Petrology And Mineralogy Of An Igneous Clast In The Northwest Africa 1685 (Ll4) Chondrite: Comparison With Alkali-Rich Igneous Clasts In Ll-Chondritic Breccias

We have conducted petrological and mineralogical studies on an igneous clast in the Northwest Africa (NWA) 1685 (LL4) chondrite. This meteorite was described in the Meteoritical Bulletin as containing clasts similar to alkali-rich clasts in LL chondritic breccias Yamato (Y)-74442 (LL4), Bhola (LL3-6), and Krahenberg (LL5). We carefully compared the textures, as well as mineral and matrix compositions, of the NWA 1685 clast with those of the previously described alkali-rich clasts in the LL chondritic breccias. Olivine grains are embedded in glassy matrix and have no chemical zoning. Shock melt veins and fractures were observed only in olivine grains and did not continue into matrix. Potassium abundance of matrix glasses of the NWA 1685 clast is lower than those of alkali-rich igneous clasts in Y-74442, Bhola, and Krahenberg, indicating that the igneous clasts in NWA 1685 are different from the alkali-rich clasts previously reported in the LL chondritic breccias. The NWA 1685 clast might have formed during an impact melting and quenching event on the LL-chondrite parent body, and then been incorporated into a breccia.