HR: 08:20h
AN: V11B-02    [PDF]
TI: A Magma Genesis Model to Explain Growth History of Hawaiian Volcanoes: Perspectives of 2001-2002 JAMSTEC Hawaii Cruises
AU: * Takahashi, E
EM: etakahas@geo.titech.ac.jp
AF: Earth & Planetary Sciences, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguroku, Tokyo, 152-8551 Japan
AB: The 2001 and 2002 JAMSTEC Hawaii cruises have been carried out using RV-Kairei with ROV-Kaiko and RV-Yokosuka with submersible Shinaki-6500, respectively. The main focus of these cruises is 1) to clarify the growth history of Hawaiian volcanoes through geological study on deep submarine exposures, 2) to understand the nature of submarine rifts, 3) to understand the nature of magmas erupted on the deep ocean floor away from the center of the Hawaiian plume. The geologic reconstruction of gigantic landslides (Moore et al., 1989) provided opportunities to study the long-term growth history of Hawaiian volcanoes, approaches complimentary to those by HSDP. Using this approach, we studied the growth histories of Kilauea (Lipman et al., 2002), Koolau (Moore \& Clague, 2002; Yokose, 2002), and Mauna Loa (Yokose et al, this conference). The geochemical reconstruction of Koolau volcano showed a secular variation in basalt magma types; from Kilauea-like to Mauna Loa-like and finally the silica-rich Koolau-type tholeiites (Shinozaki et al. 2002). These chemical changes are associated with significant changes in Sr, Nd and Pb isotopes (Tanaka et al., 2002). Similar changes in basalt magma types have been found in the growth history of Haleakala volcano (Ren et al., 2003) and in HSDP cores representing the growth history of Mauna Kea. Accordingly, it is plausible that the basalt magma types found among Hawaiian shield volcanoes are not representing geographic trends (e.g., Kea-trend and Loa trend) but are representing different growth stages. In order to elucidate secular changes in the geochemistry of Hawaiian volcanoes newly revealed by this project, I have carried out high-pressure melting studies at 2-3 GPa with eclogite/peridotite composite starting materials (experimental detail will be given by Takahashi, this conference V03). In eclogite/peridotite reactive melting, magmas produced above the solidus of peridotite (1480C at 2.8 GPa) are silica deficient alkalic picrites (MgO=15 wt%, SiO2=45). In temperatures slightly below the peridotite solidus (1470-1450C), olivine-rich tholeiite magmas similar to those in Kilauea (MgO=13-15, SiO2=46-48) are produced. At temperatures well below the peridotite solidus (1450-1400C), opx reaction bands are formed at eclogite/peridotite interfaces and the partial melts formed in eclogite domains (saturated only with cpx and garnet) increase in SiO2 and decrease in MgO. These experimental results suggest that the systematic change in SiO2 composition from Loihi, Kilauea and Mauna Loa may represent lowering temperatures in magma feeding zones. The very silica-rich tholeiite that appeared at the final growth stage of Koolau volcano would have been produced in an eclogite pocket chemically isolated from ambient peridotite. The silica-undersaturated basanite and nephelinite magmas occurring in the post-shield stage and in the Hawaiian Arches may represent incipient partial melts rich in volatiles (H2O and CO2) and low in temperatures (1300-1400C). If these magmas are derived from the tail of the Hawaiian plume, large volumes of the plume must consist of entrained asthenospheric peridotite judging from their depleted Nd and Sr isotopes.
DE: 1025 Composition of the mantle
DE: 3630 Experimental mineralogy and petrology
DE: 3640 Igneous petrology
DE: 8400 VOLCANOLOGY
DE: 8434 Magma migration
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting