Inhibited eclogite formation: The key to the rapid growth of strong and buoyant Archean continental crust

Abstract

Subduction is the principal mechanism by which the hydrosphere and interior of 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, it is unlikely that the first continental crust 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 in 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.

Pervasive Eclogitization Due to Brittle Deformation and Rehydration of Subducted Basement: Effects on Continental Recycling?

“…Where deformation and fluid flow are mostly localized, such densification is unlikely, and fragments of dominantly mafic composition, once foundered, are likely to escape upward as well. Our density modeling indicates that, in order to exert slab-pull into the upper mantle, even mafic crustal rocks first have to be largely transformed to eclogite (as proposed for Western Norway; Austrheim et al, 1997;Bjørnerud & Austrheim, 2004).…”

Late Miocene–Pliocene eclogite facies metamorphism, D'Entrecasteaux Islands, SE Papua New Guinea

“…, 2000; Wain et al. , 2001; Bjornerud & Austrheim, 2004; Camacho et al. , 2005), mineral assemblages preserved in these eclogites largely lack hydrous phases and evidence for extensive veining.…”

The Interplay of Eclogitization and Deformation During Deep Burial of the Lower Continental Crust—A Case Study From the Bergen Arcs (Western Norway)

“…The structures observed on Holsnøy have been studied extensively (e.g., Austrheim, ; Austrheim & Griffin, ; Bhowany et al, ; Bjørnerud & Austrheim, ; Boundy et al, ; Camacho et al, ; Cohen et al, ; Griffin, ; Jamtveit et al, ; Jamtveit, Ben‐Zion, et al, ; Jolivet et al, ; Raimbourg et al, ). A general consensus is that the complex exposed on Holsnøy constitutes an excellent example of dry granulite‐facies lower continental crust, where the deformation and eclogitization processes during HP‐LT metamorphism in a continental collision or subduction setting can be studied at the surface (e.g., Jackson et al, ; Jolivet et al, ; Putnis et al, ).…”