A chemical study of the minerals from L-6 chondritic meteorites Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/m613n0943

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  • Major and minor minerals have been separated from three L-6 chondritic meteorites (Alfianello, Colby (Wisconsin), and Leedey). Olivine, hypersthene, diopside, feldspar, troilite, chromite, phosphate minerals, and metallic minerals were analyzed for major, minor and trace elements using instrumental neutron activation analysis and radiochemical neutron activation analysis techniques. Concentrations df the following elements were determined in some or all of the minerals which were separated: Al, Na, Ca, K, Rb, Cs, Cr, V, Mn, Mg, Zn, Sc, Fe, Ni, Co, Au, Se, S, Cl, La, Ce, Nd, Sm, Eu, Tb, Tm, Yb, Lu. The major element abundances were used primarily to evaluate contamination between minerals. An attempt was made to quantify the cross contamination and correct the trace element abundances accordingly. The distribution of elements within minerals, the partitioning of elements between minerals, and the chemical-mineralogical relationships suggest that many minor and trace elements have been distributed in an orderly fashion in L-6 chondritic meteorites. The Rare Earth Elements (REE) have well defined differences in their absolute and relative abundances in the same minerals from the three meteorites. These chondrites were not in equilibrium with each other with respect to the REE. Neither heterogeneous distribution of accessory minerals nor variations in the absolute and relative abundances of REE in minerals from different meteorites can explain the subtle fractionation of REE between different L chondrites (Masuda et al., 1973). The differences in REE distributions in the same minerals from the three meteorites probably represent various degrees of attainment of equilibrium or equilibration under different physical-chemical conditions. The REE fractionations between different meteorites probably pre-date the present chemical-mineralogical relationships. The relative abundances of Ni, Co, and Au in fine grained metallic minerals differ from the abundances of these elements in the bulk of the metallic phases. Differences in the abundance of Ni can be attributed to various proportions of kamacite and taenite in the analyzed samples. Cobalt and Au abundances do not correlate with Ni; therefore, variations in the abundances of Co and Au are apparently not related to the relative proportions of the metallic minerals. The distribution of Ni and Co between non-metallic meteoritic minerals is different than the distribution between analogous terrestrial minerals. The abundances probably reflect equilibrium between metallic and non-metallic minerals. The last temperatures at which Ni was equilibrated between metal and olivine was calculated from pertinent thermodynamic relationships. The calculated temperatures indicate that the last temperature of Ni equilibration in the three meteorites decreased in the following order: Colby > Alfianello > Leedey. Rubidium and Cs are concentrated in the feldspar minerals. Rubidium is uniquely associated with the feldspars within the sensitivity of the techniques. Cesium is nearly ubiquitous among the meteoritic minerals. Quantities of Cs in minerals other than feldspar probably reflect the presence of Cs at grain boundaries and in interstitial sites. The Rb/Cs ratio in feldspars from all three meteorites vary considerably. This ratio may be an indicator of thermal events in the history of the meteorites in which Cs was volatilized to various degrees. The largest Rb/Cs ratio which has been identified in the whole rocks is nearly the same value as this ratio in the feldspar minerals. This may indicate that the interstitial Cs enriched phase is the volatile Cs component. If the Rb/Cs ratio is an indicator of some thermal event, the relative temperature in the three meteorites decreases in the following order: Leedey > Alfianello > Colby. This ordering of relative temperatures is reversed compared to the relative temperatures based on the equilibration of Ni between olivine and metallic minerals. The two temperature estimates may reflect the effective temperatures of two different thermal events in the petrogenesis of the three meteorites. Zinc and Mn have nearly identical geochemical affinities in these L-6 chondrites. There is no indication of the mechanism which depleted the abundance of Zn in ordinary chondrites relative to the abundance in C-1 carbonaceous chondrites.
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