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Speaker

Marcia Bjornerud

Lawrence University
Geology
920-832-7015
bjornerm@lawrence.edu
Inhibited eclogite formation: The key to the rapid growth of strong and buoyant Archean continental crust
Subduction is the principal mechanism by which the hydrosphere and interior of the Earth interact. Today, subduction involves the dehydration of ocean crust at depths of 60–120 km depending on the age of the slab. Release of the water leads to generation of arc magmas (future continental crust), and the slab is then transformed into denser eclogite that helps to pull more of the slab into the trench. However, the first continental crust is unlikely to have formed this way. Growing geochemical evidence indicates that large volumes of continental crust were produced over a short period of time in the Archean, when the planet was probably too hot for modern plate tectonics to operate. A significant increase in the kinetics of eclogite-forming reactions may have been the key to the transition from Archean to modern tectonics. Under the higher geothermal gradients of the Archean, tectonically buried ocean crust would have been severely dehydrated before reaching eclogite-facies pressures. Because rapid eclogitization is dependent on water as a medium for advective ion transport, the very shallow dehydration in the Archean may have inhibited the formation of eclogite facies minerals. The importance of water in eclogite metamorphism is illustrated by a complex of partly eclogitized mafic granulites on Holsnøy, western Norway, in which reaction progress was limited by the availability of water. When water is scarce or absent, metastable granulite-facies mineral assemblages can persist at eclogite-facies depths owing to the extremely slow reaction kinetics when diffusion is the only chemical transport mechanism. Such dehydrated but uneclogitized mafic crust would have been very strong and too buoyant to sink into the mantle, and it may have formed the substrate for the first continental lithosphere.
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