HR: 1330h
AN: V22A-0567    [PDF]
TI: A Geochemical Study of Magmatic Processes and Evolution along the Submarine Southwest Rift zone of Mauna Loa Volcano, Hawaii
AU: * Rhodes, J M
EM: jmrhodes@geo.umass.edu
AF: Dept. of Geosciences, University of Massachusetts, Amherst, MA 01003 United States
AU: Garcia, M O
AF: Hawaii Center for Volcanology, University of Hawaii, Honolulu, HI 96822 United States
AU: Weis, D
AF: Dept. of Earth and Ocean Sciences, University of British Columbia, Vancouver, BC V6T-1Z4 Canada
AU: Trusdell, F A
AF: Hawaii Volcano Observatory, U. S. Geological Survey, Volcano, HI 96718 United States
AU: Vollinger, M J
AF: Dept. of Geosciences, University of Massachusetts, Amherst, MA 01003 United States
AB: Mauna Loa's southwest rift zone (SWR) extends for 102 km from its summit caldera, at an elevation of 4,170 m above sea level, to submarine depths of over 4,500 m. About 65% of the rift zone is subaerial and 35% submarine. Recent sampling with the Jason II submersible of the `mile-high' (1800 m) Ka Lae submarine landslide scarp and the deepest section of the rift zone, in conjunction with previous submersible and dredge-haul collecting, provides petrological and geochemical understanding of rift zone processes, as well as a record of Mauna Loa's eruptive history extending back about 400 ka. The major and trace element trends of the submarine lavas are remarkably similar to those of historical and young prehistoric lavas ($<$31 ka) erupted along the subaerial SWR. We take this to imply that magma-forming processes have remained relatively constant over much of the volcano's recorded eruptive history. However, the distribution of samples along these trends has varied, and is correlated with elevation. There are very few picrites ($>$12% MgO) among the subaerial lavas, and compositions tend to cluster around 6.8-8.0% MgO. In contrast, picritic lavas are extremely abundant in the submarine samples, increasing in frequency with depth, especially below 1200 m. These observations support earlier interpretations that the submarine lavas are derived directly from deeper levels in the magma column, and that magmas from a shallow, steady-state, magma reservoir are of uncommon at these depths. Isotopic ratios of Pb and Sr in the submarine lavas, in conjunction with Nb/Y and Zr/Nb ratios, extend from values that are identical with subaerial historical Mauna Loa lavas to lavas with markedly lower 87Sr/86Sr and higher 206Pb/204Pb isotopic ratios. As yet, we see no correlation with depth or age, but the implications are that, in the past, the plume source of Mauna Loa magmas was more variable than in the last 31 ka, and contained a greater proportion of the Kea component. *Team members also include: H. Guillou, CEA/CNRS, France; M. Kurz and D. Fornari, WHOI; M. Norman and V. Bennett, ANU, Australia; S. Schilling, USGS; M. Chapman, Morehead State University; D. Wanless and K. Kolysko, University of Hawaii.
DE: 1040 Isotopic composition/chemistry
DE: 1065 Trace elements (3670)
DE: 3640 Igneous petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting