High-pressure experimental trace-element partitioning between clinopyroxene and basaltic melts

Abstract

Clinopyroxenes play an important role in determining the distribution of trace elements in magmatic systems. In order to evaluate the extent and source of variation of clinopyroxene-silicate melt partition coefficients ( D), experiments were conducted on natural basalts (picrite, alkaline basalt, nephelinite, high-alumina basalt; 1235–1300°C; 1–2.8 GPa) doped with a multi-element spike (Ti, Cr, V, Sr, Y, Zi, Nb, Ta, Hf, La, Ce, Nd, Sm, Dy and Yb) and water (2%). Trace-metal concentrations were determined by synchrotron XRF (SXRF) microprobe and a subset of samples were analyzed by SXRF superconducting wiggler and ion microprobe (IMP). Mid-range D-values (by IMP) for clinopyroxene in a picritic starting composition at 1300°C and 1 GPa are: D ti, 0.273; D V, 2.77; D Cr, 7.32; D Sr, 0.079; D Y, 0.376; D Zr, 0.089; D Nb, 0.003; D Hf, 0.179; D Ta, 0.010; D La, 0.03; D Ce, 0.054; D Nd, 0.112; D Sm, 0.201; D Dy, 0.280; and D Yb, 0.255. D-values for Ta and Zr are lower than adjacent D-values for REE (La and Nd, respectively) on a spider diagram; however, there is no decoupling observed between D Ti, D Hf and adjacent D REE ( D Y,Yb). Many of the experiments resulted in sector-zoned augite, and these constrain the nature of crystal chemical controls on partitioning at fixed T,P,X. Sector-zoned augite in an alkaline basalt at 1250°C and 1 GPa shows enrichment in Ti, Al, Ca, Cr, trace HFSE and REE and depletion in Si, Mg and Fe in the fast-growing (100) sector relative to the slower growing (010) sector. Reverse-zoning in incompatible elements in the (100) sector adjacent to normally-zoned (010) sectors confirms that sector zoning in augite arises from differences in surface kinetic processes (adsorption-desorption) during crystal growth. A positive correlation between IVAl and HFSE concentrations emphasizes the importance of coupled substitution involving highly charged cations in M sites and Al in T sites.