HR: 0800h
AN: V51D-1523    [Abstracts]
TI: Os isotopes and platinum-group elements (PGE) in the mantle section of the Talkeetna arc
AU: * Blusztajn, J
EM: jblusztajn@whoi.edu
AF: WHOI, Department of Geology and Geophysics, Woods Hole, MA 02543 United States
AU: Kelemen, P
EM: peterk@lamont.ldeo.columbia.edu
AF: LDEO, Columbia University, Palisades, NY 10964 United States
AU: Hart, S
EM: shart@whoi.edu
AF: WHOI, Department of Geology and Geophysics, Woods Hole, MA 02543 United States
AU: Hanghoj, K
EM: khanghoj@ldeo.columbia.edu
AF: LDEO, Columbia University, Palisades, NY 10964 United States
AB: The Mesozoic Talkeetna arc section in south central Alaska contains a sequence of mantle peridotites at the base, through lower crustal gabbroic rocks to volcanic rocks and sediments at the top. Zircon ages for Talkeetna igneous rocks range from ~ 200 to 170 Ma, with little indication of an older, inherited component. In parts of the section, Talkeetna magmatism was the last significant igneous event. The mantle exposure in the Tonsina area is up to 1 km thick, and consists of residual harzburgites and discordant dunites. A few hundred meter thick band of igneous pyroxenite (websterite, olivine websterite and clinopyroxenite) is present between residual peridotites and overlying crustal gabbros. Pyroxenite-peridotite and pyroxenite-gabbro contacts are interfingered and igneous. We analyzed 15 representative samples (harzburgites, dunites and pyroxenites) from this section. Os isotopic compositions were determined by sparging OsO4 directly into ICP-MS following a flux fusion sample preparation. Re and platinum group elements (PGE) were determined by isotope dilution ICP-MS. The present day 187Os/188Os in dunites and harzburgites varies from 0.1192 to 0.143, within the observed range for mid-ocean ridge (MOR) peridotite and basalt. Pyroxenites are more heterogeneous with 187Os/188Os between 0.133 and 0.434. Re concentrations vary in a large range from 6 ppt to 680 ppt. Due to low Re/Os in harzburgites and dunites age correction on 187Os/188Os, assuming a 175Ma age of the Talkeetna arc, is very small. There are however some pyroxenite samples with high Re/Os for which the age correction is very significant (e.g. initial 187Os/188Os=0.434, age corrected 187Os/188Os=0.205). As for MOR samples, unradiogenic Os isotopic compositions probably record an ancient melting event that took place ~ 0.9-1.3 Ga, long before initiation of Talkeetna arc magmatism. Os isotopic variability in Talkeetna can be explained by interaction between (a) a mantle wedge with low Re/Os and 187Os/188Os similar to MOR peridotite and (b) relatively Re- or 187Os-rich fluids released from the subducted slab, coupled with ingrowth of 187Os over the past 170 to 200 Ma. PGE show large variations in concentrations and ratios. For example Ir content varies from 106 ppt to 7.8 ppb, Pt from 167 ppt to 447 ppb and Os/Ir from 0.06 to 1.08. The majority of samples are characterized by nonchondritic fractionated PGE patterns. Results from Talkeetna confirm that PGE are mobile in the arc environment.
DE: 8413 Subduction zone processes (1031, 3060, 3613, 8170)
DE: 8416 Mid-oceanic ridge processes (1032, 3614)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005