Abstract:
Solidus temperatures of quartz–alkali feldspar assemblages in the haplogranite system (Qz-Ab-Or) and subsystems in the presence of H2O-H2 fluids have been determined at 1, 2, 5 and 8 kbar vapour pressure to constrain the effects of redox conditions on phase relations in quartzofeldspathic assemblages. The hydrogen fugacity (f H2) in the fluid phase has been controlled using the Shaw membrane technique for moderately reducing conditions (f H2 < 60 bars) at 1 and 2 kbar total pressure. Solid oxygen buffer assemblages in double capsule experiments have been used to obtain more reducing conditions at 1 and 2 kbar and for all investigations at 5 and 8 kbar. The systems Qz-Or-H2O-H2 and Qz-Ab-H2O-H2 have only been investigated at moderately reducing conditions (1 and 5 kbar) and the system Qz-Ab-Or-H2O-H2 has been investigated at redox conditions down to IW (1 to 8 kbar). The results obtained for the water saturated solidi are in good agreement with those of previous studies. At a given pressure, the solidus temperature is found to be constant (within the experimental precision of ± 5°C) in the f H2 range of 0–75 bars. At higher f H2, generated by the oxygen buffers FeO-Fe3O4 (WM) and Fe-FeO (IW), the solidus temperatures increase with increasing H2 content in the vapour phase. The solidus curves obtained at 2 and 5 kbar have similar shapes to those determined for the same quartz - alkali feldspar assemblages with H2O-CO2- or H2O-N2-bearing systems. This suggests that H2 has the behaviour of an inert diluent of the fluid phase and that H2 solubility in aluminosilicate melts is very low. The application of the results to geological relevant conditions [HM (hematite-magnetite) > f O2 > WM] shows that increasing f H2 produces a slight increase of the solidus temperatures (up to 30 °C) of quartz–alkali feldspar assemblages in the presence of H2O-H2 fluids between 1 and 5 kbar total pressure.