Partitioning of phosphorus between olivine, clinopyroxene and silicate glass in a spinel lherzolite xenolith from Yemen

The melting reaction: amphibole+apatite=olivine+cpx+spinel+liquid is documented in spinel lherzolite xenoliths found in Plio-Quaternary alkali basalts from Yemen (Ataq Volcano). All the phases involved in this reaction, including the melt, have been preserved within millimeter-size glassy melt pockets. In a previous study, ion microprobe analyses performed on these phases allowed determination of trace-element partition coefficients between apatite, clinopyroxene, amphibole and glass. In this study, we focus on the phosphorus distribution between apatite, clinopyroxene, olivine and glass. Silicate/melt partition coefficients for this element (Dsilicate/melt) are presently poorly known and the Yemen mantle xenoliths represent a good opportunity to obtain new data and to discuss the behaviour of phosphorus during mantle melting. Electron microprobe (EMP) analyses of the JK3 sample and two of its melt pockets in particular, yield P2O5 concentrations within the range of 0.75–1.3 wt.% in the glass, 0.07–0.15 wt.% in olivine, 0.035–0.07 wt.% in amphibole and 0.015–0.06 wt.% in clinopyroxene. Silicate/melt partition coefficients derived from EMP data on newly formed melt pocket phases (Dolivine/melt=0.1, Dcpx/melt=0.05), as well as from the inspection of the phosphorus distribution among the matrix minerals, were used in a simple batch-melting calculation. The composition of most natural basalts is well-matched for MOR basalt and an alkali basalt (and OIB) source containing 90 and 250 ppm P, respectively. From these data, and by using relevant Nd source compositions, the model is also able to reproduce the P/Nd ratio of about 70 that is typically observed in natural basalts including MORB and alkali basalts (and OIB). The relatively high phosphorus-content of the mantle silicates studied here, the corresponding new silicate/melt partition coefficients and the high solubility of apatite in basaltic melts strongly suggest that apatite is not required to host phosphorus in most upper-mantle sources of natural basalts.