Partitioning of transition elements between orthopyroxene and clinopyroxene in peridotitic and websteritic xenoliths: new empirical geothermometers

The partitioning of transition elements (Sc, Ti, V, Cr, Mn, Co, Ni) between orthopyroxene (opx) and clinopyroxene (cpx) in carefully selected garnet peridotite, spinel peridotite and garnet websterite xenoliths was determined by electron probe microanalyses (EPMA) and secondary ion mass spectrometry (SIMS). Xenoliths studied cover a wide compositional range and equilibrated under variable upper mantle conditions at temperatures between about 760 and 1370°C (two-pyroxene thermometer based on the enstatite-diopside solvus) and pressures between about 0.8 and 3.6 GPa (Al-in-opx and Ca-in-olivine barometers). We found that the partitioning of transition elements between opx and cpx (expressed as D_M = concentration of element M in opx cations per formula unit]/concentration of M in cpx cations per formula unit]) is mainly controlled by temperature and to a much lesser degree by pressure. Variations in major element compositions of pyroxenes (e.g., variable X_Mg, Al^IV or Na) have no influence on D_M. For Sc, V, Cr, Mn, and Co, our data result in good correlations between ln D_M and reciprocal absolute temperature, with correlation coefficients (r) between 0.950 and 0.981. It is therefore possible to use the partitioning of these elements between opx and cpx from peridotites and websterites as geothermometers. On the basis of our data, we suggest the following empirical thermometer equations: T_Sc = (17.64 · P + 5663)/(3.25 − ln D_Sc)], T_V = (18.06 · P + 3975)/(2.27 − ln D_V)], T_Cr = (11.00 · P + 2829)/(1.56 − ln D_Cr)], T_Mn = (−0.20 · P − 2229)/(−1.37 − ln D_Mn)], T_Co = (−4.31 · P − 2358)/(−0.98 − ln D_Co)], where T is the absolute temperature in Kelvin and P the pressure in kilobars. For Ti and Ni observed correlations between ln D_M and 1/T are less well defined.