EXPERIMENTAL INVESTIGATION OF THE GLAUCOPHANE-TREMOLITE MISCIBILITY GAP

Field studies of metabasaltic eclogite- and blueschist-facies rocks have indicated the presence of a miscibility gap (solvus) between glaucophane (Na₂Mg₃Al₂Si₈O₂₂(OH)₂ = Gl) and tremolite (Ca₂Mg₅Si₈O₂₂(OH)₂ = Tr). Because glaucophane and tremolite are key minerals of blueschist- and greenschist-facies rocks, respectively, it is important to determine the location of this miscibility gap in temperature-composition space.

Several types of compositional re-equilibration experiments were done. First, equal molar mixtures of end-member glaucophane (Jenkins & Corona, 2006, Am. Min.) and tremolitic amphibole (Trem₉₀MgCum₁₀) were treated over the range of 500 – 800 °C at a constant pressure of 1.62 GPa for durations of 120-424 hours and with only a small amount of water (~3 wt%) to avoid nucleation of vermiculite. There was strong compositional re-equilibration at 600 °C and higher, readily discernible from changes in the unit-cell volumes and the proportions of the two amphiboles. Second, mixtures of omphacite, talc, and tremolitic amphibole or omphacite, talc, and glaucophane were made with bulk compositions of Tr₆₀Gl₄₀ and Tr₄₀Gl₆₀, respectively. These were treated in the range of 600-800 °C at 1.6 GPa with ~3 wt% water. Changes in the compositions of the amphiboles were monitored using a calibrated volume-composition curve for amphiboles synthesized along this join as well as by electron microprobe analyses of individual amphibole grains from polished chips of the run products. It should be noted that the pronounced positive deviation from ideality in the volume-composition curve is consistent with the presence of a miscibility gap along this join.

The results clearly indicate the asymmetry of the miscibility gap, with greater solid solution existing for Tr-rich compared to Gl-rich amphiboles. The miscibility gap can be modeled using a two-parameter asymmetric solvus, where mixing of Na and Ca on the two M4 sites is assumed to be coupled with Al and Mg on the two M2 sites, giving values of W_NaCa = 11.6 kJ and W_CaNa = 20.4 kJ. The critical point is approximately at 820 °C and amphibole composition of 65 mol% Gl. The asymmetric nature of this solvus is in agreement with that of Reynard & Ballevre (1988, J Meta. Petrol.) although the critical point in this Fe-free system is at least 200 °C higher.