Experimental Investigation of the Interaction of Amphibole with Highly Salted H₂O-NaCl-KCl Fluid at 750°C, 700 MPa: Application to Alkaline Metasomatism of Amphibole Rocks

Abstract

The article presents experimental data on the interaction of amphibole with NaCl-H2O and (K, Na)Cl-H2O solutions at varying salt content. When interacting with the H2O-NaCl fluid, amphibole remains the predominant mineral in all experiments. In addition to it, Na-phlogopite, plagioclase, and nepheline/sodalite are formed. At \({{a}_{{{{{\text{H}}}_{{\text{2}}}}{\text{O}}}}}\) 0.6, amphibole melts. When amphibole interacts with the H2O-NaCl-KCl fluid at \({{X}_{{{{{\text{H}}}_{{\text{2}}}}{\text{O}}}}}\) 0.40 and ХKCl/(ХKCl + ХNaCl) in the fluid, defined as ХNaCl = 0.506 – 0.84ХKCl, the amphibole is replaced by an association of nepheline with acid plagioclase, sodalite, and biotite. At ХKCl/(ХKCl + ХNaCl) 0.3, nepheline, sodalite, and plagioclase become unstable, K-feldspar appears, and biotite, clinopyroxene, and amphibole remain. At ХKCl/(ХKCl + ХNaCl) 0.5, the association Cpx + Bt + Kfs + Grt (grossular–andradite) is stable. Thus, the grossular–andradite garnet is an indicator of high potassium activity in fluids, while nepheline is an indicator of high sodium activity. Na → K isomorphism is observed in amphibole and biotite, and Ca → Na isomorphism in clinopyroxene, but in general these minerals (unlike nepheline and garnet) remain stable in the range of wide K/Na variations in the fluid. Clinopyroxene in experiments corresponds to Ca-Fe-Mg compositions with varying, sometimes high aluminium content, amphiboles belong to the pargasite-hastingsite composition. With an increase in \({{a}_{{{{{\text{H}}}_{{\text{2}}}}{\text{O}}}}}\) (\({{X}_{{{{{\text{H}}}_{{\text{2}}}}{\text{O}}}}}\) 0.57), i.e. a decrease in the gross salinity of the fluid, melts appear, the composition of which varies from trachyte to phonolitic. As ХKCl/(ХKCl + ХNaCl) increases in fluids, the alumina content of melts decreases. An increase in the total salinity of the fluid leads to an increase in the content of potassium in the melt and a decrease in the content of chlorine in it. Experiments have shown that the interaction of amphibole with fluids containing high concentrations of NaCl and KCl leads to the formation of mineral associations typical of alkaline metasomatism of amphibole rocks and concomitant HCl enrichment of the fluid phase. The replacement of fluids with a high saline component by highly acid ones leads to the leaching of Ca, Mg, Fe from metamorphic rocks, their transport and redeposition. Thus, in some cases, a significant removal of FeO, MgO, CaO is a consequence of the interaction of host rocks with water-salt solutions.