The physical state of the Earth during the Hadean was characterized by 1) heavy bombardment episodes; 2) extremely high internal heat production in crust and mantle rocks, and 3) high temperatures left over from Earth and core formation processes. Taken together, a strong case is made for extremely high internal temperatures, low internal viscosities, and extremely vigorous mantle convection. Previous studies of mixing in high-Rayleigh number convection indicate that chemically heterogeneous mantle anomalies should have efficiently remixed into the mantle on timescales of less than 100Myr. However, Nd isotope studies indicate that heterogeneous mantle domains survived, without mixing, for over 2Gyr -- at odds with mixing rates expected. Similarly, PGE concentrations in Archaean komatiites, purportedly due to the laterveneer of meteoritic addition on the Earth, only achieve current levels at 2.7Ga -- indicating a time lag of almost 1-2Gyr in mixing this material thoroughly in the mantle. Plausible mixing retardation mechanisms include layering of mantle convection, anomalously viscous blobs of material, or the cessation of tectonic resurfacing mechanisms. Here we show that layered convection is limited in models with strong tectonic plates, and argue that high viscosity blobs are undemonstrated. In contrast, a number of lines of evidence suggest resurfacing in the Archaean was episodic, and extending these models to Hadean times implies the Hadean was characterized by long periods of tectonic quiescence. We show that mixing in stagnant lid regimes is, at the extreme, over an order of magnitude less efficient than mobile lid mixing, which may explain unduly long mixing times recorded in the Hadean/early Archaean.
|Number of pages||1|
|Publication status||Published - 2012|
|Event||International Geological Congress (34th : 2012) - Brisbane, Australia|
Duration: 5 Aug 2012 → 10 Aug 2012
|Conference||International Geological Congress (34th : 2012)|
|Period||5/08/12 → 10/08/12|