PARTITIONING OF ELEMENTS BETWEEN SILICATE MELT AND H2O-NACL FLUIDS AT 1.5 AND 2.0 GPA PRESSURE: IMPLICATIONS FOR MANTLE METASOMATISM

Abstract

Partition coefficients between a synthetic andesite melt and 1.5 and 3.0 molal (m) NaCl-H2O fluids have been measured at 1.5-2.0 GPa and 1250°C. Use of the double-capsule method allowed collection of silicate glass and of solute quenched from the fluid. Measured solubilities of silicate melt in both 1.5m and 3.0m NaCl fluid are 51 wt% at 1.5 GPa and 53 wt% at 2.0 GPa, much higher than solubilities measured for pure H2O at the same conditions. Wet chemical analysis of the run products yielded fluid/melt partition coefficients (Dfluid/melt) ranging from 0.43 to 1.31 for Ca, Mg, Na, K, Rb, Sr, La, Sm, Y, Tm, and Ti, suggesting a near-congruent dissolution of the melt in fluid. D values for many elements increase in direct proportion to melt solubility, as expected for congruent solution. Concentrations of alkalis in the fluid positively correlate with total Cl concentration, suggesting that Cl complexes with alkalis in the fluid. Results imply that NaCl-bearing aqueous fluids can dissolve large amounts of silicate material, but do not strongly fractionate elements in equilibrium with silicate melts. Thus, the geochemical signature left by metasomatic H2O-NaCl fluids will not be distinctly different from that of silicate melts.