Over the past two decades it has become clear that the continental crust can be heated to extreme, ?ultrahigh? temperatures (UHT) in excess of 900 °C (about 1650 °F) over large regions during metamorphism and mountain building events. How metamorphic rocks of the crust reach such extreme temperatures remains a fundamental unresolved problem in the geosciences, with major implications for rock strength, generation of magmas (which may ascend from depth to produce ore deposits and volcanic eruptions), heat transfer, and Earth?s overall heat budget. This project will focus on newly-discovered UHT rocks in northeastern Connecticut, the first regional UHT locality found in the United States. The rocks underwent extreme temperature conditions of roughly 1000 °C (more than 1800 °F) at depths of at least 35 km (about 22 miles) below the surface during Appalachian mountain building in the Devonian Period. This discovery will force a rethinking of how the deep rocks of the Appalachian mountains were formed, and shed new light on how UHT conditions can be reached. Moreover, the rocks contain abundant evidence for the former presence of CO2-rich fluids at ultrahigh temperatures, and thus could have important implications for carbon cycling in the crust and the long-term carbon cycle.
Fundamental hypotheses for UHT metamorphism will be tested including: rapid exhumation from the mantle; underthrusting of radiogenic crust; asthenospheric upwelling; mechanical strain heating; heat input from mantle-derived magmas; and heat transfer by fluids. Basic data collection will involve field mapping of geologic relations; electron microprobe analyses of minerals; bulk chemical analyses of rocks; U-Pb dating of zircons by ion microprobe; and micro-Raman analyses of mineral and fluid inclusion types. These data will be used to determine pressure-temperature-time histories for the rocks; crustal fluid compositions; radiogenic heat production due to the decay of uranium, thorium, and potassium; and other geologic variables critical for hypothesis testing. The project will support and train Yale Ph.D. and undergraduate students, and integrate research results into public science outreach programs at the Yale Peabody Museum of Natural History.
