HADEAN-ARCHEAN CRUSTAL EVOLUTION CONSTRAINED BY TRACE ELEMENT ABUNDANCES AND LU-HF SYSTEMATICS OF DETRITAL ZIRCONS

Ancient detrital zircons are important proxies for the very limited Hadean rock record. Elemental and isotopic measurements of U, Pb, Lu, Hf, O, etc. in these chemically and physically robust grains are helping place more rigorous constraints on the early evolution of the solid earth, including the origins of the earliest continents (proto-continents) and the depleted mantle (DM). Here we report new U-Pb and trace element data obtained by ion microprobe (SHRIMP-RG), and Lu-Hf systematics (LA-MC-ICP-MS) of 3.3-4.0 Ga detrital zircons from the northern Wyoming Province. Critical observations from a subset of carefully screened zircons (Th/U ratios; Ca and Fe contents) include: 1) REE, U, and Y abundances, including Eu anomalies, indicate crystallization from moderately evolved (not primitive) magmas; 2) magmatic temperatures based on Ti abundances indicate a range from 660-820°C; 3) moderately high Hf contents (8,000-12,000 ppm) also suggest crystallization in at least moderately evolved magmas; and 4) Li abundances are mostly in the range 10-200 ppm and not indicative of derivation from mafic magmas. Significantly, variations in these parameters do not correlate with the ages of the individual zircons over the entire range of ages nor with their initial Hf isotopic compositions. We interpret these results in terms of a model in which early continents formed over mantle upwellings along with their deep tectospheres. A second stage of evolution, which lasted several hundred million years, was characterized by significant intra-crustal recycling. This second stage ended with an increasing flux of juvenile crust (mantle-derived), which may represent the initiation of plate tectonics in the modern sense. The formation of proto-continents over mantle upwellings and their ultimate modification by subuduction-related magmatism was likely diachronous on a global scale and reflects a changing balance between the ancient equivalents of mantle plumes (anhydrous mantle melting) and the mid-ocean ridge-subduction zone system (hydrous mantle melting) as the primary mode of crustal growth. The initiation of subduction along the margins of the proto-continents provides the mechanism for recycling of ancient enriched components into the mantle and younger crust.