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Brine-assisted anatexis: Experimental melting in the system haplogranite-H2O-NaCl-KCl at deep-crustal conditions

  • Author(s): Aranovich, LY
  • Newton, RC
  • Manning, CE
  • et al.
Abstract

The first-melting temperature of haplogranite in the presence of H O-NaCl-KCl solutions was determined experimentally at deep-crustal conditions of 6-14kbar and 700-900°C. Minimum melting occurs at fluid K/(K+Na) of 0.2-0.25. Melting temperature rises strongly with increasing salinity: at 10kbar, first melting occurs at 800°C with a H O mole fraction (XH2O) of 0.62±0.02, similar to the salinity of fluid inclusions in minerals of some granites and migmatites. At 900°C and 10kbar, XH2O is 0.33±0.01 for first melting. All minimum melts are granitic over the entire P-T-XH2O range, with SiO melt concentrations of 74±2wt% on a H O-free basis and Al O and alkalis typical for granites. Normative quartz is near 30% for all liquids. The K/(K+Na) ratios of minimum melts increase strongly with decreasing H O activity: at 10kbar and XH2O=1, K/(K+Na) in the melt is 0.25, whereas at XH2O=0.34 it is 0.55. This "brine trend" is similar to, but more pronounced than, the trend described by decreasing H O activity in H O-CO fluids, and it better explains the compositions of K-rich granites. Minimum-melting curves in the presence of brines of constant XH2O have strongly positive dP/dT slopes at P>2kbar, in contrast to vertical or negative dP/dT slopes for those in the presence of H O-CO fluid. There is therefore a wide P-T range over which migrating low-H O-activity brines may generate subsolidus, deep-crustal metasomatism in the form of K O and silica enrichment. Moreover, once melting occurs, rising accumulations of granitic magma may be fluid saturated and even increase their melting capacity with decreasing depth because of the strong pressure dependence of H O activity in salt solutions. Our results offer an explanation for mid-crust migmatization and granite production: rising hot brines may provoke rock melting at some threshold of decreasing depth in the range 15-20km. Because of their enhanced capacity for metasomatism, leading to eventual melting at granulite facies conditions of temperature, pressure and H O activity, concentrated brines should be considered as possibly important agents in crustal evolution. © 2013 Elsevier B.V. 2 2 2 2 2 3 2 2 2 2 2 2 2 2 2 2

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