Wed 21st January @ noon, Rm 210.104

Abstract:

Topic 1: A 4463 Ma apparent zircon age from the Jack Hills resulting from ancient Pb mobilization

Hadean (≥4.0 Ga) zircon grains provide the only direct record of the first half billion years of Earth’s history. Determining accurate and precise crystallization ages of these ancient zircons is a prerequisite for any interpretation of crustal evolution, surface environment and geodynamics on the early Earth, but this may be compromised by mobilization of radiogenic Pb due to subsequent thermal overprinting. Here we report a detrital zircon from the Jack Hills (Western Australia) with 4486 – 4425 Ma concordant ion microprobe ages that yield a concordia age of 4463 ± 17 Ma (2σ), the oldest zircon age recorded from Earth. However, scanning ion imaging reveals that this >4.4 Ga apparent age resulted from incorporation of micrometre-scale patches of unsupported radiogenic Pb with extremely high 207Pb/206Pb ratios and >4.5 Ga 207Pb/206Pb ages. Isotopic modeling demonstrates that these patches likely resulted from redistribution of radiogenic Pb in a ~4.3 Ga zircon during a ~3.8 Ga or older event. This highlights that even a concordia age can be spurious and should be carefully evaluated before being interpreted as the crystallization age of ancient zircon.

Topic 2: Remnants of Eoarchean continental crust derived from a subducted proto-arc

Eoarchean [3.6 – 4.0 billion years ago (Ga)] tonalite – trondhjemite – granodiorite (TTG) are the major component of Earth’s oldest remnant continental crust, thereby holding the key to understanding how continental crust originated and when plate tectonics started in the early Earth. TTGs are mostly generated by partial melting of hydrated mafic rocks at different depths, but whether this requires subduction remains enigmatic. Recent studies show that most Archean TTGs formed at relatively low pressures (≤1.5 GPa) and do not require subduction. Here we report a suite of newly-discovered Eoarchean tonalitic gneisses dated at ~3.7 Ga from the Tarim Craton, northwestern China. These rocks are probably the oldest high-pressure TTGs so far documented worldwide. Thermodynamic and trace element modelling demonstrates that the parent magma may have been generated by water-fluxed partial melting of moderately enriched arc-like basalts at 1.8 – 1.9 GPa and 800 – 830 ℃, indicating an apparent geothermal gradient (400 – 450°C GPa-1) typical for hot subduction zones. They also locally record geochemical evidence for magma interaction with a mantle wedge. Accordingly, we propose that these high-pressure TTGs were generated by partial melting of a subducted proto-arc during arc accretion. Our model implies that modern-style plate tectonics was operative, at least locally, at ~3.7 Ga and was responsible for generating some of the oldest continental nuclei.

Short bio:

Rongfeng Ge obtained a joint PhD from Nanjing University (China) and Curtin University in 2014 – 2015. He has been a research associate working with Prof. Simon Wilde at Curtin since April 2016 on the Jack Hills zircons, aiming to extracting a primary record of the Hadean zircons using various methods. He has also being working on the Precambrian geology of the Tarim Craton, with an emphasis on the origin and evolution of continental crust, as well as reconstruction of the Tarim Craton in the Rodinia and Columbia/Nuna supercontinents.