中酸性硅酸盐熔体-水体系氢同位素分馏的压力效应

Pressure effect on hydrogen isotope fractionation between intermediate- acidic silicate melts and water

对 0.2～ 2 000 MPa条件下钠长石熔体、钾长石熔体以及 0.2～ 150 MPa条件下流纹岩熔体-水体系的氢同位素分馏实验数据进行了等压拟合 ,发现硅酸盐熔体与水之间的氢同位素分馏存在显著的压力效应。在 800、 1 000和 1 200℃条件下对钠长石熔体-水体系和流纹岩熔体-水体系氢同位素分馏压力方程进行的等温拟合表明 ,只有在特定的压力条件下才可以用钠长石熔体-水体系来近似流纹岩熔体-水体系的氢同位素分馏行为；当压力超过临界值时 ,硅酸盐熔体-水体系氢同位素分馏会发生变化。本文拟合的硅酸盐熔体-水体系氢同位素分馏等值线在 P- T空间的形态变化特征与矿物-水体系存在较大差异。依据流纹岩熔体与水之间氢同位素分馏的压力效应，成功地模拟了美国西部 Glass Creek流纹岩δ D值和水含量变化规律与岩浆去气之间的关系。

Experimental data for hydrogen isotope fractionations between albitic, K- feldspar, and rhyolitic melts and water vary systematically with pressure from 0.2 to 2 000 MPa. Pressure equations for albitic and rhyolitic melt- water systems are fitted accordingly under isothermal conditions of 800, 1 000, and 1 200℃ . The results show that the behavior of hydrogen isotope fractionation in the albitic melt- water system cannot be analogously applied to that in the rhyolitic melt- water system except at particular pressures. There is crossover in hydrogen isotope fractionation between silicate melts and water when the experimental pressure is higher than the critical value. The contours of hydrogen isotope fractionation between silicate melts and water are constructed in P- T space, which are apparently different from those for brucite- water system. In terms of pressure effect on hydrogen isotope fractionation in the rhyolitic melt- water system, the unusual variability in theδ D values and H2O contents of the Glass Creek rhyolites from western United States are successfully modeled in the course of magmatic degassing.