Formation of Archean continental crust by Nicholas (Nick) Arndt, Emeritus Professor ISTerre, University Grenoble Alpes
The process that generated the continental crust has long been debated. One school argues that, through most of geological time, the crust formed at convergent margins like most modern granitoids. The other school promotes a model in which, from the Hadean to at least 2.5-3.0 Ga, the continental crust grew within a stagnant or sporadically mobile lid. In one version of this model, the lower part of thick mafic crust partially melts to produce felsic magma. In another, a process called sagduction operates: the lower part of a thick pile of mafic rocks converts to eclogite, sinks into the mantle, and partially melts to form the felsic magmas that build the continental crust.
In this paper, I argue that the latter processes are implausible. The lower part of thick oceanic crust consists of ultramafic cumulates and these rocks are dry. Missing is basalt and water, both of which are needed to produce granitic melt. Hydrated volcanic rocks never are deeply buried, and any water released during their dehydration migrates upwards into cooler rocks where it cannot trigger partial melting.
Subduction probably operated on Earth from at least 3.8 Ga. Archean subduction zones may have been shallow, short-lived and transient but they were capable of transporting hydrated oceanic crust to depths of 100 km, which is all that is needed for the formation of granitoid magmas. The presence of subduction zones does not necessary imply the operation of plate tectonics. A hybrid situation is possible in which a mobile or partially mobile lid covered most of the planet, but localized, shallow, transient subduction zones operated to generate the felsic magmas that built the continental crust.
All the changes that are said to signal a major change in geodynamic regime around 3 Ga are better explained as the nexus of several ongoing processes: (1) progressive growth of felsic crust, particularly during major mantle-generated pulses at 2.7 and 2.5 Ga; (2) the emergence of land that allowed erosion and generation of detrital sediments; and (3) a decline of temperatures in mantle and crust, which changed the style of intracrustal deformation.
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