[S-IT18] GEOPHYSICAL PROPERTIES AND TRANSPORT PROCESSES IN THE DEEP CRUST AND MANTLE

[SIT18-P03] Pressure dependence of water speciation in albitic melts determined in-situ at high pressures and high temperatures

*Shigeru Yamashita¹ (1.Institute for Planetary Materials, Okayama University)

Keywords:magma, water, infrared spectroscopy

Experimental results from in-situ observation of water speciation in albitic melts at pressures to 2.4 GPa and at temperatures to 900 °C are presented. Near infrared (NIR) absorption bands assigned to the combination mode of the fundamental Si-O + O-H stretch vibrations (~4500 cm^-1), the combination mode of the fundamental H-O-H bend + stretch vibrations (~5200 cm^-1), and the first overtone of O-H stretch vibration (~7000 cm^-1) were investigated using an externally heated hydrothermal diamond anvil cell (HDAC) fitted to micro-FTIR and confocal micro-Raman spectrometers. A rough cylindrical wafer of hydrous albitic glass containing 11 wt.% H₂O and a piece of ¹³C diamond aggregate were loaded together in the sample chamber of the HDAC. Pressure in the HDAC was monitored with the pressure- and temperature-dependent Raman shift of the ¹³C diamond (Mysen and Yamashita, 2010). To ensure the highest precision of the Raman shift, the emission of the 585 nm Ne line was recorded simultaneously as reference. Temperature was controlled to ±1 °C with chromel-alumel thermocouples in contact with the anvils near the sample chamber. In the experiments, the sample was first brought to 900 °C, then NIR spectra of the melt were recorded every 100 °C while cooled (thus decompressed) at the rate of 1 °C/sec until H₂O started to be separated from the melt as bubbles. This process was repeated along different isochore paths to cover a wide coverage of pressures at high temperatures. In the melts, the 4500 cm^-1 band decreased in the intensity while the 5200 cm^-1 band grew as temperature fell. At 900 °C, the NIR spectra of the melts were indistinguishable from each other over the pressure range from 1.5 to 2.4 GPa. Similarly at 800 °C, the NIR spectrum taken at 1.3 GPa was identical with that taken at 1.9 GPa. When ideal mixing was assumed, the equilibrium constant ln K of the water section reaction H₂O + O = 2OH was obtained from the 4500 cm^-1 and 5200 cm^-1 band intensities as 0.1 ± 0.1 at 1.5 - 2.4 GPa and 900 °C. These values are in agreement with results from previous in-situ experiments for aluminosilicate melts at 1.0 GPa (Shen and Keppler, 1995). These observations suggest that the water speciation equilibrium in aluminosilicate melts is insensitive to pressure. This in turn implies that the standard volume change of the water speciation reaction is negligibly small, which is consistent with dilatometric observation of hydrous aluminoslicate melts (Ochs and Lange, 1997).
References: Ochs and Lange, Contrib. Mineral. Petrol., 129, 155-165, 1997; Mysen and Yamashita, Geochim. Cosmohim. Acta 74, 4577-4588, 2010; Shen and Keppler, Amer. Mineral., 80, 1335-1338, 1995.