CORRELATED SEM-EDX AND μXRD SURVEY OF THE TAGISH LAKE C2 CARBONACEOUS CHONDRITE

Micro X-ray diffraction (μXRD) is a new technique in meteoritics which extends X-ray diffraction to the micron scale, allowing both in situ examination of crystal structure and correlation with other microanalytical techniques. μXRD can also provide microstructure, texture and strain information via 2-D detection of the diffracted X-ray pattern. μXRD is particularly valuable when combined with a chemical analysis technique such as SEM-EDX. The two in situ survey techniques are highly complimentary: μXRD provides crystal structure, SEM-EDX the chemistry, thus enabling more confident mineral identification. These coordinated surveys also provide an opportunity to identify areas of interest for further, more detailed study.

A point-correlated μXRD and SEM-EDX survey of the Tagish Lake (C2, ungrouped) carbonaceous chondrite has produced both new insights and new questions: 1) Some ferromagnesian silicate inclusions (amoeboid olivine aggregates (AOA), chondrules, fragments) are rimmed by fine grained sulphides (pyrrhotite, minor pentlandite), and some are not. These are not from different brecciated fragments of the meteorite, nor do the type and composition of the silicates vary systematically between rimmed and unrimmed clasts; 2) Many large sulphide grains appear to be enriched in P. These sulphide grains may have formed by sulphidation of Fe-Ni metal in the presolar nebula, or by direct condensation in a high fS region of the nebula. In either case the P enrichment implies that these sulphides are primary; 3) An assemblage of magnesioaluminate spinel, dolomite and aluminous phyllosilicates may be an aqueously altered Ca-Al rich inclusion (CAI) that was processed in high temperature fluids, at a spatially limited scale or at some time before its final inclusion in the matrix.

A previously-noted similarity between Tagish Lake and CR chondrite matrix has also been explored by in situ μXRD analysis of Tagish Lake and CR 2 chondrite Gao-Guenie B matrix. Several key differences have been found: Carbonates are much more abundant in Tagish Lake matrix with siderite predominant over calcite. Anhydrous silicates detected in Gao-Guenie B matrix are forsteritic olivine, clinopyroxene, and anorthite, whereas only forsteritic olivine was detected in Tagish Lake matrix.