Hadean-Archean Transitions
My dissertation was titled "Hadean-Archean Transitions: Constraints from the Jack Hills Detrital Zircon Record." Most of my research focused on analyzing various isotopic systems in the Jack Hills zircons, a detrital population that ranges up to nearly 4.4 billion years (Ga) in age. Most previous investigations of the zircons naturally focused on the Hadean (>4 Ga) population, but I have demonstrated that the period 4.0-3.6 Ga contains several crucial events in the evolution of this early crust discernible in both the Lu-Hf isotopic system and in zircon trace element contents. In addition, the xenon isotopic compositions of Jack Hills zircons (measured in collaboration with the Manchester Isotope Group) may record alteration perhaps billions of years after their formation. Although most of my work focused on analyzing the zircons themselves, I also collaborated with Prof. Dave Stegman (at Scripps Instiution of Oceanography/UCSD) to build numerical models for subduction under warmer mantle conditions appropriate for the early Earth. We investigated potential geodynamic regimes given various crustal thicknesses, mantle temperatures, and mantle Rayleigh numbers to better constrain the conditions that could be expected for subduction-like regimes in the early Earth.
Is There a Terrestrial Record for the Hypothesized Late Heavy Bombardment?
Although the very earliest period of our solar system's history must have seen a very high rate of meteorite impacts (i.e., effecting planetary accretion), how impact rates in the inner solar system and Earth/Moon system evolved from this earliest period to the more quiescent solar system of the present day is uncertain. A spike in impact rates several hundred million years after solar system formation has been hypothesized based on reset isotopic systems in some lunar rocks. This so-called Late Heavy Bombardment (LHB) period is classically hypothesized to have occurred at ca. 3.9 Ga, although some recent models suggest multiple spikes in bombardment rates beginning much earlier. However, these are all based on ages from extraterrestrial samples, either meteorites or samples returned from the Moon. Earth's sparse rock record during this period has yielded limited opportunities to test whether such an event is evident in Earth's geologic record.
However, many Jack Hills zircons do indeed fall into the age range of the various LHB estimates, and we have looked at their geochemistry in detail to determine potential changes in geologic conditions during this period. A group of zircons with unusual trace chemistry consistent with solid-state recrystallization occurs between 3.92 and 3.84 Ga, potentially pointing to metamorphic heating of the source terrane near this time. Using the zircon record, we cannot distinguish between exogenic (meteorite bombardment) versus endogenic (e.g., tectonic events or magmatism) reasons for a heating event at this time. However, the coincidence with some LHB estimates is very intriguing and bears further study of Eoarchean samples elsewhere on Earth to help distinguish whether this was a global or merely local event.
Paper on provenance transitions ca. 3.9 Ga based on trace elements.
When was crust formed and destroyed in the Jack Hills source terrane?
176Lu decays to 176Hf with a half life of ~37 Ga, and differences in the melting behavior of Lu and Hf in the Earth has led the mantle to develop a higher Lu/Hf ratio than the crust. Over time this has led to much higher ratios of 176Hf/177Hf (177Hf is a stable, non-radiogenic isotope) in the mantle than in the continental crust. The quantity εHf is used to denote a sample's 176Hf/177Hf relative to that of a model bulk Earth, for which we use the 176Hf/177Hf ratio of chondrites. ("DM": depleted mantle; "Forbidden": original solar system εHf evolved forward in time with no 176Hf ingrowth.)
My Lu-Hf isotopic work on the Jack Hills zircons has demonstrated that some ancient crust was lost from the source of the zircons at around 3.9-3.7 Ga. Very unradiogenic (negative εHf) material seen before 3.7 Ga largely doesn't occur in the record after 3.7 Ga, suggesting loss of some amount of crust. This occurs at the same time that new juvenile (melts from the mantle) material appears, which is reminiscent of the crustal evolution seen in subduction settings today. Because of this I argued that this εHf pattern may be due to recycling by a subduction-like process. Either way, it points to an important event in the evolution of the crust that formed the Jack Hills zircons at ca. 3.9-3.7 Ga.
