A new lens into the Hadean Eon: Zircons from the Green Sandstone Bed, South Africa

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, long-lived 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. Zircon trace element signatures start resembling those of zircons formed in Archean tonalite–trondhjemite–granodiorite rocks (TTGs), which form by partial melting of hydrated basaltic rock. This shift mirrors changes in Hf isotopes and trace element geochemistry in other Archean terranes between ~3.8 to 3.6 Ga and supports the notion that the global onset of pervasive crustal instability and recycling – whether representing an incipient form of plate tectonics or other proposed mechanisms such as drip-and-plume tectonics – occurred in that time period.

About this Series: The Department Lecture Series at EAPS at MIT is a series of weekly talks given by leading thinkers in the areas of geology, geophysics, geobiology, geochemistry, atmospheric science, oceanography, climatology, and planetary science. For more information and Zoom password please contact Madelyn Musick: mmusick@mit.edu