The compositional variation of synthetic sodic amphiboles at high and ultra-high pressures

Abstract

Sodic amphiboles in high pressure and ultra- high pressure (UHP) metamorphic rocks are complex solid solutions in the system Na2O-MgO-Al2O3-SiO2- H2O (NMASH) whose compositions vary with pressure and temperature. We conducted piston-cylinder experi- ments at 20-30 kbar and 700-800 ∞C to investigate the stability and compositional variations of sodic amphi- boles, based on the reaction glaucophanea 2jade- ite+talc, by using the starting assemblage of natural glaucophane, talc and quartz, with synthetic jadeite. A close approach to equilibrium was achieved by performing compositional reversals, by evaluating compositional changes with time, and by suppressing the formation of Na-phyllosilicates. STEM observations show that the abundance of wide-chain structures in the synthetic amphiboles is low. An important feature of sodic amphibole in the NMASH system is that the assemblage jadeite-talc a quartz does not fix its com- position at glaucophane. This is because other amphi- bole species such as cummingtonite (Cm), nyboite (Nyb), Al-Na-cummingtonite (Al-Na-Cm) and sodium anthophyllite (Na-Anth) are also buAered via the model reactions: 3cummingtonite + 4quartz + 4H2Oa 7talc, nyboite + 3quartza 3jadeite + talc, 3Al-Na-cumming- tonite + 11quartz + 2H2Oa 6jadeite + 5talc, and 3 sodium anthophyllite + 13quartz + 4H2Oa 3 jade- itea 7talc. We observed that at all pressures and tem- peratures investigated, the compositions of newly grown amphiboles deviate significantly from stoichiometric glaucophane due to varying substitutions of Al IV for Si, Mg on the M(4) site, and Na on the A-site. The deviation can be described chiefly by two compositional vectors: (Na A Al IV ) (h A Si) (edenite) toward nyboite, and (Na (M4) Al VI ) (Mg (M4) Mg VI ) toward cummington- ite. The extent of nyboite and cummingtonite substitu- tion increases with temperature and decreases with pressure in the experiments. Similar compositional vari- ations occur in sodic amphiboles from UHP rocks. The experimentally calibrated compositional changes therefore may prove useful for thermobarometric applications.