HR: 0830h
AN: V11E-0540    [PDF]
TI: Helium Isotope Signatures of Peridotites and Basalts from Atlantis Bank, Indian Ocean
AU: * Kumagai, H
EM: hkumagai@whoi.edu
AF: Department of Geology and Geophysics, Woods Hole Oceanograpic Institution, MS#8, 266 Woods Hole Rd., Woods Hole, MA 02543 United States
AU: * Kumagai, H
EM: hkumagai@whoi.edu
AF: Deep-Sea Research Department, JAMSTEC, 2-15 Natsushima-cho, Yokosuka, 237-0061 Japan
AU: Kurz, M D
EM: mkurz@whoi.edu
AF: Department of Marine Chemistry and Geochemistry, Woods Hole Oceanograpic Institution, MS#25, 266 Woods Hole Rd., Woods Hole, MA 02543 United States
AU: Curtice, J M
AF: Department of Marine Chemistry and Geochemistry, Woods Hole Oceanograpic Institution, MS#25, 266 Woods Hole Rd., Woods Hole, MA 02543 United States
AU: Arai, S
EM: ultrasa@kenroku.kanazawa-u.ac.jp
AF: Department of Earth Sciences Faculty of Science, Kanazawa University, Kakuma-machi, Kanazawa, 920-1192 Japan
AU: Dick, H J
EM: hdick@whoi.edu
AF: Department of Geology and Geophysics, Woods Hole Oceanograpic Institution, MS#8, 266 Woods Hole Rd., Woods Hole, MA 02543 United States
AB: Helium isotopic compositions have been measured in clinopyroxene separates from a suite of abyssal peridotites including a harzburgite, a websterite and 5 lherzolites, collected from Atlantis Bank during R/V Kairei KR00-06 cruise in 2000. The Atlantis Bank is an oceanic core complex, flanking the Atlantis II Fracture Zone at $57\deg$E on the Southwest Indian Ridge (SWIR), Indian Ocean. Along the AII-Fracture Zone, a massive mantle section is exposed at $32\deg$50'S, between 5000 and 3000 m water depth. The samples were selected from two ROV-Kaiko dive traverses: the uppermost section of Dive 173 and the entire length of Dive174 with intervals of several hundreds meters. Throughout the sample suite, the range of $^{3}$He/$^{4}$He ratios overlap with that of MORB, falling between 7.9 and 8.5 R$_{A}$. In a plagioclase-free lherzolite 10K173R20, $^{3}$He/$^{4}$He ratio was not determined due to a very low helium concentration (5x10$^{-10}$cm$^{3}$STP/g). With the exception of this helium depleted sample, $^{4}$He concentrations vary from 3.0x10$^{-8}$ to 6.5x10$^{-7}$cm$^{3}$STP/g. Duplicate measurements of clinopyroxenes of lherzolite 10K173R23 show significant variation of $^{4}$He content, exceeding a factor 4. Comparison of He released by crushing in vacuo with furnace melting of crushed powders (hereafter powder melting), shows that most of the helium was released by powder melting in the sample. This suggests that helium may not be contained in fluid inclusions, and that helium is heterogeneously distributed, e.g. dissolved in melt inclusions distributed throughout the clinopyroxenes. For comparison, two basalts were selected from neighboring localities. One came from the present-day spreading center; the other came from a ring volcano located on 1Ma seafloor to the south in the transform. Both areas were investigated by a manned submersible. Both glass samples have helium isotope signatures close to MORB: 8.69R$_{A}$. The glass from the south contains 3x10$^{-5}$cm$^{3}$STP/g of $^{4}$He, a very high concentration that is almost equivalent to gas-rich 'popping rocks'. The helium isotopic differences between the basalts and the related peridotite/clinopyroxene separates are very small. We have found no evidence of the extreme low $^{3}$He/$^{4}$He values (down to 6.2R$_{A}$) that were recently reported from the western region of SWIR (Georgen et al., 2003). These results indicate that an area of 'normal' helium isotope signature found around the Rodriguez T.J. and the SE Indian Ridge, with values $>$ 8R$_{A}$, may extend at least as far as the AII Fracture Zone, in contrast to the low helium isotopic signature from $9\deg$E to $25\deg$E on the western SWIR.
DE: 1025 Composition of the mantle
DE: 1040 Isotopic composition/chemistry
DE: 3035 Midocean ridge processes
DE: 8499 General or miscellaneous
DE: 9340 Indian Ocean
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting