FLUIDS IN EQUILIBRIUM WITH PERIDOTITE MINERALS: IMPLICATIONS FOR MANTLE METASOMATISM

Abstract

Solubilities of oxides in H2O and H2O-CO2 fluids in equilibrium with peridotites and with several single minerals have been determined at 15–20 kbar pressure and 600–1100°C.Solutes from H2O fluids are rich in normative quartz and feldspars, rather than in alkalies alone, and are not enriched in Mg, Fe, or Ti. Fluids in equilibrium with phlogopite peridotite contain much more solute than fluids in equilibrium with amphibole peridotite. Typical reactions for solution of phlogopite peridotite and amphibole peridotite in H2O fluid are, respectively, in weight proportions: 1.05 phlogopite + 1.33 orthopyroxene + 8.62 H2O = 9.62 fluid + 1.38 olivine0.59 amphibole + 1.64 orthopyroxene + 0.05 spinel + 32.33 H2O = 33.33 fluid + 1.85 olivine + 0.43 diopsideExperiments on amphibole peridotite and on Jadeite peridotite indicate that solutes in H2O-CO2 fluids have molar , unlike H2O solutes. By analogy, H2O-CO2 solute in equilibrium with phlogopite peridotite is less peraluminous than H2O solutes. These relations suggest alkali-CO3 complexing in fluids. Solute contents of H2O-CO2 fluids are far less than of H2O fluids, however, even for low ratios.Fluids that equilibrate with peridotitic wallrocks in subsolidus continental lithosphere, at depths in excess of about 70 km, are constrained by phase equilibria to be H2O-rich and to coexist with phlogopite and carbonate. Upward-moving solute-undersaturated hydrous fluids may leach that portion of the lithosphere; the greatest relative depletion, among major elements, will be in K. Fluids must remain saturated during precipitation reaction with wallrocks, however, and hence large fluid/rock ratios are required for mantle metasomatism at depths below 70 km. Melts are more efficient metasomatic agents, in the sense that only small ratios of crystallizing melt/rock are required.Hydrous fluids that flow upward through the phlogopite-carbonate region of the lithosphere will precipitate, at depths shallower than about 70 km, by amphibolitization-carbonatization of peridotite wallrocks. That process will add small amounts of major elements, notably K, relative to amphibole produced (on the order of 1 K2O to 3700 amphibole). Nevertheless, integrated over time, the process may produce a metasomatized zone that represents a possible source for alkaline magmatism.