The Properties of Carbonated Fluids in the Systems Na2CO3-H2O and K2CO3-H2O to 1000°C and 20 Kbar

Abstract

Fluids and melts are generally accepted to be the principle agents of metasomatism in the Earth's crust and upper mantle. In the lithospheric mantle, for example, they are assumed to be responsible for the 'cryptic' enrichment in light REE and large-ion lithophile elements of many peridotite xenoliths hosted by alkali basalt (e.g. Harte et al., 1987) and for modal metasomatism linked to the production of phlogopite, amphibole and Fe-Ti rich phases in xenoliths found in kimberlites (e.g. Dawson, 1987). In the mantle wedge overlying subduction zones hydrous fluids or melts appear to transport material with high B, U/Th and Pb/Ce (Brenan et al., 1995) while CQ-rich fluids and melts have been connected with enrichment in the rare earth elements without concomitant enrichment in their high field strength elements (Ti, Zr, Nb, Ta etc.). In practice, however, these ideas about the effects of fluids are based mainly on petrographic and geochemical evidence; they are empirical and of dubious validity. The problem of how elements, particularly incompatible trace elements, are transported in the mantle and in the lower crust is geologically very important because these elements are the ones used to deduce temperatures and patterns of melting beneath midocean ridges, island arcs and ocean islands (e.g. McKenzie and O'Nions, 1992). If the stabilities of the fluids and melts and their potential to transport material are poorly known, then deductions about the melting processes are poorly constrained. Natural and synthetic carbonate melts appear to produce strong metasomatic enrichment in the light R E E and Ba, high Zr/Hf and low Ti/Eu (e.g. Rudnick et al., 1993). However, little is known about the nature of carbonate melts in the Earth's mantle and crust. In this study phase relations of the carbonateH20 fluids axe presented.