Experimental study on the pseudobinary H2O + NaAlSi3O8 at 600–800 °C and 0.3–2.4 GPa

Abstract

The pseudobinary H2O+NaAlSi3O8 was studied at compositions between 35 and 66 mass% NaAlSi3O8, at temperatures between 600 and 804°C, and at pressures ranging from 0.3 to 2.41GPa using a hydrothermal diamond-anvil cell (HDAC), cold-seal hydrothermal pressure vessels (CSPVs), and a piston-cylinder apparatus (PC). We propose a revised pseudosection for bulk compositions near 40 mass% NaAlSi3O8, the critical composition at 750°C obtained from the HDAC experiments. The corresponding critical pressure was 1.56GPa, as determined directly using zircon as Raman spectroscopic pressure sensor. Pressures at the melt-fluid homogenization temperature and along six isochoric P–T paths obtained from the Raman spectrum of zircon were close to or slightly lower than the pressure calculated from the EoS of H2O based on the vapor dissolution or ice melting temperature, which demonstrates that the effect of albite or NaAlSi3O8 melt dissolution is small. In an experiment up to 700°C and 2.28GPa, no clear evidence was found from optical observation for the hypothetical reappearance of melt-fluid immiscibility at higher pressure.The results consistently point to incongruent behavior at elevated pressures by the formation of paragonite. The HDAC experiments showed that paragonite nucleation was generally sluggish and occurred usually within several hours to days. For the critical composition of 40 mass% NaAlSi3O8, the temperature limit of paragonite stability was determined to a pressure of 1.2GPa using CSPVs and a PC and found to increase significantly with pressure. There was no discernible difference in its P–T location from that of the paragonite breakdown reaction to corundum, albite, and water reported in the literature for pressures to 0.7GPa. The paragonite stability limit intersected the critical curve at 764°C, 1.53GPa. Hence, our experiments indicate the existence of three previously not recognized stability fields in pseudosections for bulk compositions near 40 mass% NaAlSi3O8, namely (1) paragonite+albite+fluid, (2) paragonite+melt+fluid, and (3) paragonite+fluid. Formation of a substantial fraction of paragonite at these NaAlSi3O8 concentrations implies that the aqueous fluid must have a peralkaline composition, which has important implications for mass transfer in the crust an upper mantle, e.g. enhancement of the solubility of high field strength elements.