oxide ratio of 1.5, significantly lower than the ratios typically observed for chondritic meteorites. This may indicate a higher degree of aqueous alteration in the studied meteorites, or hint at a heterogeneous distribution of presolar silicates and oxides in the solar nebula. Nevertheless, the low O-anomalous grain abundance is consistent with aqueous alteration occurring in the protosolar nebula and/or on the respective parent bodies. Six O-rich presolar grains were studied by Auger Electron Spectroscopy, revealing two Fe-rich silicates, one forsterite-like Mg-rich silicate, two Al-oxides with spinel-like compositions, and one Fe-(Mg-)oxide. Scanning electron and transmission electron microscopic investigation of a relatively large silicate grain (490 nm x 735 nm) revealed that it was crystalline akermanite (Ca2Mg[Si2O7]) or a an akermanite-diopside (MgCaSi2O6) intergrowth.