THE INFLUENCE OF H2O-H-2 FLUIDS AND REDOX CONDITIONS ON MELTING TEMPERATURES IN THE HAPLOGRANITE SYSTEM

Solidus temperatures of quartz-alkali feldspal assemblages in the hapl ogranite system (Qz-Ab-Or) and subsystems in the presence of H2O-H-2 f luids have been determined at 1, 2: 5 and 8 kbar vapour pressure to co nstrain the effects of redox conditions on phase relations in quartzof eldspathic assemblages, The hydrogen fugacity (f(H2)) in the fluid pha se has been controlled using the Shaw membrane technique for moderatel y reducing conditions (f(H2) < 60 bars) at 1 and 2 kbar total pressure , Solid oxygen buffer assemblages in double capsule experiments have b een used to obtain more reducing conditions at 1 and 2 kbar and for al l investigations at 5 and 8 kbar. The systems Qz-Or-H2O-H-2 and Qz-Ab- H2O-H-2 have only been investigated at moderately reducing conditions (1 and 5 kbar) and the system Qz-Ab-Or-H2O-H-2 has been investigated a t redox conditions down to IW (1 to 8 kbar), The results obtained for the water saturated solidi are in good agreement with those of previou s studies, At a given pressure, the solidus temperature is found to be constant (within the experimental precision of +/- 5 degrees C) in th e f(H2) range of 0-75 bars, At higher f(H2), generated by the oxygen b uffers FeO-Fe3O4 (WM) and Fe-FeO (IW), the solidus temperatures increa se with increasing H content in the vapour phase. The solidus curves o btained at 2 and 5 kbar have similar shapes to those determined for th e same quartz - alkali feldspar assemblages with H2O-CO2- or H2O-N-2-b earing systems. This suggests that H-2 has the behaviour of an inert d iluent of the fluid phase and that H-2 solubility in aluminosilicate m elts is very low, The application of the results to geological relevan t conditions [HM (hematite-magnetite) > f(O2) > WM] shows that f(H2) p roduces a slight increase of the solidus temperatures (up to 30 degree s C) of quartz-alkali feldspar assemblages in the presence of H2O-H-2 fluids between 1 and 5 kbar total pressure.