HR: 0800h
AN: V41C-1465    [Abstracts]
TI: Isotopic Variations in the Post-Shield Lavas of Mauna Kea, Hualalai and Kohala: Evidence for a Koolau Component
AU: * Aciego, S
EM: aciego@eps.berkeley.edu
AF: Department of Earth and Planetary Science, University of California, Berkeley, Berkeley, CA 94720 United States
AU: DePaolo, D J
EM: depaolo@eps.berkeley.edu
AF: Department of Earth and Planetary Science, University of California, Berkeley, Berkeley, CA 94720 United States
AU: Eiler, J M
EM: eiler@expet.gps.caltech.edu
AF: Division of Geological Planetary Sciences, California Institute of Technology, Pasadena, CA 91125 United States
AU: Hanano, D
EM: dhanano@eos.ubc.ca
AF: Earth and Ocean Sciences, University of British Columbia 6339 Stores Road, Vancouver, BC V6T-1Z4 Canada
AU: Weis, D
EM: dweis@eos.ubc.ca
AF: Earth and Ocean Sciences, University of British Columbia 6339 Stores Road, Vancouver, BC V6T-1Z4 Canada
AB: We have measured O, He, Pb, Sr, and Nd isotope ratios in olivine separates and whole rock powders from post-shield lavas of the island of Hawaii. These small-volume eruptions, which presumably correspond to small-volume source regions in the mantle, serve as high resolution probes of geochemical heterogeneity to complement data available from shield-stage tholeiites that originate in the primary melting region. He, Sr, and Nd ratios show small amplitude variations, well within the range of previously published data indicating little evidence for isotopically anomalous material in the melting region - either recycled sediments or oceanic crust. These data suggest that the plume does not contain large-amplitude isotopic variations at the scale of the source volume of post-shield lavas, which is likely to be about 100 times smaller than the volume averaged during main stage tholeiite production. However, the oxygen and lead isotope data indicate consistent, large amplitude differences between Hualalai and the other volcanoes. Samples from Hualalai have high δ¹⁸O (+5.5) and low ²⁰⁶Pb/²⁰⁴Pb (17.888-18.028). The only other volcanoes in the Hawaiian chain with such correlated extreme values are Koolau and Lanai, which have enriched radiogenic isotopic Sr and Nd compositions not seen in the data from Hualalai. Furthermore, correlated low helium values (8-10R/Ra)- high δ¹⁸O from Hualalai are consistent with a Koolau component in the source region for this volcano. The high δ¹⁸O contra-indicates a significant lithospheric interaction that would probably decrease δ¹⁸O rather than increase it to the values we measure. Samples from Mauna Kea and Kohala are consistently lower in δ¹⁸O (+4.8-5.12) than those from Hualalai, and fall within the range of previously measured values for late shield-stage lavas. The multiple isotope systems measured on the same post-shield samples indicate that simple models for Kea and Loa trends based on main stage tholeiitic lavas do not capture some of the most significant geochemical features. In particular, there are large temporal variations in the Loa trend, so that the difference between the Loa and Kea trends varies with time.
DE: 1025 Composition of the mantle
DE: 1040 Radiogenic isotope geochemistry
DE: 1041 Stable isotope geochemistry (0454, 4870)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005