by Kent Condie, Professor of Geochemistry
Kent Condie and his graduate students are involved in several studies related to the question on when plate tectonics began on planet Earth. Although geodynamic models support coherent plates throughout Earth history, just when and how plates became negatively buoyant is not yet clear. Part of the problem relates to our uncertainty about the early thermal history of the mantle. Perhaps the most exciting method of tracking plate tectonics through time using modern plate tectonic rock associations and subduction P-T regimes. Clearly the arc assemblage (basalt-andesite-dacite-rhyolite-graywacke and associated minor rock types) is widespread to at least 2 Ga, common to 3 Ga, and is found locally in crust older than 3 Ga. Also, dual thermal regimes (low-P, high-T and high-P, low-T) characteristic of plate tectonics are identified at least to 2.8 Ga and possibly to 3.3 Ga. In addition, collisional orogens are recognized to about 2 Ga and accretionary orogens to ≥ 3.5 Ga. Both types of orogens contain terranes with distinct terrane boundaries.
At New Mexico Tech, we are studying several Archean greenstone belts using geochemical indices to constrain the tectonic settings in which they formed. Results of these studies should be valuable in constraining when modern-style plate tectonics began.
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Pillow basalts from at 2.7 billion year old greenstone with geochemical affinities to modern island arcs |
We are also involved in geochemical studies Precambrian granitoids to better understand the evolution of the continental crust and associated mantle with time. Numerous geochemical changes have been reported to occur near the end of the Archean some 2.5 billion years ago, yet most of these are not well documented. Kent Condie and his graduate students are involved with detailed geochemical and isotopic studies of granitoids on both sides of the Archean/Proterozoic boundary in Colorado, Wyoming, Canada and elsewhere. If verified, geochemical differences across this boundary may indicate fundamental changes in tectonic regime at this time, a conclusion that would be very important in terms of silicate planetary evolution.
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A complex Archean granitoid in the central Wind River Mountains in Wyoming |
Isotopic ages from the continental crust suggest that it has been extracted from the mantle in short pulses, with a large proportion crust extracted around 2.7 and 1.8 billion years ago. We are currently involved with U/Pb zircon dating of orogenic granitoids and detrital zircons to better monitor crustal growth and understand more fully the processes involved. It is possible to recognize juvenile continental crustal sources using Hf isotopes in detrital zircons, and when coupled with U/Pb zircon ages on the same zircons, we can recognize major periods of continental growth in the geologic past. Kent Condie and his research group are currently involved in age and Hf isotopic studies of zircons from modern stream sediments and from ancient sediments, to better track the extraction rate and geographic distribution of continental growth.
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| Estimates of the volume of continental crust produced globally throughout geologic time |
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