Complementary element relationships between chondrules and matrix in Rumuruti chondrites

Abstract

The complimentary chemical composition of chondrules and matrix has so far only been studied in carbonaceous chondrites. We have extended these studies to the matrix-rich Rumurutis We have determined the chemical composition of 27 bulk chondrules and 100 matrix spots in unequilibrated fragments of three different Rumuruti (R) chondrites (NWA 2446, NWA 753, Hughes 030). Also, the bulk chemical composition of NWA 753 was determined. Bulk R chondrites have about CI chondritic (= solar) ratios of Fe/Mg (1.89), Al/Ti (18.62), and Al/Ca (0.94), while chondrules and matrix are complementary to each other. Mean Fe/Mg ratios in chondrules are 0.43 (NWA 2446), 0.36 (NWA 753), and 0.34 (Hughes). Chondrules are depleted in Fe, while matrices are enriched in Fe with respective values of 2.59, 2.45, and 2.39. Bulk Fe/Mg ratio of NWA 753, the only sample which is unaffected by terrestrial weathering, is reproduced by careful mass balance calculations using compositions and abundances of chondrules, matrix and sulphides from the same thin section. Refractory element ratios are also complementary: Al/Ti and Al/Ca are sub-chondritic in chondrules (Al/Ti: 10.43 in NWA 753, 12.24 in NWA 2446 and 11.47 in Hughes 030; Al/Ca: 0.66 in NWA 2446, 0.53 in NWA 753, and 0.46 in Hughes 030), while matrices generally have super-chondritic Al/Ca and Al/Ti ratios (Al/Ti: 28.00 in NWA 2446, 23.20 in NWA 753 and 19.00 in Hughes 030; Al/Ca: 2.30 in NWA 2446, 1.90 in NWA 753, and 0.41 in Hughes 030). Calcium is enriched in the Hughes 030 matrix due to terrestrial weathering. These complementary element ratios and the CI chondritic bulk strongly suggest a common reservoir from which all R chondrite components formed, similar to carbonaceous chondrites. Further, super-chondritic Si/Mg, and CI chondritic Fe/Mg ratios in bulk R chondrites require addition of Si to their reservoir, most probably before chondrule formation.