Abstract

The structure of phosphorus-bearing, H2O-saturated silicate melts, silicate-saturated aqueous fluids, and silicate-rich single phase (supercritical) liquids has been characterized in situ to 800 °C and 1486 MPa in an Ir-gasketed hydrothermal diamond-anvil cell (HDAC) with the aid of both confocal microRaman and FTIR spectroscopy. Temperature and pressure in the HDAC were recorded with thermocouples (±1 °C uncertainty) and pressure- and temperature-dependent Raman shift of 13C diamonds (±40 MPa uncertainty). Starting materials were aluminum-free Na2O·4SiO2 (NS4) and with 10 mol% Al2O3 (NA10) substituting for SiO2, both with 5 mol% P2O5.

Aluminosilicate species of Q0, Q1, Q2, and Q3 type exist in coexisting fluid and melt and in single phase liquid together with phosphate species, PO4, P2O7, and QnP. In the QnP species, n O atoms bridge between Si4+ and P5+, whereas there is no Si-to-P linkage in the PO4 and P2O7 species. In melts, the abundance of the most depolymerized silicate species, Q0, is positively correlated with temperature and pressure, whereas that of the most polymerized species, Q3, decreases with temperature and pressure. In the silicate solute of aqueous fluids, the opposite relationship exists with Q0 abundance decreasing and Q3 (and Q1 and Q2) abundance increasing with increasing temperature and pressure. The silicate melts, therefore, become increasingly depolymerized and the silicate solute in aqueous fluids decreasingly depolymerized because increasing H2O solubility in melts causes melt depolymerization, whereas increasing silicate solute abundance in fluids results in silicate polymerization. The P2O7 and QnP are the dominant phosphate species in fluid, melt, and single phase liquid with orthophosphate, PO4, playing a subordinate role. The fluid/melt partition coefficients for P2O7 and QnP species are in the 0.15–0.7 range with that of QnP being greater than that of P2O7. The PO4 fluid/melt partition coefficients are <0.2. In all cases, the partition coefficients increase with increasing temperature and pressure. There is no clear influence of Al2O3 on partition coefficients. Hence, it appears that P-bearing complexes in fluids and melts are associated with Na+ that becomes available as silicate species polymerize. The mobility of phosphorus during metamorphic processes is principally governed by the availability of alkali metals (and perhaps alkaline earths).

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