Trace element and Lu-Hf systematics in Hadean-Archean detrital zircons; implications for crustal evolution

The very limited Hadean and Eoarchean rock record has greatly affected our understanding of the earliest evolution of the planet but has also spurred a strong interest in using the chemical characteristics of ancient detrital zircons as proxies for this missing record. In the northern Wyoming Province, detrital zircons with U-Pb ages from 3.3 to 4.0 Ga exhibit trace element and U-Pb systematics that suggest minimal, if any, disturbance to the Lu-Hf system since magmatic crystallization. The (super 207) Pb/ (super 206) Pb ages of these zircons define a continuum of ages within analytical error from 3.5 to 4.0 Ga. Initial Hf isotopic compositions outline a trend that begins at approximately 4.1 Ga, with values characteristic of primitive or slightly depleted mantle followed by a steady decrease in minimum initial values to about -9 at 3.5 Ga, characteristic of a reservoir with Lu/Hf of <0.1. Initial Hf compositions are variable, however, and indicate mixing with magmas derived from a primitive or depleted mantle that begins to increase substantially at approximately 3.5 Ga. These observations characterize the evolution of a system with extensive internal recycling that originated by melting of primitive mantle under anhydrous conditions, probably associated with a zone of mantle upwelling or diapirism. The combination of the continuum of ages and the likely Lu/Hf of the enriched reservoir from which these zircons were derived suggest that subduction-related magmatism did not play a significant role in crustal evolution in this region during the period 3.5-4.0 Ga. The noticeable increase in contributions from less enriched sources after 3.6 Ga suggests an increasing role for subduction-derived magmas in crustal growth. The contrasts between the negative initial eHf values at approximately 3.7-3.8 Ga that characterize detrital zircons from Wyoming (and Australia) and the higher values reported for ancient gneisses from Greenland for the same period suggest that crustal genesis occurred simultaneously in Hadean and Eoarchean analogues of both modern plume and subduction environments. The composition of zircons <3.5 Ga also provide some insight into the fate of these early enriched reservoirs.?