Trace element partitioning on the Tinaquillo Lherzolite solidus at 1.5 GPa

Abstract

The partition coefficient ( D) for any trace element in the mantle is a function of pressure, temperature and phase composition, the latter of which is controlled by the relative fertility of the peridotite. Models of mantle melting therefore require accurate determination of partition coefficients along the peridotite solidus. We have experimentally determined partition coefficients for clinopyroxene, orthopyroxene and olivine in equilibrium with melt, close to the anhydrous solidus of refractory Tinaquillo Lherzolite at 1.5 GPa, 1315 °C. All trace elements, apart from Sc, are incompatible in clinopyroxene, and the maximum D REE is D Er at 0.76. All trace elements are incompatible in orthopyroxene; the most compatible REE is Lu ( D Lu=0.18). Partition coefficients for olivine are generally <0.005, and only Ti and Li are moderately compatible ( D Ti=0.10; D Li=0.29). Compared to a previous study of trace element partitioning at the solidus of a more fertile peridotite (mid-ocean ridge basalt (MORB)-pyrolite-90, MPY-90) at 1.5 GPa, it is shown that, at fixed pressure, partition coefficients, especially for clinopyroxene, generally increase with increasing source fertility. D U/ D Th for clinopyroxene is 0.86±0.02 and for orthopyroxene is 1.49±0.03, significantly lower than D U/ D Th ratios previously determined for MPY-90 at 1.5 GPa. When compared with pyroxene partition coefficients experimentally determined on the MPY-90 solidus at 3.0 GPa, the partition coefficients for the majority of trace elements increase as pressure decreases along the mantle solidus. Notable exceptions are Li and Na which become more compatible as pressure increases along the solidus. The combined effects of pressure, temperature and source fertility mean that partition coefficients increase with decreasing pressure during decompression mantle melting beneath mid-ocean ridges. By expressing the variation in D as a function of melt fraction ( F), the new data can be used to model decompression melting taking into account changes in partition coefficients as melting proceeds. We show that the trace element composition of residual (or “trapped”) melts is sensitive both to source mineralogy (i.e. garnet–lherzolite versus spinel–lherzolite) and to the depth range over which melting occurs. The variation in partition coefficients during mantle melting therefore provides a valuable tool in the interpretation of melt inclusions. The variable partition coefficients have much less impact on the composition of aggregated melts.