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Niihara, Takafumi*; Yokoyama, Tatsunori; Arai, Tomoko*; Misawa, Keiji*
Meteoritics & Planetary Science, 56(8), p.1619 - 1625, 2021/08
Times Cited Count：1 Percentile：10.87(Geochemistry & Geophysics)We have conducted petrological and mineralogical studies on an igneous clast in the Northwest Africa (NWA) 1685 (LL4) chondrite. In an earlier description, the meteorite contained similar clasts in the LL chondritic breccias Yamato (Y)-74442 (LL4), Bhola (LL3-6), and Kraehenberg (LL5). We carefully compared their textures as well as mineral and matrix compositions with those of alkali-rich clasts in the LL chondritic breccias. Olivine grains are embedded in glassy matrix and have no chemical. Shock melt veins and fractures were observed only in olivine grains and did not continue to matrix. Major and minor element compositions of olivine grains in the clast are homogeneous. Potassium abundance of matrix glasses of the NWA 1685 clast is lower than those of alkali-rich igneous clasts in Y-74442, Bhola, and Kraehenberg, indicating that the igneous clasts in NWA 1685 are different from the alkali-rich clasts previously reported in the LL chondritic breccias and that they could have formed during an impact melting event, quenched on the LL-chondrite parent body, and finally incorporated into breccia.
Niihara, Takafumi*; Tsuzuki, Yuki*; Misawa, Keiji*; Yokoyama, Tatsunori; Yoneda, Shigekazu*
no journal, ,
Some brecciated chondrites contain alkaline-rich igneous clasts. Their formational process is explained as early nebular condensates followed by shock melting. The Beardsley chondrite (H5) possesses large amounts of Rb compared to other H chondrites. However, detailed petrological and mineralogical signatures of Beardsley are still lacking and there is no petrological and mineralogical evidence for alteration such as presence of halite/sylvite or other alkaline-rich phases contained in other H chondrites. Here we conduct petrological and mineralogical study for the meteorite, especially to clarify secondary effect(s) and distribution of alkaline elements on the chondrites and discuss the petrogenesis of Beardsley. Petrological and mineralogical features of Beardsley are similar to those observed in impact melted H-chondrites, implying that the grey lithology is a product of impact melting event(s). On the other hand, the higher K
O content in the grey lithology cannot be explained only by an impact melting process but is still unclear at this stage.
