Clement Conditions in the "Hadean"

Presenter: John Valley, University of Wisconsin, Madison
When: September 27, 2010 11AM PDT

Detrital igneous zircons from Western Australia yield concordant U-Pb ages up to 4.4 Ga and provide the only direct evidence of conditions on the Early Earth. Parent rocks were destroyed by weathering and erosion; ages >4 Ga are known only from isolated crystals. These zircons and their mineral inclusions represent very small rocks and analysis is a technical challenge. These time capsules have been imaged by cathodoluminescence (CL) and back-scattered electron detecting (BSE), and analyzed for U-Pb age; isotope ratios of O, Li, Si, & Hf; trace elements; and inclusions.

What is the genesis of the pre-4 Ga zircon suite? In situ analyses argue against meteorites including the Moon, and also the Earth’s mantle. The question of oceanic vs. continental crust is more contentious. Zircons are common in oceanic plagiogranites and oxide gabbros leading to speculation that the pre-4 Ga samples originated in dominantly mafic crust. However, oceanic zircons are significantly different in trace elements, [Li] and δ7Li, δ18O, and mineral inclusions. There is no known modern oceanic or ophiolitic analog for the pre-4 Ga zircons. In contrast, similarities are strong to Archean continental crust, esp. tonalite–trondhjemite–granodiorite (TTGs). We see no compelling evidence for true granites.

Was the surface of Early Earth Hadean? Mildly elevated values of δ18O(Zrc) indicate low temperature interaction of protoliths with liquid water during weathering or diagenesis. Thus, steam atmospheres condensed to liquid water oceans (possibly ice) before 4.3 Ga, in agreement with geophysical estimates, and the period from 3.8 to 4.3 Ga is not distinctively “Hadean”. We propose that condensation of early steam atmospheres before 4.3 Ga should mark the start of the Archean.

Why are there no known rocks older than 4 Ga, but zircons have survived? Common thought suggests that rocks were destroyed by intense meteorite impacting and melting, or special tectonic processes, but the new δ7Li(Zrc) results suggest instead that a CO2-rich atmosphere and extensive weathering may have disaggregated the protoliths.

When did Earth become habitable to life? Both O and Li isotope ratios suggest that conditions were sufficiently clement before 4.325 Ga, possibly at 4.4 Ga. The first life may have been almost 1 byr earlier than the first microfossil evidence.


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