HR: 12:05h
AN: T22B-08    [Abstracts]
TI: Radiogenic Heat Generation and the Evolution of Continental Crust During the First Half of Earth History
AU: * Morgan, P
EM: paul.morgan@dmns.org
AF: Denver Museum of Nature & Science, Dept. Earth Sciences, 2001 Colorado Blvd., Denver, CO 80205, United States
AB: Reports during the past few years that many early Proterozoic terrains are reworked Archean crust support the interpretation by Morgan (1985, JGR, v 90 Suppl., C561-C570) that "low" heat flow in modern Archean cratons is a manifestation of selective survival of Archean continental crust with low intrinsic radiogenic heat production. The importance of intrinsic crustal radiogenic heat production in the reworking of early continental crust has been extended to explain the lack of preservation of crust from the first 550 Ma of Earth history (the Hadean: Morgan, 2006, Eos, Trans. AGU, 87 (52), Fall Meet. Suppl., Abstract VC11D-0630). Low crustal heat production implies thicker lithosphere relative to higher crustal heat production and does not preclude the recording of chemical evolution of the mantle lithosphere through time, as indicated by xenolith data. However, the thick lithosphere models do not explain the generally low measured heat productions values in Archean cratons, nor the statistically chemically distinct composition of Archean upper crust, including low U, Th, and K (for this discussion the Archean-Proterozoic boundary is taken as a transition from ~2.2 to ~2.8 Ga based on general crustal characteristics rather than a globally synchronous chronologically boundary at 2.5 Ga). Based on radiogenic heat production arguments alone, global heat loss must have been higher in the Archean than at present. Although the partitioning between continental and oceanic heat flow in the Archean cannot be assumed, any mechanism that assumes more efficient oceanic heat loss in the Archean requires higher asthenospheric temperatures, which are likely to have affected continental as well as oceanic lithosphere. Assuming that radiogenic heat producing elements have remained essentially immobile in Archean cratonic crust since the stabilization of these cratons, with the exponential decay of radioactive decay with time, ancient geotherms should have been hotter than modern geotherms. This conclusion suggests that xenoliths do not record ancient geotherms, nor do they constrain the thickness of ancient continental lithosphere.
DE: 1020 Composition of the continental crust
DE: 5134 Thermal properties
DE: 8125 Evolution of the Earth (0325)
DE: 8130 Heat generation and transport
DE: 9623 Archean
SC: Tectonophysics [T]
MN: 2007 Fall Meeting