HR: 1340h
AN: V33A-1156    [Abstracts]
TI: Lack of Correlated Isotopic and Compositional Variations in Mauna Loa Lavas: A Serious Problem for Pyroxenite/Eclogite Plume Source Models
AU: * Rhodes, J M
EM: jmrhodes@geo.umass.edu
AF: J. Michael Rhodes, Dept. of Geosciences, University of Massachusetts, Amherst, MA 01002, United States
AU: Weis, D
EM: dweis@eos.ubc.ca
AF: Dominique Weis, Dept. of Earth and Ocean Sciences, University of British Columbia, Vancouver, BC V6T 1Z4, Canada
AU: Norman, M D
EM: Marc.Norman@anu.edu.au
AF: Marc D. Norman, Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia
AU: Garcia, M O
EM: garcia@soest.hawaii.edu
AF: Michael O. Garcia, Dept. of Geology and Geophysics, University of Hawaii, Honolulu, HI 96822, United States
AB: The long held notion that basaltic magmas are produced by decompressional melting of peridotite is under challenge. Recent models for the Hawaiian and other plumes argue that they consist of a heterogeneous mix of peridotite and discrete eclogite blobs, the latter derived from recycled subducted crust. Eclogite melting produces relatively siliceous magmas (dacite to andesite) which either mix with picritic melts from the peridotite, or, more plausibly, react with the peridotite to produce pyroxenite. Melting of varying proportions of the peridotite/pyroxenite mix is thought to produce the correlated compositional and isotopic characteristics of Hawaiian volcanoes. Magmas from Mauna Loa and Koolau volcanoes are thought to contain more of the recycled component; those from Loihi and Kilauea volcanoes contain less. A simple test of these mixed source models examines whether isotopic changes within the long magmatic history of a single volcano are accompanied by corresponding changes in major and trace element characteristics. Mauna Loa, where we have sampled around 400 - 500 ka of the volcano's eruptive history, provides an excellent opportunity for such a test. During this time, Mauna Loa will have traversed almost half the Hawaiian plume. According to the models, it should have erupted magmas produced from a range of pyroxenite/peridotite mixes with corresponding differences in both isotopic ratios and major and trace elements. Our data show that there is only minor isotopic (Sr, Pb, Nd, Hf) diversity in young lavas (<100 ka), but older lavas are highly diverse, ranging from modern values to those that are close to, and overlap with, those of Loihi volcano. If this isotopic diversity is a consequence of different proportions of pyroxenite and peridotite in the plume source, as the new models predict, we should expect to see correlated changes in bulk composition, particularly. in normalized SiO2, CaO/Al2O3, FeO/MgO and Ni - MgO relationships, as well as changes in Ni - Sc - V relationships. We do not. These parameters remain remarkably uniform over the 400 to 500 ka magmatic history of the volcano, with no correlated variation with isotopic ratios. We conclude that the isotopic heterogeneity within the Hawaiian plume is intrinsic to the peridotite plume source and not dependent on variable contributions from entrained, lithologically-discrete units.
DE: 1025 Composition of the mantle
DE: 1037 Magma genesis and partial melting (3619)
DE: 1040 Radiogenic isotope geochemistry
DE: 1065 Major and trace element geochemistry
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting