HR: 1330h
AN: V32A-0991    [PDF]
TI: Pb Isotopic Evolution of Koolau Volcano (Oahu, Hawaii)
AU: * Fekiacova, Z
EM: fekiacov@mail.uni-mainz.de
AF: Max-Planck-Institut fuer Chemie, Postfach 3060, Mainz, 55020 Germany
AU: * Fekiacova, Z
EM: fekiacov@mail.uni-mainz.de
AF: Institut fuer Geowissenschaften Universitaet Mainz Abt. Geologie, Becherweg 21, Mainz, 55099 Germany
AU: Abouchami, W
EM: wafa@mpch-mainz.mpg.de
AF: Max-Planck-Institut fuer Chemie, Postfach 3060, Mainz, 55020 Germany
AB: High precision Pb isotopes in Hawaiian shield lavas have revealed the existence of source heterogeneities between volcanoes, as well as within a single volcano during its temporal evolution, e.g. Mauna Kea [1, 2]. The Koolau Scientific Drilling Project (KSDP) was initiated in order to evaluate the long-term evolution of Koolau volcano (Oahu), whose subaerial Makapuu stage lavas define the isotopically enriched endmember of Hawaiian shield lavas. We report Pb triple spike data on KSDP main shield-stage lavas (depth range: 304-632 mbsl) and post-erosional Honolulu volcanics. KSDP lavas show a small range of Pb isotopic compositions ($^{206}$Pb/$^{204}$Pb=18.02-18.15; $^{207}$Pb/$^{204}$Pb=15.44-15.46; $^{208}$Pb/$^{204}$Pb=37.82-37.87). Pb isotope ratios increase with depth until $\sim$450 m and then decrease again to a depth of 616 m. Superimposed on this "bell" trend, $^{206}$Pb/$^{204}$Pb ratios oscillate at depth intervals of $\sim$10m. The Honolulu volcanics display, at a given $^{206}$Pb/$^{204}$Pb ratio, similar $^{207}$Pb/$^{204}$Pb but lower $^{208}$Pb/$^{204}$Pb ratios than KSDP lavas. In $^{208}$Pb/$^{204}$Pb-$^{206}$Pb/$^{204}$Pb space, KSDP and Honolulu lavas define two distinct linear arrays which converge at the radiogenic end. However, in $^{207}$Pb/$^{204}$Pb-$^{206}$Pb/$^{204}$Pb space, KSDP and Honolulu lavas form a single array, with Honolulu lying at the radiogenic end of the array. While KSDP lavas have more radiogenic Pb isotopic compositions than Makapuu stage lavas [1], they show close resemblance to Nuuanu 1 and Nuuanu 2 landslide blocks [3]. The distinct Pb isotopic features of subaerial, main-shield and post-erosional lavas reflect compositional source changes during the growth of Koolau volcano. The mixing lines defined by KSDP and Honolulu lavas in $^{208}$Pb-$^{206}$Pb space require the presence of three distinct Pb isotopic components. While the enriched "Koolau" component is predominantly sampled during the subaerial stage, its contribution during the main shield building stage has been waxing and waning. The radiogenic Pb endmember common to Honolulu and KSDP lavas samples a plume component which apparently persisted during the post-erosional stage. The unradiogenic Pb component present in Honolulu lavas has Pb isotope characteristics that are clearly distinct from those of East Pacific Rise MORB [4], suggesting that it may not be derived from the underlying Hawaiian lithosphere. Available isotopic data on the oceanic crust basement near Hawaii [5] are unreliable, making a direct comparison difficult. While the Hawaiian lithosphere hypothesis cannot be entirely ruled out, we suggest that the unradiogenic Honolulu Pb component reflects material thermally entrained by the upwelling plume during its ascent and is essentially sampled during the post-erosional stage of Hawaiian volcanism. 1.Abouchami, W. et al., Eos Trans. AGU, 81 (48), Fall Meet. Suppl., Abstract V21D-03, 2000b 2.Eisele, J. et al., Geochem. Geophys. Geosyst., 4(2), 8710, doi:10.1029/2002GC000339, 2003 3.Tanaka, R. and Nakamura, E., Geophys. Monograph Ser., vol. 128, 311-332, 2002 4.Galer, S.J.G. et al., Eos Trans. AGU, 80 (46), Fall Meet. Suppl., Abstract V11E-08, 1999 5.King et al., Proc. ODP, Sci. Results., 136, 1993, 107-118
DE: 1040 Isotopic composition/chemistry
DE: 1749 Volcanology, geochemistry, and petrology
DE: 9355 Pacific Ocean
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting