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Terra Nova, 25, 130–136, 2013 Abstract High‐Al2O3 tonalite, trondhjemite and granodiorite (TTG) magmas characterise felsic Archaean crust, yet are uncommon in the post‐Archaean rock record. Consequently, understanding the petrogenesis of these rocks provides valuable insights into early Earth processes. Fluid‐absent slab melting represents the dominant hypothesis for the origin of these rocks; however, the absence of voluminous magmas of intermediate composition formed concurrently with these TTGs is incompatible with expectations of slab water loss prior to slab melting. This study demonstrates that for reasonable Archaean mantle temperatures, slab‐derived water is captured by an anatectic zone near the slab surface, which melts via reactions that consume quartz, clinopyroxene and water to produce high‐Al2O3 Archaean trondhjemite. Late in the Archaean, the mantle cooled sufficiently to prevent wet melting of the slab, allowing slab water to migrate into the wedge and produce intermediate composition magmatism, which has since been associated with subduction zones.

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