THICK OCEANIC CRUST, HOT MANTLE, AND PLATE TECTONICS IN THE ARCHAEAN EARTH

Phanerozoic and Archaean suprasubduction zone (SSZ) ophiolites represent remnants of ancient oceanic lithosphere evolved in discrete, pre-collision subduction rollback cycles. Their geochemical features reflect repeated melting of a MORB mantle source in a subarc-forearc wedge that was modified by fluids released from subducting oceanic slab and sediment-derived melts. However, Archaean ophiolites show komatiitic-picritic bulk compositions, indicating elevated potential mantle temperatures (>1500°C), which caused higher degrees of partial melting. These conditions resulted in the formation of thicker oceanic crust (>20 km) in the Archaean than at present (~5–10 km). Typical Archaean oceanic crust included a lower layer of anorthosites, gabbros, and mafic lavas with ultramafic sills and an upper layer of gabbroic-dioritic sills, rare sheeted dikes, basaltic-to-picritic lavas, high-Mg andesites and IAT volcanics, and dunitic-to wehrlitic sills. The lower layer represents the first stage of oceanic crust formation during the initial stages of subduction, whereas the upper layer characterizes the products of adiabatically upwelling ultramafic picritic melts intruding into the overlying volcanic rocks as sills and dikes at later stages of subduction in the presence of slab rollback-induced extension. The rare occurrence of or the lack of sheeted dikes was a result of much higher spreading rates and higher mantle temperatures in the Archaean than at present or in the Phanerozoic. The occurrence of SSZ ophiolites in the Archaean record (as early as 3.8 Ga in the Isua greenstone belt, Greenland) suggests that Phanerozoic-type seafloor spreading and subduction zone processes were operating at this time, and that hydrothermal cooling beneath spreading centers and sinking of rigid lithospheric slabs via subduction were already established by then. This inferred plate tectonic regime in the Eoarchaean marks a significant shift from the earlier thermal state (Hadean-style thermal convection) of the Earth.