Compositions of Fluids in Equilibrium with Peridotite: Implications for Alkaline Magmatism-Metasomatism

Abstract

Estimates of the oxidation state of upper mantle source regions for alkaline magmas suggests that those regions are moderately oxidized, with fO 2 between the QFM and MW buffers. At such fO 2 's, mantle fluids should be H 2 O-CO 2 rather than CH 4 -H 2 O. Solubilities of oxides in H 2 O and H 2 O-CO 2 fluids in equilibrium with amphibole peridotite, with phlogopite peridotite, and with several single minerals have been determined at 15-20 kbar pressure and 600°-1100°C. Hydrous fluids dissolve much more solute than fluids containing CO 2 . In general, the solutes are rich in normative quartz and feldspars, rather than in alkalies alone, and are not particularly enriched in Mg, Fe, or Ti. Fluids in equilibrium with phlogopite peridotite contain much more solute than fluids in equilibrium with amphibole peridotite. Experiments on amphibole peridotite and on jadeite peridotite indicate that solutes in equilibrium with H 2 O-CO 2 have molar Na/Al > 1, unlike H 2 O solutes, suggesting Na 2 CO 3 complexing. Solubility of SiO 2 in H 2 O-CO 2 fluids is suppressed relative to other oxides. Regions of the subcontinental mantle and, possibly, the suboceanic mantle at a depth near 70 km, which corresponds to the lower pressure limit of carbonate stability and the upper limit of amphibole stability, can be predicted to be regions of concentrated metasomatic enrichment. These regions could yield alkaline and carbonate-rich magmas upon melting.