Effect of melt structure on trace-element partitioning between clinopyroxene and silicic, alkaline, aluminous melts

Abstract

We have investigated trace-element partitioning between clinopyroxene and silicic melts ranging from 62 to 69 wt% SiO 2 in piston-cylinder experiments at 1.1 GPa and 1145-1155 °C. Run products consist of diopsidic clinopyroxene (with 3-5 wt% Al 2 O 3 and ∼1 wt% Na 2 O) coexisting with silicic, aluminous, and alkaline melts. Clinopyroxene-melt partition coefficients for rare earth elements (REE) are significantly higher (e.g., D Sm = 0.76 to 3.1) than previous studies of clinopyroxene-basaltic melt partitioning. The maximum D REE is also shifted toward middle REE (e.g., Tb). Although the partition coefficients of Sr and Ti (D Sr = 0.23-0.39, D Ti = 0.28-1.0) are also elevated relative to clinopyroxene-basaltic melt, partition coefficients for Zr and Li (0.14-0.23 and 0.16-0.25, respectively) are within the range of literature values. The high partition coefficients for REE, Sr, and Ti in this study do not reflect crystal-compositional effects. For instance, no relationship is observed between IV Al and these partition coefficients. REE and Ti partition coefficients correlate poorly with jadeite component of the clinopyroxene; however, the elevated Na 2 O contents of both the melt and clinopyroxene make it difficult to discriminate fully the effect of jadeite component from the effect of changes in melt structure. The non-bridging oxygen to tetrahedral cation ratio (NBO/T; Mysen et al. 1985) varies from 0.08 to 0.17 and roughly correlates with epx/melt D REE , consistent with a recent study (Gaetani 2004) that concluded that melt structure significantly affects partitioning when NBO/T < 0.49. Notably, we find a strong correlation between the molar Ca 2+ /(M + + M 2+ ) of the melt and the optimum D (D 0 ) for the REE, indicating that melt structure, and not jadeite content, is the dominant control on clinopyroxene-melt partitioning in these experiments. The partition coefficients for the REE, Ti, Zr, and Sr are in good agreement with observed clino-pyroxene-silicic glass partitioning within mantle xenoliths (Chazot et al. 1996; Vannucci et al. 1998), indicating these results may apply to lithospheric melting. Furthermore, the origin of hump-shaped REE patterns observed in residual clinopyroxene from abyssal peridotites could be explained if shallow depleted melts of peridotite are silica-rich. A two-stage fractional melting model assuming clinopy-roxene-silicic melt partitioning at shallow mantle depths from 10 to 30 km beneath a mid-ocean ridge can reproduce the observed hump-shaped REE patterns. If so, hump-shaped REE patterns may provide evidence for the existence and extraction of silica-rich melt created in the shallow mantle.