Metasomatic Phase Relations in the System CaO-MgO-SiO2-H2O-NaCl at High Temperatures and Pressures

An equation of state of solute silica in NaCl brines at 500 to 900°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, mx, divided by the molality in pure H2O at the same P and T, mo) is independent of temperature in the evaluated range. These two properties are expressed in the relation: log(mx/mo)∗ = A + BX(NaCI), where log(mx/mo)∗ 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 × 10−3P + 5.923 × 10−4P2 −9.243 × lO−5P3, and A = 0 at P>10 kbar, whereas A = 0.6131 − 0.1256P + 6.431 × 10−3P2 at P≤10 kbar, as derived from fits to experimental data (Newton and Manning, 1999). The parameter A decreases from 0.214 to 0 from 4 to 9.5 kbar, and remains zero to 15 kbar; B decreases from −1.373 to −1.571 from 4 to 15 kbar. With the above relationship defining a variable X(NaCl)-T-P standard-state of solute silica, the activity of SiO2 can be replaced by its molality for calculations of mineral-fluid equilibria over most of the conditions for metasomatism in the deep crust and upper mantle. Significant departures from ideality occur only at the lowest pressures, and low salinities. Calculations on peridotite mineral stability in the simple system CaO-MgO-SiO2-H2O-NaCl at high T and P show that antigorite, brucite, and diopside are stable at 500°C and pressures of 5 to 15 kbar in the presence of concentrated NaCl solutions at low SiO2 activities. At 700°C, anthophyllite is stable over a wide range of salinities at 5 kbar with tremolite but not with diopside. The presence of anthophyllite buffers silica solubility at a high, salinity-independent value close to quartz saturation. At 10 and 15 kbar and 700°C, talc replaces anthophyllite as the stable hydrate, and talc-trem-olite assemblages buffer SiO2 fluid concentrations at high values nearly independent of salinity. At 900°C hydrates are unstable and diopside again becomes stable and coexists with enstatite in peridotites. These stability calculations correspond well to the observed progressive metamorphic sequence in peridotite bodies in the Central Alps. This method of analysis may be useful in interpretation of metamorphosed ultramafic bodies in general, including the basal portions of obducted ophiolitic mantle lithosphere and the mantle wedge above subduction zones. More detailed calculations, including rocks containing feldspars, must take into account the more soluble major components of rocks, especially alkalis, as these will affect the activity coefficient of SiO2 in NaCl solutions. The solubility of silica in the presence of minerals containing these components must be determined by additional measurements.