HR: 09:15h
AN: H41H-06    [Abstracts]
TI: The Relationship Between Carbonate Weathering, Hydrothermal Activity, and Strontium Isotopes of River Waters in Steady State Orogens
AU: * Chamberlain, C P
EM: chamb@pangea.stanford.edu
AF: Department of Geological and Environmental Science, Stanford University, Building 320, Stanford, CA 94305 United States
AB: We present a model for steady-state chemical weathering. This model is applicable to the case where uplift provides a constant supply of fresh rock to the bottom of the weathering zone that is balanced by physical erosion from the surface. This condition of steady-state chemical weathering should exist in steady state orogens, at certain temporal and spatial scales. We have applied this model to the weathering of granitic composition rocks that contain trace amounts of calcite to examine the relationship between calcite weathering and rock uplift. Trace amounts of hydrothermal calcite are ubiquitous in rocks of granitic composition exposed in active collisional mountain belts. These calcite veins form as a result of hydrothermal systems that occur in areas of high uplift rates and surface elevation. The calcite veins are often enriched in 87Sr which complicates interpretation of the relationship between the marine strontium isotope record, silicate weathering, and long-term drawdown of carbon dioxide. The strontium isotope composition of weatherable material will be strongly influenced by the abundance of calcite and the uplift rate. The strontium isotope composition of rock at the Earth's surface will be set by the strontium isotopic composition of the constituent minerals at their closure temperature and the length of time it takes for the rock to move from the closure depth to the weathering zone. Our calculations show that calcite dominates the flux of strontium in active orogens, and holds the strontium isotope composition of river waters essentially constant over a range of tectonic conditions from active orogens to temperate cratons. In steady state orogens the strontium flux and the 87Sr/86Sr values should vary in an inverse fashion and this close coupling provides information on the types of orogens supplying strontium to the oceans.
DE: 4885 Weathering
DE: 1030 Geochemical cycles (0330)
DE: 1040 Isotopic composition/chemistry
DE: 1065 Trace elements (3670)
DE: 0330 Geochemical cycles
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting