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 (NM ASH) whose compositions vary with pressure and temperature. We conducted pi ston-cylinder experiments at 20-30 kbar and 700-800 degrees C to investigat e the stability and compositional variations of sodic amphiboles, based on the reaction glaucophane = 2jadeite + talc, by using the starting assemblag e 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 for mation of Na-phyllosilicates. STEM observations show that the abundance of wide-chain structures in the synthetic amphiboles is low. An important feat ure of sodic amphibole in the NMASH system is that the assemblage jadeite-t alc +/- quartz does not flu its composition at glaucophane. This is because other amphibole species such as cummingtonite (Cm), nyboite (Nyb), Al-Na-c ummingtonite (Al-Na-Cm) and sodium anthophyllite (Na-Anth) are also buffere d via the model reactions: 3cummingtonite + 4quartz + 4H(2)O = 7talc, nyboi te + 3quartz = 3jadeite + talc, 3Al-Na-cummingtonite + 11quartz + 2H(2)O = 6jadeite + Stale, and 3 sodium anthophyllite + 13quartz + 4H(2)O = 3 jadeit e + 7talc. We observed that at all pressures and temperatures investigated, the compositions of newly grown amphiboles deviate significantly from stoi chiometric 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: [(NaAlIV)-Al-A]< = > [square(A)Si] (edenite) toward nyboite, and [(NaAlVI)-Al-(M4)]< = >[(MgMgVI)-Mg-(M4)] toward cummi ngtonite. The extent of nyboite and cummingtonite substitution increases wi th temperature and decreases with pressure in the experiments. Similar comp ositional variations occur in sodic amphiboles from UHP locks. The experime ntally calibrated compositional changes therefore may prove useful for ther mobarometric applications.