HR: 1340h
AN: U33A-0006    [Abstracts]
TI: Radiogenic ³He/⁴He Estimates and Their Effect on Calculating Plio-Pleistocene Cosmogenic ³He Ages of Alluvial-Fan Terraces in the Lower Colorado River Basin, USA
AU: * Fenton, C
EM: crfenton@gfz-potsdam.de
AF: GeoForshungsZentrum Potsdam, Telegrafenberg Haus B, Potsdam, D-14473 Germany
AU: Pelletier, J
EM: jon@geo.arizona.edu
AF: Department of Geosciences University of Arizona, Gould-Simpson Building 1040 E. Fourth St., Tucson, AZ 85721 United States
AB: Several alluvial-fan terraces near Topock, AZ were created by successive entrenchment of Pliocene and Pleistocene alluvial-fan gravels shed from the adjacent Black Mountains along the lower Colorado River corridor below Hoover Dam. These fans interfinger with and overlie main-stem Colorado River sands and gravels and grade to terrace levels that correspond with pre-existing elevations of the Colorado River. Absolute dates for the ages of Quaternary deposits on the lower Colorado River are rare and cosmogenic ³He age estimates of these surfaces would help constrain the timing of aggradation and incision in the lower Colorado River corridor. We analyzed individual basalt boulders from several terrace surfaces for total ³He/⁴He concentrations to calculate cosmogenic ³He ages of each fan terrace; ³He/⁴He values, expressed as R/Ra where Ra is the ³He/⁴He of air, range from 0.29 to 590. Black Mountain volcanic rocks have reported K-Ar ages between 15 and 30 Ma and basalt samples from adjacent alluvial fans contain 0.42 to 47× 10¹² at/g of ⁴He, which has likely accumulated due to nuclear processes. The amount of radiogenic ³He/⁴He can be significant in old rocks with young exposure ages and can complicate determination of cosmogenic ³ He content. Alpha-decay of U, Th, and their daughter isotopes produces large amounts of ⁴He, whereas significant amounts of radiogenic ³He are only produced through the neutron bombardment of Li and subsequent beta-decay of tritium. We measured Li, U, Th, major and rare-earth element concentrations in whole-rock basalts and mineral separates. These concentrations are used to estimate the ratio of radiogenic helium contributed to the total helium system in our samples. Li concentrations typically range from 6 to 17 ppm, with one outlier of 62 ppm. U contents range from <0.1 to 2.7 ppm and Th contents range from 0.4 to 15.3 ppm. Based on these values, our calculations predict that the average radiogenic helium (R/Ra) contributed to the total helium in Black Mountain basalt samples is 0.011. Other noble gas studies have shown that radiogenic ³He/⁴He is independent of the U content, nearly independent of the Th content, and strongly influenced by the Li content of a rock; we find the same results. It is assumed that mantle gases are released when the sample is crushed into a fine powder before melting in a furnace under vacuum. To correct for the possible presence of mantle gases in our age-calculations, we crushed two samples under vacuum to measure the R/Ra value (7.9 and 16.03) of mantle helium trapped in fluid inclusions in olivines and pyroxenes. Based on our ³He corrections and calculations, boulders on these alluvial fans range in age from 10 ka to 2.7 Ma.
DE: 1105 Quaternary geochronology
DE: 1150 Cosmogenic-nuclide exposure dating (4918)
SC: Union [U]
MN: Fall Meeting 2005