HR: 10:20h
AN: DI42A-01 INVITED    [Abstracts]
TI: Earth's deep H cycle: H isotope evidence from the Manus Basin for complementary recycled reservoirs
AU: * Shaw, A M
EM: ashaw@whoi.edu
AF: Geology & Geophysics Dept., WHOI, Woods Hole, MA 02543, United States
AU: Hauri, E H
EM: hauri@dtm.ciw.edu
AF: DTM, CIW, Washington, DC 20015, United States
AU: Hilton, D R
EM: drhilton@ucsd.edu
AF: SIO, UCSD, La Jolla, CA 92093, United States
AU: Macpherson, C
EM: colin.macpherson@durham.ac.uk
AF: Dept. of Earth Sciences, University of Durham, Durham, DH1 3LE, United Kingdom
AU: Sinton, J M
EM: sinton@soest.hawaii.edu
AF: SOEST, University of Hawaii, Honolulu, HI 96822, United States
AB: Determining the H isotope composition of Earth's reservoirs is critical for evaluating the origin of water on Earth and the extent to which surface and mantle reservoirs have exchanged water over time. The hydrogen isotope composition of the upper mantle (MORB) is relatively well constrained (δD = -80 ± 10‰¹). However, ocean island basalts (OIB) show significantly more variability. In particular, if we consider OIBs proposed to have mantle sources containing recycled subduction-related components, we find both higher (e.g., Samoa², Salas y Gomez³, Iceland⁴) and lower values (Koolau⁵) than MORB. We propose that this difference reflects whether the subduction component represents recycled mantle wedge peridotite or the slab itself. Here we present new H isotope data for Manus Basin glasses and show how complementary H isotope reservoirs could be created. The Manus back-arc basin is a complex region where erupted lavas show superimposed plume, MORB and subduction-related components. He isotope studies of submarine glasses⁶ show high ³He/⁴He ratios (up to 15R_A) consistent with derivation from a lower mantle plume, thought to originate at the core- mantle boundary. Subsequent work⁷ found anomalously low δ¹⁸O values in the high ³He/⁴He samples, which could reflect interaction of an ancient recycled slab component with the Manus plume. δD values of Manus glasses with the highest ³He/⁴He and the lowest δ¹⁸O are extremely low (down to δD = -126‰). We argue that the low δD values reflect a recycled slab component in the mantle source, rather than degassing processes (based on CO₂ abundances and He-Ar systematics). Based on our studies of hydrogen isotopes in Mariana arc melt inclusions⁸, we find high δD values, consistent with experimental dehydration-induced fractionations which predict release of a D-enriched fluid from the slab into the mantle wedge. Thus, as dehydration proceeds, the slab will evolve to progressively lower δD values, while the wedge will be fluxed with D-enriched water. We suggest that ocean island basalts (OIB) with recycled slab components should be characterized by low δD, while high δD signatures could be derived from OIB containing recycled mantle wedge peridotite. ¹Kyser and O'Neil, GCA (1984) 48 2123-2133 ²O'Leary, GCA v. 71, Suppl. 1 A737 ³Kingsley et al. (2002) G-cubed 3, U23-U48 ⁴Poreda et al. (1986) EPSL v.78 1-17 ⁵Hauri (2002) Chem. Geol. 183 115-141 ⁶Macpherson et al. (1998) Geology 26 1006-1010 ⁷Macpherson et al. (2000) EPSL 176 171-183 ⁸Shaw et al. (2007), submitted
DE: 1031 Subduction zone processes (3060, 3613, 8170, 8413)
SC: Study of the Earth's Deep Interior [DI]
MN: 2007 Fall Meeting