ABSTRACT

The partition coefficients of rare earth elements (REE) and Sr between olivine and silicate melt and those of Ga, In and TI between metallic phase and silicate melt were experimentally determined under atmospheric pressure using L6 chondrite (ALH-76009) as a starting material. Temperature and oxygen fugacity were finely controlled; temperature = 1320, 1370, 1420 and 1440°C and oxygen fugacity = 0.5, 1.0, 1.5 and 2.0 log units lower than the Fe-FeO boundary on the iron-oxygen phase diagram. Olivine formed at 1320°C shows REE partition patterns with horizontal or heavy-REE-enriched features, having partition coefficients from 0.02 to 0.08. Inclination of light-REE increases with increase of temperature of olivine formation. Finally, at the temperature of 1440°C near the liquidus of the chondrite, olivine/liquid partitioning of REE displays a V-shaped feature with partition coefficients of 0.15-0.8 for La, 0.08-0.3 for Lu and 0.05-0.09 for Dy and Er. The V-shaped REE patterns are known to be characteristic of olivine in the main group pallasites, for which our REE results imply the formation at higher temperatures near the liquidus of olivine. Our results of major element partitioning also support the idea that such pallasites were formed at a temperature near the liquidus. Experiments of this study show that compositions of olivine formed at 1440°C are Fa₈-Fa₁₃ comparable to those of the main group pallasites, while those formed at 1320°C are Fa₁₅-Fa₁₈. The REE concentrations in the ferrous olivine of two pallasites, Springwater (Fa_18.0) and Eagle Station (Fa_20.0), were determined here. It was found that their chondrite-normalized REE patterns also show V-shaped features, as observed for the olivine of the main group pallasites, the Brenham (Fa_12.5) and Thiel Mts. (Fa_13.0) pallasites. These results suggest that the olivines of Springwater and Eagle Station were also formed at temperatures near liquidus around 1450°C. The difference in major element compositions between the main group pallasites (Brenham and Thiel Mts.) and the other pallasites (Springwater and Eagle Station) may be due to differences in major element compositions of their sources, oxygen fugacity at their formation and/or the cooling history of the pallasites. In the last case, it is reasoned that at lower temperatures (e.g., ca. 1300°C), Fe and Ni in olivine of non-main group pallasites were reequilibrated with those in the metal phases, but V-shaped REE patterns were retained because the diffusibility of REE is lower than those of Fe and Ni. The main group pallasites are considered to be free from the lower temperature reequilibrium processes.