HR: 0800h
AN: V51B-0548    [Abstracts]
TI: A Combined He and Os Isotopic Study of the HSDP-2 Core from Mauna Kea, Hawaii
AU: * Ireland, T J
EM: tireland@geol.umd.edu
AF: Department of Geology, University of Maryland, College Park, MD 20742 United States
AU: Walker, R J
EM: rjwalker@geol.umd.edu
AF: Department of Geology, University of Maryland, College Park, MD 20742 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
AU: Kurz, M D
EM: mkurz@whoi.edu
AF: Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, MA 02543 United States
AB: Combined osmium and helium isotope systematics of hotspot lavas have the potential to reveal information about the deep Earth. A high $^{3}$He/$^{4}$He ratio may represent an undegassed reservoir, generally associated with the lower mantle. There are two Os isotopes that can be studied to help to further elucidate the problem. The decay of $^{187}$Re to $^{187}$Os is the more frequently cited system; however, in terms of lower mantle processes, the decay of $^{190}$Pt to $^{186}$Os may be extremely useful. Both of these Os isotopes are enriched in the core relative to chondritic values. In a previous study, Brandon {\it et al}. (1999) examined several Hawaiian volcanoes for both He and Os isotopes. A correlation was noted between the $^{3}$He/$^{4}$He, $^{187}$Os/$^{188}$Os and $^{186}$Os/$^{188}$Os ratios. In terms of $^{3}$He/$^{4}$He and $^{187}$Os/$^{188}$Os space, the three commonly cited Hawaiian end-members (Kea, Koolau and Loihi members) were clearly defined. A strong positive correlation was also observed for $^{186}$Os/$^{188}$Os versus $^{3}$He/$^{4}$He. These correlations were interpreted as a possible signature of core-mantle interaction. There were some limitations to previous studies. Only 2-3 samples from each volcano were studied, with these samples generally being subaerially erupted. The He data utilized were often not for the same samples for which the Os data were collected (volcano averages for He were used on some samples). With the introduction of data from the Hawaiian Scientific Drilling Project (HSDP-2), which drilled 2.84 km into the Mauna Kea volcanics (DePaolo {\it et al}., 2000), an extensive history of a single volcano can be observed (from the early submarine stages to the later subaerial rocks). In the current study a detailed Os isotopic analysis of several samples that span a large depth range of the HSDP-2 core, in conjunction with previously collected He isotopic data (Kurz {\it et al}., 2004), was conducted. The samples define a relatively narrow range of slightly suprachondritic $^{187}$Os/$^{188}$Os ratios (0.12865-0.13056), despite having a large spread in $^{3}$He/$^{4}$He (8.5-23.5 Ra). This result indicates that the $^{187}$Os/$^{188}$Os ratios for Mauna Kea may reflect contributions from a relatively constant source in terms of Os, while the He characteristics of the source appear to be highly variable. Anticipated $^{186}$Os measurements will likely add additional insight to the causes of the isotopic heterogeneities.
DE: 1025 Composition of the mantle
DE: 1040 Isotopic composition/chemistry
DE: 1000 GEOCHEMISTRY (New field, replaces Rock Chemistry)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting