Detrital zircons record rigid continents after 2.5 Ga

Date & time

12.30–1.30pm 6 March 2017

Location

Green Room (J5)

Speakers

M. Linda Iaccheri

Contacts

 Joelle Ducommun

The Neoarchean-Paleoproterozoic boundary at ca. 2.5 Ga represents a turning point in Earth’s history and is marked by fundamental changes in the mantle, crust and atmosphere-hydrosphere compositions. These transitions show that the evolution of Earth’s deep interior and its exterior are linked, but the processes that lie behind the global transitions are cryptic. Using the isotopic signatures of detrital zircons to compare the nature of felsic magma source before and after 2.5 Ga may provide insight into the processes driving secular change. Here, new oxygen and Hf isotope compositions of detrital zircons from Paleoproterozoic meta-turbidites from the North Australian Craton are presented. The detrital zircons have, in part, local provenances and reflect a section of continental crust that records three magmatic events at 2.7 Ga, 2.5 Ga and 1.9 Ga. These ages straddle the Neoarchean-Paleoproterozoic boundary and correlate with the major peaks in the global zircon record. At 2.7 Ga, bimodal zircon eHf (+6 to +4, 0 to -7) and mantle-like d18O indicate both crustal growth and the reworking of older crustal components. At 2.5 Ga, a wide range in zircon eHf (+7 to -12) and d18O (5 to 7‰) reflects reworking of infracrustal and supracrustal components of various age, with limited juvenile addition. At 1.9 Ga the eHf array contracts markedly (from +3 to -8) and is coupled with isotopically heavy oxygen (d18O from 7 to 9.5‰), requiring the partial melting of supracrustal sources. The lack of continuity of zircon Hf and oxygen isotope compositions from 2.5 Ga to 1.87 Ga indicates that there is no clear and simple crustal evolutionary trend from the Neoarchean to the Paleoproterozoic. The shift in the composition of the magma sources after 2.5 Ga, from dominantly infracrustal to dominantly supracrustal, implies a change in the mechanical behaviour of the lithosphere from soft to rigid, which may be responsible for the transition in the composition of the continental crust at the Neoarchean-Paleoproterozoic boundary. This finding implies that the processes of crust formation and reworking in the Archean were different from today.

Updated:  23 October 2017/Responsible Officer:  RSES Webmaster/Page Contact:  RSES Webmaster