2008 Joint Meeting of The Geological Society of America, Soil Science Society of America, American Society of Agronomy, Crop Science Society of America, Gulf Coast Association of Geological Societies with the Gulf Coast Section of SEPM

Paper No. 2
Presentation Time: 8:00 AM-6:00 PM

Plate Boundary Processes on the Eoarchean Earth

MOJZSIS, Stephen J. and CATES, Nicole L., Department of Geological Sciences, Univ of Colorado, UCB 399, 2200 Colorado Avenue, Boulder, CO 80309-0399, mojzsis@colorado.edu

The time of the onset of plate interactions on Earth is unknown. It is important to place temporal constraints on the initiation of this process at least for considerations of planetary heat-loss timescales, element recycling and habitats for the origin of life. We report direct evidence for plate boundary processes in the generation of melt compositions and sediments before 3.7 Ga. These data are from the geochemistry of the oldest supracrustal rocks, which include paragneisses, orthogneisses and mafic schists from Eoarchean terranes in West Greenland (≥3.83 Ga), northern Quebec (≥3.75 Ga) and a Hadean rock from northwestern Canada (ca. 4.03 Ga). Compositions for many of these lithologies strongly resemble those generated by some contemporary arc and back-arc volcanosedimentary systems. The metamorphic equivalents of tonalite-trondhjemite suites captured in the oldest terranes further show that subduction-like settings where partial melting of a subduction slab (or thickened mafic lower crust?) were widespread on Earth since at least ca. 3.83 Ga. Yet earlier geochemical indicators from oxygen- and hafnium isotopes, mineral inclusions and rare earth elements in Hadean detrital zircons (up to 4.38 Ga) suggest that some form of plate boundary process was operative to within 100 Myr of the Moon-forming event.