THE UNREALIZED BEHAVIOR OF MAGMATIC WATER IN A TEMPERATURE GRADIENT: IMPLICATIONS FOR THE MAGMATIC-HYDROTHERMAL TRANSITION IN ARC MAGMATIC SYSTEMS

The magmatic-hydrothermal transition remains one of the poorest understood areas of the ore formation process. Conceptually, the magmatic regime is often distinctly separated from the region of hydrous fluid flow; new experiments dramatically impact this view.

Our recent temperature gradient experiments (all at 0.5 GPa) over large temperature ranges (1300-350°C) show that wet silicates undergo compositional differentiation to temperatures <400°C while nominally anhydrous experiments show no differentiation occurring below a normal silicate solidus. For instance, Huang et al. (GCA, 2009) show that AGV-1 andesite with 4% H2O in a large temperature gradient (950 to 350°C) evolved to a crystalline granitic composition at the cold end of the gradient. A similar, 25 day long rhyolite experiment (RGM-1) with 4 wt% water evolved to a crystalline quartz plus muscovite cold end. In contrast, two basalt (BCR-1) experiments with no water added and variable durations (7 and 45 days) showed little compositional change at the cold end despite prominent mineral layering developed at higher temperatures. The difference between wet and dry experiments reflects component transport occurring through a continuum of melt compositions down to melts that may contain 30-50% water; such peralkaline hydrothermal fluid-like melts were shown to coexist with qtz and Kspar at <400°C (Tuttle and Bowen, 1958).

Spatial samples from all 4 charges were analyzed for δ18O. Despite almost entirely crystalline cold ends, δ18O in the AGV-1 and RGM-1 charges systematically changes by 24 and 28‰, respectively, reflecting the isotopic fractionation by thermal diffusion occurring as water diffused through the charge. In contrast, δ18O in dry experiments varies less (by 5‰) and only in the hot glassy portion of the charge. The difference in isotopic behavior reflects the ability of molecular water dissolved in a silicate melt and subject to a T gradient to move through and rapidly exchange isotopes with coexisting mineral grains. This finding implies that an arc magmatic/hydrothermal system is better viewed as a continuum rather than discrete units.