HR: 0800h
AN: V41D-1485    [Abstracts]
TI: Implications of new High ³He/⁴He Values from the Samoan Hotspot
AU: * Jackson, M G
EM: mjackson@whoi.edu
AF: Woods Hole Oceanographic Insitution, 266 Woods Hole Rd., Woods Hole, MA 02543 United States
AU: Kurz, M D
EM: mkurz@whoi.,edu
AF: Woods Hole Oceanographic Insitution, 266 Woods Hole Rd., Woods Hole, MA 02543 United States
AU: Hart, S R
EM: shart@whoi.edu
AF: Woods Hole Oceanographic Insitution, 266 Woods Hole Rd., Woods Hole, MA 02543 United States
AU: Workman, R
EM: rworkman@whoi.edu
AF: Woods Hole Oceanographic Insitution, 266 Woods Hole Rd., Woods Hole, MA 02543 United States
AB: We report new olivine phenocryst helium measurements from Ofu Island, American Samoa; the ³He/⁴He ratios range from 19.5 to 33.7 times atmospheric (R/Ra), significantly expanding the observed range for Samoa. The highest ³He/⁴He ratio of 33.7 Ra was measured in olivines from an ankaramite dike. Relatively high helium concentrations (4.4*10⁸ cc/g) in this sample, crushing and fusion measurements, coupled with sampling from a roadcut exposure, all ensure that the isotope ratio is not affected by in situ cosmogenic ³He. A second basaltic dike yielded a ³He/⁴He ratio of 29.6 Ra, and measurements on 9 other basalt samples from Ofu range from 19.5 to 26.4 Ra. Previous high ³He/⁴He measurements (~25 Ra) from the Samoan hotspot were also obtained from basaltic dikes, but were from Tutuila Island (Farley et al., 1992). The new high ³He/⁴He ratios from Samoa are similar in magnitude to the high ratios found at Iceland (~37 Ra) and Hawaii (~35 Ra). However, the Ofu basalts have ⁸⁷Sr/⁸⁶Sr > 0.7044, which is significantly more radiogenic than Iceland or Hawaii. The combined Sr-He isotopic data are broadly consistent with mixing between an enriched mantle source (EM2) and the putative common high ³He/⁴He component (FOZO, as best represented by Baffin Island Picrites, Stuart et al., 2003). Assuming that the overall isotopic variations are produced by mixing processes, we attempt to place constraints on the relative helium concentrations in the FOZO, EM2 and Depleted MORB mantle (DMM) endmembers. In addition to using the shape of the plausible mixing lines, we employ new estimates for the trace element concentrations in the DMM and Samoan EM2 sources (Workman et al., 2004; Workman and Hart, 2005) to get at relative helium concentrations in these reservoirs. We assume that high ³He/⁴He basalts from the mid-Atlantic ridge North of Iceland (Schilling et al., 1999, and others) are a mixture between DMM and FOZO. We further assume that the N. Iceland ridge-FOZO and Ofu-FOZO mixing arrays observed in ⁸⁷Sr/⁸⁶Sr ³He/⁴He isotope space are a result of subsolidus mixing (i.e., before helium degassing). As a limit, the FOZO source has trace element characteristics between DMM and EM2. In ⁸⁷Sr/⁸⁶Sr ³He/⁴He isotope space, calculated mixing lines between FOZO and DMM best match the N. Iceland ridge MORB data trends when the helium concentrations of FOZO are 3-5 times lower than DMM. Surprisingly, the same degree of helium depletion in FOZO relative to EM2 is required for the calculated FOZO-EM2 mixing lines to match the Ofu basalt data. Despite the assumptions of the mixing model this result is remarkable, given the different bulk-rock chemistries (tholeiitic vs. alkaline) and tectonic settings of North Iceland ridge and Samoan basalts. If the EM2 reservoir formed by aging recycled, metasomatized oceanic lithosphere (Workman et al., 2004), it is possible that the DMM and EM2 sources have similar helium concentrations. Assuming that the low helium concentrations in the high ³He/⁴He FOZO end-member is not a model-dependent feature, the challenge remains in explaining how the high ³He/⁴He FOZO reservoir could have lower helium concentrations than the DMM and EM2 reservoirs, which would not be consistent with conventional isotopic evolution models involving an undegassed source for Samoa.
DE: 1009 Geochemical modeling (3610, 8410)
DE: 1025 Composition of the mantle
DE: 1038 Mantle processes (3621)
DE: 1040 Radiogenic isotope geochemistry
DE: 1065 Major and trace element geochemistry
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005