Petrology and mineralogy of an igneous clast in the Northwest Africa 1685 (LL4) chondrite; Comparison with alkali-rich igneous clasts in LL-chondritic breccias

Niihara, Takafumi*; Yokoyama, Tatsunori   ; Arai, Tomoko*; Misawa, Keiji*

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.