Amorphous Materials: An experimental study of phosphorous and aluminosilicate speciation in and partitioning between aqueous fluids and silicate melts determined in-situ at high temperature and pressure

Abstract

The structure of phosphorus-bearing, H 2 O-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 13 C diamonds (±40 MPa uncertainty). Starting materials were aluminum-free Na 2 O·4SiO 2 (NS4) and with 10 mol% Al 2 O 3 (NA10) substituting for SiO 2 , both with 5 mol% P 2 O 5 . Aluminosilicate species of Q 0 , Q 1 , Q 2 , and Q 3 type exist in coexisting fluid and melt and in single phase liquid together with phosphate species, PO 4 , P 2 O 7 , and Q n P. In the Q n P species, n O atoms bridge between Si 4+ and P 5+ , whereas there is no Si-to-P linkage in the PO 4 and P 2 O 7 species. In melts, the abundance of the most depolymerized silicate species, Q 0 , is positively correlated with temperature and pressure, whereas that of the most polymerized species, Q 3 , decreases with temperature and pressure. In the silicate solute of aqueous fluids, the opposite relationship exists with Q 0 abundance decreasing and Q 3 (and Q 1 and Q 2 ) 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 H 2 O solubility in melts causes melt depolymerization, whereas increasing silicate solute abundance in fluids results in silicate polymerization. The P 2 O 7 and Q n P are the dominant phosphate species in fluid, melt, and single phase liquid with orthophosphate, PO 4 , playing a subordinate role. The fluid/melt partition coefficients for P 2 O 7 and Q n P species are in the 0.15–0.7 range with that of Q n P being greater than that of P 2 O 7 . The PO 4 fluid/melt partition coefficients are 2 O 3 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).