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GSA Annual Meeting, November 5-8, 2001

Paper No. 0
Presentation Time: 4:15 PM

OSMIUM ISOTOPE SYSTEMATICS: OBSERVATIONS FROM THE CASCADE ARC


HART, Garret L.1, JOHNSON, Clark M.1 and SHIREY, Steven B.2, (1)Department of Geology and Geophysics, Univ of Wisconsin, 1215 W. Dayton St, Madison, WI 53706, (2)Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broadbranch Road, N.W, Washington, DC 20015, glhart@geology.wisc.edu

Radiogenic Os isotope compositions have been recognized for several convergent arcs, including the Andes, Mexico, Java, and Cascade arcs. Debate has centered on the origin of such radiogenic compositions, with models advocating either subducted slab components or interaction with the crust. New data for intermediate- to silicic-composition volcanic rocks from the Cascade Arc display a wide range of high Os isotope compositions (gamma-Os=+30 to +264), which are substantially more radiogenic than mantle values. The high Os isotope compositions cluster into two groups, one group with an elevated but limited range (gamma-Os=+30 to +50), and a second group extending to much more radiogenic values (gamma-Os=+50 to +264). The osmium contents of magnetite separates from these rocks are quite variable, extending from sub-ppt levels to >150 ppt. Rhenium contents vary even more widely, from ppt to ppb levels. In general the elevated Os isotope compositions show no arc-wide correlation with Os or Re content, although some correlation is observed within individual rock suites. Radiogenic Os isotope compositions also do not correlate with any existing Sr, Nd, or Pb data. The osmium isotope analyses indicate that a high gamma-Os component is involved in the generation and/or evolution of the magmas. The high gamma-Os values of the Cascade arc, coupled with other isotopic and elemental data that indicate little apparent subduction influence, suggest that the radiogenic Os component may lie within the crustal column, and record a crustal process that is not otherwise recorded or observed through the use of only Sr, Nd, and Pb isotope data in young mafic arcs. This high gamma-Os component may be young, fractionated, mantle derived material that has developed high Os isotope compositions due to high Re/Os ratios. Such fractionated material is thought to be present in the lower crust throughout the arc, and may be a common feature of orogenic arcs that is uniquely suited to detection using Os isotopes.

Session No. 129

Igneous Petrology II
Wednesday, 7 November 2001: 1:30 PM-5:30 PM
Geological Society of America Abstracts with Programs. Vol. 33, No. 6, October 2001, p.1806
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