EXPERIMENTALLY DETERMINED RARE-EARTH ELEMENT AND Y-PARTITIONING BEHAVIOR BETWEEN CLINOPYROXENE AND BASALTIC LIQUIDS AT PRESSURES UP TO 20 KBAR

A series of experiments using five natural basalts was conducted to investigate the effects of pressure, temperature and composition on REE: and Y partitioning between clinopyroxene and melt. The starting materials span the compositional range from alkali basalt to island are basalts to mid-ocean ridge basalt. They were doped with wt% levels of Y, La, Sm, Cd and Lu in order that the charges could be analyzed by electron microprobe. Our analysis showed positive correlations between D-REE and a variety of parameters including Ca concentration in pyroxene, D-Ti, and Al concentrations in pyroxene and melt. Partition coefficients for the REE vary by over a factor of 2 for any given bulk composition, and by up to a factor of 10 over the range of compositions and conditions investigated. This wide range of partitioning behavior, and the sensitivity of partitioning to composition, demonstrates that care must be taken in selecting partition coefficients for petrogenetic models. To numerically describe REE partitioning behavior, we parameterized the data in terms of pressure, temperature and compositional variables using multiple linear regression analysis. The resulting equations reproduce the calibration data set within 10-32% relative error. The magnitude of the regression constants indicates that temperature and the phase composition have large effects on partitioning behavior. The regression constant for pressure was near zero, probably because the effects of pressure are included in the compositional and temperature regression constants.