Metasomatic Phase Relations in the System CaO-MgO-SiO_2-H_2O-NaCl at High Temperatures and Pressures

摘要

An equation of state of solute silica in NaCl brines at 500 to 900 deg C and 4 to 15 kbar is formulated by making use of two experimentally determined properties of quartz solubility: the silica molality decreases in direct proportion to the logarithm of the NaCl mole fraction (X(NaCl)) at pressures approaching 10 kbar, and the relative silica molality (molality at a given NaCl mole fraction, m_x divided by the molality in pure H_2O at the same P and T, m_0) is independent of temperature in the evaluated range. These two properties are expressed in the relation:log(m_x/m_0)* = A + BX(NaCI),where log(m_x/m_0)* denotes the logarithm of the ideal molality ratio, and A and B are functions of pressure, but not temperature or salinity, such that B = -1.730 - 1.431 x 10~(-3)P + 5.923 x 10~(-4)P~2 -9.243 x 10~(-5)P~3, and A = 0 at P>10 kbar, whereas A = 0.6131 -0.1256P + 6.431 x 10~(-3)P~2 at P