Experimental study of amphibole interaction with H2O–NaCl Fluid at 900°C, 500 MPa: toward granulite facies melting and mass transfer

Аннотация

Interaction between natural pargasite [Prg, SiO₂ = 43.89 wt %, FeO/(FeO + MgO) = 0.35, (Na + K)_A = 0.51] and H₂O–NaCl fluid, whose composition (NaCl mole fraction) varied within the range X_NaCl = NaCl/(NaCl + H₂O) = 0–0.45, was experimentally studied in an internally heated apparatus at 900°C and 500 MPa. Natural pargasite begins to melt at a temperature 120–150°C lower than its synthetic analogue. In the presence of pure H₂O, the subliquidus mineral assemblage involves amphibole Hbl₁, whose composition is closely similar to the starting Prg, clinopyroxene Cpx, calcic plagioclase Pl, and minor amounts of hercynite-magnetite spinel. With increasing X_NaCl, the subliquidus assemblage systematically changed: calcic plagioclase disappeared and more Fe- rich amphibole Hbl₂ appeared at X_NaCl = 0.07; Cpx disappeared at X_NaCl = 0.14; and appearance of Na-Phl compositionally close to wonesite and almost complete disappearance of Hbl₁ was observed at X_NaCl = 0.31. The composition of the melt also changed: its Na₂O gradually increased (from 1.5 to 9–10 wt %), and CaO and SiO₂ decreased(from 8.6 to 2 wt % and from 64 to 60 wt %, respectively, in recalculation to the anhydrous basis); at X_NaCl ≥ 0.35, the melt was transformed from quartz- to nepheline-normative. The maximum Cl concentration of 1.2 wt % was measured in the melt poorest in SiO₂. The experimental products contained spherical objects less than 10 μm in diameter that consisted of material that precipitated from the quenched fluid. These particles are richer than the melt in SiO₂ (62–80 wt %) and poorer in Al₂O₃ (11–19 wt %) in experiments with X_NaCl ≤ 0.24, but the differences between the compositions of the melt and particles decreased with increasing XNaCl. The relatively high concentrations of aluminosilicate material in the fluid is most likely explained by the high solubility of the melt in the fluid phase, with the formation in the fluid aqueous Si, Al–Si, Na–Al–Si, and other polymeric species. It is suggested that interaction of host rocks with such fluids, rich in granitic components, might be responsible for granitization (charnockitization) of mafic, and, particularly, ultramafic rocks described in the literature.