2/21/2012 - Evidence for the Late Heavy Bombardment in 4.0-3.8 Ga Jack Hills zircons?


12:00 PM - 1:00 PM
4460 Geology

Presented By:
Beth Ann Bell - UCLA


The Late Heavy Bombardment (LHB) is a hypothesized period of intense bombardment of the inner solar system at ca. 3.9 Ga, inferred from disturbed lunar ages. Major thermal effects to the Earth's crust are expected from LHB-impact scenarios, but unambiguous terrestrial evidence of this event is unknown, probably owing to the sparse geologic record from this period. We investigated the trace element chemistry of the detrital zircon record from Jack Hills, Western Australia for the period 4.0 ? 3.8 Ga in search of apparent changes in thermal conditions consistent with the LHB. The Ti-in-zircon temperature (Txlln) distribution is well established for Hadean detrital grains, clustering about an average value of ~680°C likely indicating near water-saturated granitic melting conditions. The average Txlln does not change appreciably through the period 4.0-3.8 Ga, but between 3.91-3.84 Ga, there is a notable group of low-Ti zircons with apparent Txlln extending well below the granite solidus. Further investigation revealed that this period contains two groups of zircons with clearly distinguishable trace element signatures. Group I resembles the Hadean Jack Hills zircons in Ti, Hf, Ce, U, and Th/U, whereas Group II contains lower Ti, Ce, and Th/U along with higher Hf and U. Group II also displays a high degree of U-Pb concordance compared to the 4.0-3.8 Ga Jack Hills zircons as a whole, despite their high U contents. We interpret Group II as originating from the solid-state recrystallization of originally magmatic (perhaps even Group I-like) zircon during a thermal event ca. 3.9 Ga. This thermal excursion is also seen in epitaxial growths on Hadean zircon cores. Although an endogenic cause for this apparent thermal event cannot be definitively ruled out, these observations may constitute the first terrestrial evidence for the LHB.

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