Experimental study on rare-earth element partitioning in minerals formed at 20 and 30kb for basaltic systems

Abstract

Crystal/glass partition coefficients of rare-earth elements (REE), Ba and Sr were precisely determined for (he systems prepared by partial melting and partial solidification of high-alumina basalt at 30kb and alkali olivine basalt at 20 and 30kb. At 30kb partial melting produced garnet and clinopyroxene as the solid phase, while partial solidification at the same pressures produced only clinopyroxene as an essential mineral. The difference in mineral composition of the solid phase appears to be due to the difficulty of nucleation of garnet from the melts. Because of the higher tendency of garnet to enrich heavy REE compared with clinopyroxene, the bulk solid phase produced by partial melting shows much higher enrichment of heavy REE compared with that produced by partial solidification. At 20kb both partial melting and partial solidification produced only clinopyroxene as the essential mineral. However, partition patterns for the systems produced by partial melting and those by partial solidification show small but systematic differences. REE partition patterns are terrace-shaped for clinopyroxenes, which are formed by partial solidification at 30kb and by partial melting at 20kb, while REE partition pattern is modified peak-shaped for clinopyroxene formed by partial solidification at 20kb. But, rectilinear character with atomic number is commonly observed for these two REE partition patterns for clinopyroxenes. In partial melting at 30kb which produced garnet and clinopyroxene as the solid phase, on the other hand, REE partition pattern for the total solid is smoothly curved with atomic number and schanze-shaped. The results of the present work suggest that liquid magmas produced by partial melting and partial solidification at the same temperatures and pressures from the initial material with the same chemical composition could show widely different trace element abundances. It should also be emphasized that such kinetic effect on both mineral assemblage in the solid phase and on the (bulk solid)/(liquid) partition coefficients must be taken into account in discussing the origin and evolution of magmas on the basis of trace element partitioning. Further, it should be added that, in partial solidification, formation of clinopyroxene essentially free from other minerals prevails in wide fields of chemical composition of liquid and physical conditions of pressure and temperature. It is emphasized in this study that rectilinear natures of REE partitioning for this clinopyroxene are very favorable in interpreting REE patterns of many igneous rocks. © 1980, GEOCHEMICAL SOCIETY OF JAPAN. All rights reserved.