Geocheminar winter-2016

Water in granitic magmas: critical phenomena, activity measurements, and evidence for 5 wt% water during batholith formation

Water plays a fundamental role in how we view processes that take place inside the deep Earth. In this talk, I will discuss recent developments on the role of water in granitic systems. First, I will discuss the complications that arise when granitic systems reach critical conditions and will show some field examples where it is inferred supercritical fluids have existed. Secondly, I will address the controversy of dehydration melting versus fluid present melting as models for genesis of A and I type granites at deep crustal conditions. Reversed liquidus measurements on a rock of haplogranite composition at deep crustal pressures of 1 GPa ( ~30km depth) indicate that it takes approximately 5 wt% H2O to completely melt a granite at 850 oC. This is much more water than is present in a tonalite, which contains only ~0.8 wt% H2O in the form of hydrous minerals biotite and amphibole and as a result, would only produce ~16 wt% granitic magma. We suggest that there must be an external influx of water (as well as heat) into the lower crust, likely from a crystalizing basalt that contains ~4wt% H2O (Plank, 2013). Lastly, I consider new H2O activity measurements on albitie and granitic melts and show how that for a granitic liquid to form from a granulite assemblage in equilibrium, it must contain ~5 wt% H2O at an H2O activity of ~0.4 at deep crustal pressures of 1.0 GPa.