Granite is an important component of Earth's continents, and represents a terminal stage of the magmatic processes that have distilled the continents from the planet's mantle over geologic time. In many areas of granite formation, the magmas erupt at the surface as well as cooling undeground. The origin of granites and equivalent volcanic rhyolites is due to magmatic processes (redistribution of mass in the partially molten state) within pre-existing crust, and until now most workers have agreed that the magmas are produced by melting of earlier rocks in the middle to lower continental crust, where temperatures are high. In the Snake River Plain, ID, we have discovered that a large quantity of rhyolite, of comparable volume to granitic batholiths and erupted at around 11 million years ago under the influence of the Yellowstone hotspot, has oxygen isotopic compositions that are strongly influenced by rainwater. Rainwater normally does not penetrate more than several km into the crust, suggesting that the origins of these magmas may be very shallow indeed, and perhaps requiring revision of existing models of granite and rhyolite origins. The aim of this research is therefore to understand the unusual conditions that obtained during the formation of these magmas.
The average d18O value of ~7,000 km3 of rhyolite erupted from the central Snake River Plain, represented by some 30 units, is 2 per mil. This combination of degree of 18O depletion and large volume are unprecedented. We shall build on our pilot study to determine the total spatial and temporal extent of strong 18O depletion at Bruneau-Jarbidge and neighboring volcanic centers, and use trace element and isotope analysis of whole rocks and mineral phases, as well as potential crustal protoliths, in order to constrain magma sources. This will enable us to determine whether strong 18O depletion in rhyolites is a result of hydrothermal and magmatic processes associated with caldera cycles (the currently accepted explanation), recycling of hydrothermally altered Idaho batholith rocks (as is suggested by our preliminary data), or some other process. An understanding of the petrogenesis of these rocks will be an important contribution to understanding the whole phenomenon of large-scale continental silicic magmatism, and will provide interpretations valuable to other scientists examining a wide variety of questions regarding the origin and evolution of the Snake River Plain - Yellowstone hotspot track.
