Dartmouth Events

The nature of Earth’s earliest crust and crustal processes remain unresolved questions in Precambrian geology. While some hypotheses suggest that the Hadean Eon was more similar to today than previously thought, including some form of plate tectonics, others suggest that the Hadean was characterized by long-lived protocrust and an absence of significant plate tectonic processes. Recently proposed trace-element proxies for the tectono-magmatic settings in which zircons formed are a relatively novel tool to understand crustal processes in the past. I will present high-spatial resolution zircon trace and rare earth element geochemical data along with Hf and O isotope data of a new location with Hadean materials, 4.1 to 3.3 Ga detrital zircons from the 3.31 Ga Green Sandstone Bed, Barberton Greenstone Belt. Together, the hafnium isotope and trace element geochemistry of the detrital zircons record a major transition in crustal processes. Zircons older than 3.8 Ga show evidence for isolated, longlived protocrust derived by reworking of relatively undepleted mantle sources with limited remelting of surface-altered material. After 3.8 Ga, Hf isotopic evidence for this protocrust is muted while relatively juvenile source components for the zircon’s parental magmas become more prominent. Trace element signatures start resembling those of zircons formed in Archean TTGs, which form by melting of hydrated basalt, possibly at depth. This shift mirrors changes in Hf isotopes and, though only limited data is available, trace element geochemistry in other Archean terranes between ~3.8 to 3.6 Ga and may indicate the notion of a global change in crustal conditions or processes at that time.

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