Oxygen and hydrogen isotope constraints on the deep circulation of surface waters into zones of hydrothermal metamorphism and melting
The purpose of this paper is to marshal the evidence and try to build a case that (1) shallow (1 to 7 km) circulation of surface waters in the Earth's crust is an extremely widespread and common phenomenon in areas of igneous activity and (2) deep (10 to 15 km) circulation of surface waters can occur in certain favorable geological situations, particularly in rift zones and areas of extensional tectonics. It is shown that very large amounts of water may interact with the rocks in such zones and that this can take place at temperatures high enough for melting and metamorphism to occur. Oxygen and hydrogen isotope studies have proven to be very useful in establishing the characteristics of such deeply circulating hydrothermal systems and in determining the origins of the aqueous fluids involved in producing granitic and rhyolitic magmas in such environments. This is mainly because oxygen-18 and deuterium are constituents of the H_2O molecule itself, and thus stable isotope signatures are by far the best way to characterize hydrothermal fluids of different origins. These conclusions are most clear-cut when low- ^(18)O, low-D meteoric waters are involved in the isotopic exchange processes, but ocean waters, sedimentary formation waters, metamorphic dehydration waters, and magmatic waters can also be distinguished from each other in favorable circumstances.