The amphibole glaucophane (Na2Mg3Al2Si8O22(OH)2) is the key index mineral of rocks that have been subjected to blueschist-facies metamorphism. Building on the recent advances made in this lab on the synthesis of ideal glaucophane, this proposal will focus on the experimental investigation of reactions that define the lower-pressure stability of glaucophane and that model the transition from blueschist to greenschist metamorphic rocks. Determining where these reactions occur in pressure-temperature space places constraints on the geological processes that produce blueschist-facies rocks which, in turn, improves our understanding of the pressure-temperature path followed by near-suface rocks during their subduction deep into Earth's mantle or exhumation of deep-seated rocks on their return to the Earth's surface.
In this proposal several reactions will be experimentally investigated which are all water-conserving, that is, they neither consume nor liberate water, in order to avoid the intervention of a sheet silicate (vermiculite). The feasibility of studying a water-conserving reaction with minimal amounts of water (1-5 wt%) has been demonstrated with the reaction: glaucophane + 2 quartz = talc + 2 albite (1), which is a crude analogue of the blueschist-to-greenschist transition. Preliminary experiments have been done on the reaction glaucophane + 2 diopside + 2 quartz = tremolite + 2 albite in the Ca-bearing system Na2O-CaO-MgO-Al2O3-SiO2-H2O, which will be continued in this proposal. A major part of this proposal is devoted to investigating the role of Fe-Mg exchange on the location of reaction (1). Thermodynamic calculations indicate that ferrous iron will lower the pressure of stability of glaucophane and talc and may define a very useful geobarometer. Other water-conserving reactions that have been identified in the system Na2O-MgO-Al2O3-SiO2-H2O which may provide a better model of this transition include: 7 glaucophane + 28 quartz + 6 chlorite + 8 kyanite = 17 talc + 14 paragonite; and 7 glaucophane + 14 paragonite + 3 talc = 28 albite + 8 kyanite + 6 chlorite.
