HR: 14:10h
AN: H33E-01 INVITED     [Abstracts]
TI: The Start of Plate Tectonics in the Eoarchean: A Tribute to Gilbert N Hanson, Pioneer in Archean Geochemistry
AU: * Shirey, S B
EM: shirey@dtm.ciw.edu
AF: Department of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, DC 20015, United States
AU: Kamber, B S
EM: bkamber@laurentian.ca
AF: Department of Earth Sciences, Laurentian University, Sudbury, ON P3E 2C6, Canada
AU: Whitehouse, M J
EM: martin.whitehouse@nrm.se
AF: Laboratory for Isotope Geology, Swedish Museum of Natural History, Stockholm, S-105 05, Sweden
AU: Mueller, P A
EM: mueller@geology.ufl.edu
AF: Department of Geological Sciences, University of Florida, Gainesville, FL 32611, United States
AU: Basu, A R
EM: abasu@earth.rochester.edu
AF: Earth and Environmental Sciences, University of Rochester, Rochester, NY 14627, United States
AB: The use of isotopic and trace element geochemistry and igneous petrology to understand the petrogenesis of Archean rocks was pioneered by Gilbert Hanson and Joseph Arth at SUNY Stony Brook in the 1970's. Extension of these approaches allows the onset of plate tectonics on Earth shortly after the end of the Hadean to be specified. Nb/Th and Th/U ratios of mafic-ultramafic rocks from the depleted upper mantle begin to change from 7 to 18.2 and 4.7 to 2.9 (respectively) at 3.6 Ga. This signals the appearance of subduction-altered slabs in general mantle circulation from subduction initiated at 3.8 Ga. Juvenile crustal rocks begin to show derivation from progressively depleted mantle with typical igneous ε_Nd:ε_Hf = 1:2 after 3.6 Ga. Cratons with stable mantle keels that have subduction imprints begin to appear at 3.5 Ga. These changes all suggest that extraction of continental crust by plate tectonic processes was progressively depleting the mantle from 3.6 Ga onwards. Neoarchean subduction appears largely analogous to present subduction except in being able to produce large cratons with thick mantle keels. The earliest Eoarchean juvenile rocks and Hadean zircons have compositions that reflect the integrated effects of separation of an early enriched reservoir and fractionation of perovskite from the Mars-size impact-derived magma ocean, rather than separation of voluminous continental crust or oceanic plate tectonics. Hadean zircons most likely were derived from a continent-absent, mafic to ultramafic protocrust that was multiply remelted between 4.4 and 4.0 Ga under wet conditions to produce evolved felsic rocks. If the protocrust was produced by global mantle overturn at ca 4.4 Ga, then the transition to plate tectonics resulted from radioactive decay-driven mantle heating. Otherwise, such protocrust would have been the typical product of mantle convection and the transition to plate tectonics resulted from cooling to the extent that large lithospheric plates stabilized.
DE: 1038 Mantle processes (3621)
DE: 1040 Radiogenic isotope geochemistry
DE: 1065 Major and trace element geochemistry
DE: 3610 Geochemical modeling (1009, 8410)
DE: 8125 Evolution of the Earth (0325)
SC: Hydrology [H]
MN: 2007 Joint Assembly