BLUESCHIST TO GREENSCHIST TRANSITION MODELED BY THE REACTION GLAUCOPHANE + DIOPSIDE + QUARTZ = TREMOLITE + ALBITE

The transition from blueschist to greenschist facies can be modeled by the water-conserving reaction glauocophane (Gl) + 2 diopside (Di) + quartz (Qtz) = tremolite (Tr) + 2 albite (Ab). This reaction has been experimentally investigated in the system H₂O-Na₂O-CaO-MgO-Al₂O₃-SiO₂ using a reversal mixture consisting of stoichiometric proportions of synthetic end-member phases, including glaucophane (Jenkins & Corona, 2006, Am. Min.).  The reversal mixture was treated in sealed Pt capsules in the presence of only 2-3 wt% water to prevent the formation of a sheet silicate previously identified by the author as vermiculite.  Treatments were made at 500 – 800 °C and 1.0 – 2.1 GPa using a ½-inch piston cylinder press with NaCl pressure medium for typical durations of 5-9 days.  The reaction direction was judged from X-ray diffractometer patterns and the compositional changes of the phases determined by electron microprobe analysis.

  The reaction was experimentally reversed placing the location of the boundary at 1.35 ± 0.25 GPa at 500 °C.  There was very little change in the compositions of the phases at 500 °C, probably owing to the sluggish kinetics of compositional re-equilibration at this temperature.  At 600 °C and pressures < 1.5 GPa, Tr and Ab grew at the expense of Gl and Di with little solid solution between the amphiboles (Tr₉₇Gl₃ and Tr₁₆Gl₈₄), but at higher pressures solid solution in the pyroxene begins to occur by the reaction of Ab with Di to form omphacite (Di₅₀-jadeite₅₀) + quartz and significant solid solution occurs between Tr and Gl.  By 700 °C there appears to be either two coexisting populations of amphibole of composition Tr₄₅Gl₅₅ and Tr₃₀Gl₇₀ or simply poor re-equilibration of Tr and Gl to a single amphibole of the composition winchite (= Tr₅₀Gl₅₀) coexisting with omphacite.  By 800 °C both pyroxene and amphibole have fully re-equilibrated to omphacite and winchite.  These results indicate that, in this Fe-free system, the studied reaction defines the blueschist-to-greenschist transition at about 1.35 GPa at 500 °C and about 1.5 GPa at 600 °C.  This boundary will most likely shift to lower P with the addition of Fe.  Above 600 °C this reaction is no longer univariant but instead is divariant.  These results also suggest that the crest of the (Fe-free) miscibility gap between Tr and Gl lies between 700 – 800 °C at 1.6 – 1.9 GPa.