HR: 13:55h
AN: V43E-02    [Abstracts]
TI: Strontium Isotopic Variations in the Koolau Volcanic Series, Oahu, Hawaii: Results from KSDP Drill Core
AU: * Smith, M M
EM: mms@eps.berkeley.edu
AF: Department of Earth and Planetary Science, University of California, Berkeley, CA 94720-4767 United States
AU: DePaolo, D J
EM: depaolo@eps.berkeley.edu
AF: Department of Earth and Planetary Science, University of California, Berkeley, CA 94720-4767 United States
AB: Surface samples of the Koolau tholeiite series, from the eastern side of the island of Oahu, Hawaii, have long been noted for their unusually high ⁸⁷Sr/⁸⁶Sr ratios (up to 0.7042) and other extreme geochemical parameters, as compared to both earlier and later Oahu lavas, values from other Hawaiian islands, and lavas from the Waianae volcano on west Oahu. It has been assumed that the geochemistry of the surface samples of Koolau applied to most of the volcano and that the extreme features were a relatively long-lived characteristic of the Hawaiian mantle plume at the time that the Koolau lavas were being erupted about 3 million years ago. The Koolau Scientific Drilling Project, which returned nearly continuous core from depths of 350 to 670 meters below sea level, provided an opportunity to probe deeper into the Koolau edifice (Haskins and Garcia, CMP, 147, 2004). We present new Sr isotope data on thirty whole rock samples from KSDP, which complement other isotopic data that have been reported recently (Salters and Blichert-Toft, submitted). The KSDP samples have variable, but generally significant, amounts of post-eruption weathering and hence the samples were strongly acid-leached before TIMS isotopic analysis in order to remove any seawater-derived strontium. The ⁸⁷Sr/⁸⁶Sr values in the core samples vary from values near 0.7040 at the top of the core to 0.7035 near the bottom. There is a general trend of increasing ⁸⁷Sr/⁸⁶Sr upsection as well as oscillations with peak-to-peak amplitude of 0.0003. The Sr isotope ratios correlate reasonably well with Nd and Hf isotope ratios. The data show that the Koolau surface samples are not representative of the volcano as a whole, and that the extreme geochemistry of the surface samples may represent only a minor component of the Hawaiian plume. The normal trend of Sr isotope ratios in the waning stages of shield building is from high values to low (as in Mauna Kea, Kohala, East Molokai and Haleakala). A trend toward higher ratios is seen elsewhere only in Late Pleistocene to recent Mauna Loa lavas. Models of lava accumulation rates for Hawaiian volcanoes suggest that the time interval of the KSDP core is probably 40 to 60 kyr, so the large shift of Sr isotope ratios happens over a short time; this is also true of the similar shift seen in Mauna Loa. The oscillations are also rapid - with cycle times of about 10 kyr. These rapid changes are difficult to account for with magma generation models; they seem to suggest either that there are very large amplitude variations in the mantle magma source, or that the magma extraction process is discontinuous. In general, there is a consistent pattern in Hawaiian volcanoes that the lava sections with the highest Sr isotope ratios also show the most internal isotopic variability. A simple interpretation of this trend is that the mantle sources with lower ⁸⁷Sr/⁸⁶Sr (ca. 0.7035) represent mantle that has been stirred by convection for a longer time than those with ⁸⁷Sr/⁸⁶Sr > 0.7038.
DE: 1040 Radiogenic isotope geochemistry
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005