The OH content of pyrope at high pressure

The OH contents of pyrope garnets synthesised at 1000 degrees C and pressures between 2 and 13 GPa have been measured by infrared spectroscopy. We find that under the same conditions of pressure, temperature, a(H2O) and a(SiO2) the OH content of pyrope is similar to that of grossular. This shows that observed differences in nature between water contents of pyrope and grossular are related to paragenesis rather than reflecting higher intrinsic solubility in grossular. In the presence of excess SiO2, the H2O content increases with pressure to 5 GPa (1000 ppm) then decreases to below the detection limit at pressures above 7 GPa. Thus garnet becomes more hydrous with pressure to some critical value, beyond which dehydration occurs even under H2O-saturated conditions. This observation is consistent with the measured partial molar volume of water in hydrogarnet which becomes greater than that in fluid water within the pressure range of this study. When corrected to upper mantle conditions we find that pyrope should dehydrate with increasing depth below about 250 km. Pyrope is unlikely therefore to be a major site of water storage in the transition zone and upper mantle. (C) 1998 Elsevier Science B.V. All rights reserved.