PETROLOGIC, HYDROLOGIC AND RHEOLOGICAL IMPLICATIONS OF A HOTTER ARCHEAN EARTH

Although liquid water may have been present on the surface of the Earth as early as 4.4 Ga, there is compelling evidence that the solid Earth was hotter in Archean time. On first consideration, a higher paleo-geothermal gradient would seem to suggest that the brittle-plastic transition was shallower in the Archean lithosphere, but this may not have been true if the distribution of fluids in the crust was significantly different than today. In the more recent geologic past (Proterozoic to present), water has been so ubiquitous in the middle and lower crust that its critical roles in dictating rock rheology and the kinetics of metamorphic reactions are difficult to separate from temperature effects. A complex of exceptionally dry Proterozoic mafic granulites in the Bergen arcs, Norway, provides insight into how differently rocks respond to deformation and metamorphic conditions in a nearly fluid-absent environment. These rocks experienced eclogite-facies pressures and temperatures during Caledonian crustal thickening but equilibrated only locally to these conditions. Areas of uncoverted granulite contain pseudotachylytes with microlites of eclogite-facies minerals, showing that these rocks were strong enough to fail seismically at depths of >50 km, significantly deeper than the base of the modern continental seismogenic zone. Fracturing, possibly seismic, allowed the infiltration of limited amounts of water into the complex and facilitated the delayed eclogite-forming reactions. But the slightly hydrous eclogites were dramatically weaker than the granulites, and once a critical fraction of the rock mass had been converted, the process arrested itself because the rocks were no longer strong enough to fracture and allow more water to enter.

These rocks may be good analogs for thickened Archean lithosphere with a steeper geotherm. Any rocks that experienced eclogite facies conditions as a result of tectonic burial would first have passed through higher temperatures than today, causing devolatilization reactions to occur at shallower depths. This, together with melt extraction from water-fluxed crust, may have made the Archean lower crust exceptionally dry, strong, buoyant (owing to the persistence of metastable phases), and difficult to subduct or rehydrate.