MINERALOGY, CHEMISTRY, AND NOBLE-GAS CONTENTS OF ADZHI-BOGDO - AN LL3-6 CHONDRITIC BRECCIA WITH L-CHONDRITIC AND GRANITOIDAL CLASTS

Adzhi-Bogdo is an ordinary chondrite regolith breccia (LL3-6) which fe ll on 1949 October 30 in Gobi Altay, Mongolia. The rock consists of su bmm- to cm-sized fragments embedded in a fine-grained clastic matrix T he polymict breccia contains various types of clasts, some of which mu st be of foreign origin. Components of the breccia include chondrules, melt rock clasts (some of which are K-rich), highly recrystallized ro ck fragments (''granulites''), breccia clasts, pyroxene-rich fragments with achondritic textures, and alkali-granitoids. The composition of olivine in most fragments is in the range of LL-chondrites. However, o livine in some components has significantly lower fayalite contents, c haracteristic of L-chondrites. The bulk meteorite is very weakly shock ed (S2). Based on the bulk chemical composition, Adzhi-Bogdo is an ord inary chondrite. The concentrations of Fe and Ni are somewhat intermed iate between L- and LL-chondrites. The contents of solar gases indicat e that Adzhi-Bogdo is a regolith breccia. Most of the solar He and pro bably a part of the solar Ne of Adzhi-Bogdo has been lost. It is sugge sted that Adzhi-Bogdo experienced an (impact-induced) thermal event ea rly in its history, because most of the radiogenic Ar-40 is retained.