University ’ s repository of research publications and other research outputs NWA 4477 : A unique impact melt breccia

Copyright and Moral Rights for the articles on this site are retained by the individual authors and/or other copyright owners. For more information on Open Research Online's data policy on reuse of materials please consult the policies page. Introduction: NWA 4477 is a highly brecciated type L3-7 chondrite. It therefore offers a unique insight into the history of the L-chondrite parent body. Of the ordinary chondrites, the L class has the lowest number of brecciated members at ~10% [1], some of which are impact melts. A possible relationship between FeO-rich primitive achondrites to L7 and LL7s as potential precursors has been suggested [2]. The designation of type 7 material [3,4], thermally altered equilibrated chondritic material lacking chondrules, also loosely describes primitive achondrites. Primitive achondrite's precursors are thought to be chondritic material that underwent partial melting and crystalliza-tion without melt migration [4,5]. This study of NWA 4477 aims to (1) determine the relationship of the type 7 clast to its chondritic host and to primitive achondrites, (2) determine its formation mechanism, and (3) understand the melting of the L-chondrite parent body. Analytical Procedure: We examined one thin section from the L3-7 genomict breccia (S3 W1) NWA 4477 (Fig. 1). Quantitative analyses were performed on the Cameca SX-50 EMP at LPL using an accelerating voltage of 15 kV and a beam current of 20 nA. for metal and sulfide analyses. X-ray maps were obtained on the EMP. Volume percents (vol.%) were determined using pixel counting in Adobe Photo-shop®. Bulk oxygen isotopic analysis on the L3-6 portion and the L7 portion (henceforth termed coarse-grained clast) were undertaken by infrared laser fluori-nation [6]. Results: In cross-section, it is apparent that a coarse-grained clast composes ~20% of the thin section , and contains no chondrules. The remaining portions contain abundant chondrules and distinct chon-drule bearing clasts. The boundary between these two lithologies is distinct, and shows no evidence that the major clast experienced significant thermal processing. There are thin melt veins bearing metal spherules which cross-cut the sample, one of which is continuous across the chondrule-bearing portion and the coarse-grained clast. Overall, the meteorite contains