The transition from blueschist to greenschist fades modeled by the reaction glaucophane + 2 diopside + 2 quartz = tremolite + 2 albite

摘要

The reaction glaucophane + 2 diopside + 2 quartz = tremolite + 2 albite is proposed to model the transition from the blueschist to greenschist fades. This reaction was investigated experimentally over the range of 1.0-2.1 GPa and 500-800°C using synthetic phases in the chemical system Na_2O-CaO-MgO-Al_2O_3-SiO_-H_2O. Reversals of this reaction were possible at 500 and 550°C and growth of the low-pressure assemblage at 600°C; however, at temperatures of 600°C and higher and at pressures above 1.6 GPa omphacite nucleation (at the expense of diopside and albite) became quite strong and prevented attaining clear reversals of this reaction. Compositional changes in the amphiboles were determined by both electron microprobe analyses and correlations between unit-cell dimensions and composition. Glaucophane and particularly tremolite showed clear signs of compositional re-equilibration and merged to a single amphibole of winchite composition by about 754°C. These data were used to model the miscibility gap between glaucophane and tremolite using either the asymmetric multicomponent formulism parameters of W_(TR,GL) of 68 kJ with α_(TR) of 1.0 and α__(GL) of 0.75 or a simple two-site asymmetric thermodynamic mixing expression with Margules parameters W_(NaCa) of 13.4 kJ and W_(CaNa) of 19.3 kJ. Combination of these thermodynamic models of the miscibility gap with extant thermodynamic data for the other phases yields a calculated location of the above reaction, involving pure diopside and albite, that is in good agreement with the observed experimental reversals and amphibole compositions over the range of 0.94-1.93 GPa and 400-754°C. The calculated effect of jadeite solid solution into diopside is to reduce the dP/dT slope from 0.0028 to 0.0021 GPa/°C and decrease the pressure by 0.28 GPa at 754°C. The dP/dT slope of this reaction boundary lies close to a linear geotherm of 13°C/km and is consistent with the slopes of other solid-solid reactions that have been used to model the blueschist-to-greenschist fades transition.