The effect of H 2O on crystal-melt partitioning of trace elements

Abstract

Many experimental data demonstrate distinct differences between crystal-liquid partition coefficients D i measured under high temperature, anhydrous conditions and those determined at lower temperatures in the presence of H 2O. In this study we develop a thermodynamic method of separating the effects of H 2O from those of temperature. We then apply the method to predict partitioning of REE between clinopyroxene, garnet and silicate melt over wide ranges of temperature, pressure and H 2O content. Our initial inputs are the model of Wood and Blundy (1997) for REE partitioning between clinopyroxene and anhydrous melt and the melting temperatures of diopside on the join CaMgSi 2O 6-H 2O (Eggler and Rosenhauer, 1978). We then make the hypothesis that the effect of H 2O on the activity of REE clinopyroxene component (REEMgAlSiO 6) in the melt is the same as the measured effect on CaMgSi 2O 6 component. This leads to predictions of REE partition coefficients for clinopyroxene coexisting with hydrous melt at any P,T and H 2O content up to 45 weight %. The results agree with observed REE partition coefficients with a standard deviation which is the same as that for the anhydrous data. We conclude, therefore, that the ‘H 2O-effect’ may, in this case, be accurately predicted. We extend the approach to garnet by using the join Mg 3Al 2Si 3O 12-H 2O to estimate the effects of H 2O on all ‘garnet-like’ components in the melt. This enables calculation of garnet-melt REE partition coefficients for melts containing up to 25% H 2O. The observation that H 2O influences major and trace component activities in a similar manner enables us to make some generalisations about the combined effects on partitioning of decreasing temperature and increasing water content of the melt. The relative enthalpies of fusion ΔH f of major and trace components dictate whether trace element partition coefficients increase or decrease with H 2O addition: ΔH f trace > ΔH f major gives increasing D trace with addition of H 2O and ΔH f trace < ΔH f major leads to decreasing D trace with addition of H 2O. Note that D trace is strictly the ratio of mole fractions of charge-balanced components such as REEMgAlSiO 6 in solid to melt phases. In the cases considered here, however, D trace closely approximates D = [trace] solid [trace] melt where [trace] refers to weight concentration. For clinopyroxene ΔH f REE < ΔH f major and H 2O addition leads to substantial decreases in REE partition coefficients. For garnet, ΔH f REE ≈ ΔH f major and addition of H 2O results in little change in D REE. The consequences are illustrated with respect to Yb contents of melts of spinel lherzolite. For partial melting under anhydrous conditions, Yb is relatively compatible in clinopyroxene and the Yb contents of the product melts remain close to 5 times source values at all F (fraction of equilibrium batch melting) between 0 and 0.25. Hydrous peridotite produces melts, because of low D Yb, which are strongly enriched in Yb (up to at least 10 times source) and whose Yb concentrations depend strongly on F. This means that the low Yb concentrations observed in many island arc tholeiites cannot be due to the addition of water to fertile peridotite.