HR: 1330h
AN: V42C-0370    [PDF]
TI: Oxygen - Osmium Isotopic Compositions of West Maui Lavas and the Link to Oceanic Lithosphere
AU: * Gaffney, A M
EM: agaffney@u.washington.edu
AF: University of Washington, Dept. of Earth and Space Sciences, Seattle, WA 98195 United States
AU: Nelson, B K
AF: University of Washington, Dept. of Earth and Space Sciences, Seattle, WA 98195 United States
AU: Reisberg, L
AF: CRPG/CNRS, BP 20, Vandoeuvre-les-Nancy, 54501 France
AU: Eiler, J M
AF: California Institute of Technology, Division of Geological and Planetary Sciences, Pasadena, CA 91125 United States
AB: We use stratigraphically-controlled sequences of late shield-stage lavas from West Maui to investigate age-dependence and fine-scale complexity of chemical variation within this Kea-type Hawaiian volcano. With new O and Os isotope data we identify unique geochemical signals indicative of complex processes of mixing among source components. In W. Maui lavas, $\delta^{18}$O and $^{187}$Os/$^{188}$Os range from 4.5 to 5.2 and 0.1316 to 0.1394, respectively. One sample ($^{187}$Os/$^{188}$Os =0.158) has very low [Os] (20 ppt) and likely has been severely contaminated by shallow oceanic crust. These compositions, as well as $^{87}$Sr/$^{86}$Sr (0.7034-0.7037) and $^{206}$Pb/$^{204}$Pb (18.36-18.54), are typical of shield-stage lavas from other Kea-type volcanoes. Some W. Maui compositional variability correlates with stratigraphic height. All shallow samples have $^{187}$Os/$^{188}$Os$\geq$0.1333, and all deep samples have $^{187}$Os/$^{188}$Os$\leq$0.1330. The deep samples show a negative $^{187}$Os/$^{188}$Os - $^{87}$Sr/$^{86}$Sr correlation, but the shallow samples show no Os - Sr isotopic correlation. $^{187}$Os/$^{188}$Os correlates with no other measured isotopic compositions except for $\delta^{18}$O, for which the deep and shallow samples define two sub-parallel inversely correlated trends. $\delta^{18}$O shows no other correlation with stratigraphy or other measured isotopic tracers. The inverse O-Os isotopic correlation of W. Maui is distinct from that observed for other Hawaiian volcanoes (e.g. Mauna Kea, Koolau) or within the Hawaiian suite as a whole. Age-dependent sampling of at least three components is required to describe the O-Os-Sr isotopic variation we observe at W. Maui. Similar intra-volcano correlation in $^{87}$Sr/$^{86}$Sr - $^{206}$Pb/$^{204}$Pb - trace element compositions is consistent with mixing of small-degree partial melts of gabbroic oceanic crust with plume-derived Kea-type magmas. We infer that this gabbroic endmemember is characterized by $\delta^{18}$O $<$ 4.5 and $^{187}$Os/$^{188}$Os $>$ 0.133. The remainder of the variability is the result of short-lived chemical heterogeneities in the Kea plume endmember.
DE: 1010 Chemical evolution
DE: 1020 Composition of the crust
DE: 1025 Composition of the mantle
DE: 1040 Isotopic composition/chemistry
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting