Testing the Plateau Melting Model of TTG Formation with Combined Thermal and Geochemical Models

Abstract:

TTGs, one of the archetypical lithologies of Archean cratons, are thought to be generated by the fractional crystallization of melting of hydrated, metamorphosed basalt, based on their geochemistry. In particular, hydrated basalt must melt in a particular temperature-pressure range, of ~700-1000°C and ~10-35 kbar, to produce TTGs. Recent studies of Hf isotopes in zircons from Archean cratons show that there is a global trend of descending epsilon Hf from ~4.0-3.6 Ga, followed by a shift to 0 or positive epsilon Hf in samples younger than 3.6 Ga. One hypothesis for this descending trend is that these TTGs were formed by re-melting of a Hadean protocrust, over the course of ~ 400 Myrs. Re-melting of a long-lived protocrust is consistent with a plateau melting model of early Earth TTG formation. However, what conditions allow for continual melting of a long-lived Hadean protocrust are not well understood.

We use a simple one-dimensional model, solving the advection-diffusion equation , to calculate the geotherm in a crustal plateau; downward advection of crust in the plateau is caused by the continual eruption of mafic melt at the surface. With this geotherm, we calculate when downward advected crust melts, and the theoretical epsilon Hf of the TTGs this melting would produce, under different thermal conditions of the Archean Earth. The slope of the descending trend of epsilon Hf in the models, and the time interval over which this descending trend is seen, are determined under different conditions. These two values are then compared to geochemical observations from the Acasta Gneiss complex. We use a Monte Carlo method to test which thermal conditions give results that are consistent with the data.

Preliminary results suggest that a higher downward advection rate, v, results in a steeper negative slope and that the thickness of the pre-existing crust thickness has an important effect on the time interval over which a trend of decreasing epsilon Hf can occur. With more careful modeling and calculations, this research will provide important constraints on the thermal conditions of the Archean Earth that allow for TTG formation in a crustal plateau setting, consistent with geochemical and petrological constraints.