Volatile and trace element partitioning between apatite and alkaline melts

Abstract

Apatite readily incorporates volatile and trace elements in its structure, and thus carries a record of pre-eruptive melt-fluid chemical and physical processes that play critical roles in magmatic evolution, eruption triggering, and eruptive style. However, the pressure (P), temperature (T), oxygen fugacity (fO2), and crystal-melt composition dependencies of apatite-melt elemental partition relations are only partially understood, notably for alkaline melts. Here, we report a comprehensive dataset for partitioning relations of volatiles (CO2, H2O, F, Cl, S) and 24 trace elements (including rare earth elements—REEs) between fluorapatite and phonolitic melts, based on in situ analyses of co-existing fluorapatite and melt inclusions in anorthoclase megacrystals from Erebus volcano (Antarctica). The trace monovalent cations (Li, K, Rb) have partition coefficients (D) of ≤ 0.02, lower than divalent cations (D < 0.4 for Mg, Pb, Ba, Mn; D≈ 5 for Sr) and trivalent cations (DREE + Y≈ 5–30, with Nd being the most compatible REE). We use the measured trace element partition coefficients to establish a lattice-strain model for fluorapatite and alkaline melts. Based on these observations along with experimental data from the literature, we propose a general model for estimating DREE + Y in fluorapatite and calc-alkaline/alkaline melts under a wide range of P–T conditions. We also use the lattice strain model and the Eu contents of apatite and the melt to develop a new Eu-in-apatite oxybarometer. Applying it to the Erebus fluorapatite and phonolitic melts, we find that fO2 of the system was 0.5 log units below the QFM (quartz–fayalite–magnetite reaction) buffer, consistent with the low sulphur partition coefficient we determined for apatite, and with the reduced nature of the melt reported by previous studies. The melt inclusions we analysed are much drier than the calculated melt derived from apatite-melt hygrometry, implying hydrogen reequilibration in melt inclusions during magma ascent. This has implications for magma viscosity and density, and hence for the eruptive behaviour of Erebus, as well as other open-vent volcanoes. Our generalised REE lattice-strain model is widely applicable to investigations of magma differentiation and ore formation where fluorapatite is present.