Detrital zircon age, oxygen and hafnium isotope systematics record rigid continents after 2.5 Ga

Abstract

The Neoarchean-Paleoproterozoic boundary at 2.5 Ga is marked by fundamental changes in the composition of the mantle, crust and atmosphere-hydrosphere. These changes show that the evolution of Earth's deep interior and its exterior are linked, but the causes of the global transitions are cryptic. The isotopic signatures of detrital zircon enable the nature of felsic magma sources before and after 2.5 Ga to be compared, providing insight into the processes driving secular change. For this purpose, we present new oxygen and Hf isotope data from detrital zircon grains hosted by Paleoproterozoic metasedimentary rocks of the North Australian Craton, which record three magmatic events at 2.7 Ga, 2.5 Ga and 1.87 Ga. Scattered zircon εHf (+6 to −10) coupled with mantle-like δ18O at 2.7 Ga indicates both new crustal addition and the reworking of older materials. At 2.5 Ga, a wide range in zircon εHf (+7 to −12) and δ18O (5 to 7‰) reflects reworking of infracrustal and (subordinate) supracrustal components of various crustal residence age. The dominance of subchondritic zircon εHf suggests that depleted mantle inputs were limited. The εHf array contracts markedly (+3 to −8) at 1.87 Ga and is coupled with isotopically heavy oxygen (δ18O from 7 to 9.5‰), indicating a substantial contribution from clay-rich supracrustal sources. We attribute the contraction of the zircon εHf array at ca. 1.87 Ga to the melting of a range of Neoarchean crustal components, where the disparate Hf isotope signatures of these were partially homogenised by sedimentary processes. The shift in felsic magma sources after 2.5 Ga, from dominantly infracrustal to supracrustal, implies a change in the mechanical behaviour of the lithosphere, from soft to rigid. This may have contributed to the transition in the composition of the continents at the Neoarchean-Paleoproterozoic boundary.