Clinopyroxene + liquid equilibria to 100 kbar and 2450 K

Abstract

One of the most active issues in igneous petrology is the investigation of mantle melting, and subsequent differentiation. To evaluate alternative hypotheses for melting and differentiation it is essential to accurately predict clinopyroxene compositions in natural systems. Expressions have thus been derived that describe clinopyroxene-melt equilibria, and allow equilibrium clinopyroxene compositions to be calculated. These equations were constructed from least-squares regression analysis of experimental clinopyroxene-liquid pairs. The calibration database included clinopyroxenes synthesized from both natural and synthetic basalt compositions; experimental conditions ranged from 0 to 100 kbar and 1350 to 2450 K. Regression equations were based on thermodynamic functions. Empirical expressions were also derived, since such models yield more precise estimates of clinopyroxene compositions, and may be easily incorporated into existing liquid line-of-descent models. Such equations may be useful for calculation of high pressure liquid fractionation, or for constraining P-T conditions for basalts produced by partial melting of a pyroxene-bearing source. Models of mantle melting often rely on expressions involving simple element ratios. Partition coefficients (K _d ^cpx/liq) for the minor elements, Na and Ti, were thus also calibrated as a function of P, T and composition. K _Ti ^cpx/liq, while sensitive to composition was relatively insensitive to P and T. In contrast, K _Na ^cpx/liq increases substantially with increasing P, and exceeded 1 in some experiments. Since oceanic basalts show variations in Na/Ti ratios, the potential exists for partial melting depths to be inferred from K _Na ^cpx/liq.