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

Sodic amphiboles in high pressure and ultrahigh 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 experiments at 20±30 kbar and 700±800 °C to investigate the stability and compositional variations of sodic amphiboles, based on the reaction glaucophane ˆ 2jadeite+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 ‹ quartz does not ®x its composition at glaucophane. This is because other amphibole species such as cummingtonite (Cm), nyboÈ ite (Nyb), Al±Na-cummingtonite (Al±Na-Cm) and sodium anthophyllite (Na-Anth) are also bu€ered via the model reactions: 3cummingtonite + 4quartz + 4H2O ˆ 7talc, nyboÈ ite + 3quartz ˆ 3jadeite + talc, 3Al±Na-cummingtonite + 11quartz + 2H2O ˆ 6jadeite + 5talc, and 3 sodium anthophyllite + 13quartz + 4H2O ˆ 3 jadeite ‡ 7talc. We observed that at all pressures and temperatures investigated, the compositions of newly grown amphiboles deviate signi®cantly from stoichiometric glaucophane due to varying substitutions of Al for Si, Mg on the M(4) site, and Na on the A-site. The deviation can be described chie ̄y by two compositional vectors: [NaAl]<ˆ>[hSi] (edenite) toward nyboÈ ite, and [NaAl]<ˆ>[MgMg] toward cummingtonite. The extent of nyboÈ ite and cummingtonite substitution increases with temperature and decreases with pressure in the experiments. Similar compositional variations occur in sodic amphiboles from UHP rocks. The experimentally calibrated compositional changes therefore may prove useful for thermobarometric applications.