On the modern Earth, plate tectonics offers an efficient mechanism for mass transport from the Earth's surface to its interior, but how far this mechanism dates back in the Earth's history is uncertain.

The Kaapvaal Craton and specifically the Barberton granitoid-greenstone terrain (BGGT) provide a robust rock-based benchmark for early Earth studies because of exceptional preservation of early Archean TTG and greenstone rocks. The authors examined O isotopes in zircon grains from tonalite-trondhjemite-granodiorite (TTG), the major felsic lithology of the Archean continental crust, to evaluate the recycling of supracrustal materials in the Archean.

Zircon d18O values of the TTG rocks in the Barberton area show an overall increase in terms of ranges and averages at ca. 3230 Ma, close to the Paleoarchean-Mesoarchean boundary. The >3230 Ma TTGs have zircon d18O values mostly in the range of mantle zircon with averages from +5.07 mph to +6.02 mph , whereas the TTGs of =3230 Ma have mildly elevated zircon d18O values with the largest value close to ~8 mph (averages ranging from 5.95 mph to 7.08 mph ).

These findings from the BGGT TTGs provide the first robust evidence for significant crustal recycling to depths of >40 km from ca.

3230 Ma in ancient continents. Because 18O enrichment is a diagnostic signature of low temperature water-rock interaction on the Earth's surface, the nearly synchronous increase in the maximum zircon d18O points to the onset of widespread reworking of supracrustal materials at crustal depths of >40 km at the Paleoarchean-Mesoarchean boundary.

The authors also proposed that plate tectonics was operating through warm subduction for lithospheric thickening along converging plate boundaries. As soon as the mafic oceanic crust was transformed to garnet granulite through regional metamorphism at moderate thermal gradients, gravitational delamination (or proto-subduction) and large thrust faults would take place along previously thickened boundaries.

This may induce the partial melting of the low-T altered mafic oceanic crust to form high-d18O zircon in the resultant TTGs. Such proto-subduction may be short-lived and intermittent due to frequent slab breakoff in the hotter Archean mantle.

Research Report: "The onset of deep recycling of supracrustal materials at the Paleo-Mesoarchean boundary"

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Nanjing University
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