Department of Chemistry, Faculty of Science, Tokyo Metropolitan University
Abstract
A chemical dissolution method based on different leaching rates of kamacite, taenite and tetrataenite in concentrated HF was developed for isolating metal phases from non-magnetic minerals (mainly silicates) in chondritic meteorites, and for separating taenite from kamacite. This method was applied to an Antarctic chondrite, ALH-77231 (L6), to study the elemental distributions between metal and non-magnetic phases and between taenite and kamacite. The separated phases as well as the bulk metal were studied by Mossbauer spectrometry and by INAA. Mossbauer spectra of ^Fe show that our procedure is efficient in removing silicates from metal and separating taenite from kamacite. The existence of tetrataenite and the absence of disordered taenite in ALH-77231 metals imply that the parent body of ALH-77231 has not experienced severe shock events that redistribute elements among different phases. Thus, element distributions among different phases reflect, in general, the metamorphic history of ALH-77231. INAA results show that siderophile elements are differently distributed between taenite and kamacite. Most siderophiles have very strong to moderate affinities to taenite, while Co is enriched in kamacite. Refractory siderophile elements seem to behave as an individual group. Distribution of Ir between the metal and the non-magnetic fractions implies that a portion of Ir is present in the silicate fraction. This portion of Ir cannot be attributed to the contamination of FeNi metal in the silicates. While the Os/Ir rations in the metals of ALH-77231 are in agreement with the CI value, the Re/Ir rations are significantly higher than the CI value, indicating that Re is fractionated from Ir and Os, between the metal and the non-magnetic fractions. This may be attributable to either the later redistribution of Re between metal and non-magnetic fractions or differences in the condensation phases for Re, and for Ir and Os
