Trace Element Variation in Carbonaceous Chondrite Matrix

Introduction: Chondrite matrix is defined as 'the fine-grained, predominantly silicate, material intersti-tial to macroscopic, whole or fragmented, entities such as chondrules, inclusions and large isolated mineral (i.e., silicate, metal, sulphide and oxide) grains' [1]. Matrix materials are derived from a wide variety of sources: presolar grains, solar-nebular condensates, and finely ground lithic material from chondrules and inclusions. These fine-grained components were subsequently lithified following accretion with varying amounts of inclusions (chondrules, CAIs, DIs, AOAs etc) on asteroid parent bodies, where they experienced variable aqueous, thermal, and impact processing. The fine-grained nature of chondrite matrix, and the often substantial secondary processing that it has experienced , has made unraveling the origins and abundances of specific matrix components extremely difficult. There remains little concensus on the origin of matrix materials, even within one chondrite group. Any discussion of matrix chemistry must also be concerned with the origins of volatile and moderately volatile element depletions in chondrites. Although variable secondary processing has meant that matrix mineralogy differs substantially from meteorite to meteorite and group to group, it was long considered that matrix chemistry in unequilibrated chondrites (UCs) should be essentially the same. This was an assumption of the 2-component model proposed by Anders [2], in which a volatile-rich component (matrix, of CI composition) accretes with volatile-depleted chon-drules. Wasson and Chou [3], observing a monotonic decrease in volatile abundance with decreasing condensation temperature, proposed the incomplete condensation model, in which a solar gas is dissipated during condensation. Subsequent studies, in particular experiments by Palme and co-workers which showed that volatile fractionation patterns cannot be reproduced by heating chondritic materials (e.g. [4]), and modeling of nebula conditions by Cassen (e.g. [5]), have supported the incomplete condensation model, and it is now broadly favoured. Although both models were based on measurements of trace element abundance in bulk chondrites, they make specific predictions about the composition of matrix. More recently, the x-wind model [6] has also made predictions about matrix composition. In the x-wind model, high-temperature processing of CAIs