HR: 0800h
AN: T31A-1281    [Abstracts]
TI: Evolution of Composition, Thermal Structure, and Resistance to Deformation of Continental Lithosphere Through Time: Why Archean Diamonds Are Probably Proterozoic in Age.
AU: * Morgan, P
EM: Paul.Morgan@nau.edu
AF: Northern Arizona University, Department of Geology, Box 4099, Flagstaff, AZ 86011 United States
AU: * Morgan, P
EM: Paul.Morgan@nau.edu
AF: National Key Center for Geochemistry and Metallogeny of Continents, Department of Earth and Planetary Sciences Macquarie University, Sydney, NSW 2109 Australia
AB: Ductile deformation of lithosphere is temperature-dependent and is also controlled by composition. Understanding evolution of continental lithospheric strength through time, and its related resistance to deformation, thus depends on understanding on the thermal history of continental lithosphere and changes in composition through time. Composition and temperature also control lithospheric density on which gravitational body forces are dependent. Studies of mantle xenoliths of different ages and geotherms in lithospheres from which they originate indicate decreasing stability in mantle lithosphere to Rayleigh-Taylor convection from Archean through Proterozoic to Phanerozoic continental lithospheres. In extrapolating lithospheric conditions back in time, probably the most important parameter to consider is heat flow. Both secular cooling and the exponential decrease in energy release from radioactive decay require that heat loss from the early Earth was significantly greater than from the modern Earth. Although most modern heat loss is through the ocean floor, not all additional heat loss from the early Earth can be accommodated by increased oceanic heat loss, and some increase in continental geotherms associated with increased asthenosphere temperatures is predicted. In addition, unless there has been significant change in the incompatible element concentration and/or distribution within the surviving fragments of Archean lithosphere, the ancient stabilized geotherms in these columns of lithosphere must have been higher than today because of the exponential decay of their intrinsic radioactive heat production. These factors require average continental geotherms in the Archean to be significantly higher than modern geotherms, although the maximum geotherm was probably the same, buffered by the lithospheric solidus. Although gem-quality diamonds are dated by their inclusions to be Archean in age, the continental geotherms probably did not cool into the diamond stability field until Proterozoic times.
DE: 7218 Lithosphere and upper mantle
DE: 8110 Continental tectonics--general (0905)
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 8125 Evolution of the Earth
DE: 8130 Heat generation and transport
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting