HR: 10:50h
AN: V22A-03    [Abstracts]
TI: Melting a high ³He/⁴He source in a heterogeneous mantle and its importance to the isotopic characteristics of hotspot and mid-ocean ridge basalts
AU: * Ito, G
EM: gito@hawaii.edu
AF: Dept. of Geology & Geophysics, Univ. Hawaii, 1680 East-West Rd. POST 810, Honolulu, HI 96822 United States
AU: Mahoney, J J
EM: jmahoney@hawaii.edu
AF: Dept. of Geology & Geophysics, Univ. Hawaii, 1680 East-West Rd. POST 810, Honolulu, HI 96822 United States
AB: Parameterized models of flow and melting of a heterogeneous mantle are used to explore the causes of He, Sr, Nd, and Pb isotopic variations in ocean island and mid-ocean ridge basalts (OIB, MORB). Isotopic variations along the Hawaiian hotspot chain support the possibility that lithospheric thickness strongly affects the proportions of melt contributed by mantle materials with different solidi. Calculations show that the materials with the lowest assumed solidi--enriched mantle (EM) and a high ³He/⁴He source (C)--can dominate lava compositions for most of the chain, particularly if the Hawaiian mantle plume was cooler in the past. The compositions of the oldest seamounts, however, can only be matched if a depleted mantle (DM) component makes up ~90% of the plume. For Hawaii and ten other hotspots, models predict elongate Nd-Pb-Sr isotope arrays that are similar to those observed and align along trajectories joining mantle end-members EM, C, DM, and HIMU (high ²³⁸/²⁰⁴Pb). In contrast, correlations involving ³He/⁴He are predicted to be variable and weak, and to often project away from the end-member compositions; this is a consequence of having only one end-member (C) with high ³He/⁴He. For a range of mantle compositions, calculations can reproduce most of the OIB isotopic characteristics, assuming that melting occurs under appropriately thick lithosphere and hotspot-type conditions of elevated mantle temperature and rapid mantle flow through the deepest portion of the melting zone. Predicted MORB isotopic characteristics that would be generated from the same mantle compositions but under mid-ocean ridge conditions (normal temperature, thin lithosphere, and uniform flow in the melting zone) fall within the observed MORB ranges if two conditions are met: DM is >~85% of the mantle, and the concentration of He in DM is comparable to, or greater than, that in the other sources. Both the relatively low ratios and small variability in MORB ³He/⁴He are therefore predicted to be natural consequences of sampling a statistical mantle assemblage. Such a heterogeneous mixture in a single mantle layer could therefore feed both hotspot and mid-ocean ridge volcanism.
DE: 1009 Geochemical modeling (3610, 8410)
DE: 1025 Composition of the mantle
DE: 1040 Radiogenic isotope geochemistry
DE: 3619 Magma genesis and partial melting (1037)
DE: 8121 Dynamics: convection currents, and mantle plumes
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005