HR: 0800h
AN: U21B-0406    [Abstracts]
TI: Geodynamic Regimes of the Mantle - Constraints From a He-Inclusive Global Data Compilation
AU: * Class, C
EM: class@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory, POB 1000 RT 9W, Palisades, NY 10964, United States
AU: Goldstein, S L
EM: steveg@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory, POB 1000 RT 9W, Palisades, NY 10964, United States
AB: 27 years after the introduction of Chemical Geodynamics by Allègre (1982), fundamental issues about the geodynamic evolution of the Earth's mantle remain to be resolved. Geophysical evidence for whole mantle convection combined with the difficulty to preserve mantle heterogeneity since the early Earth contrast with geochemical evidence for longterm isolation of parts of the Earth's mantle. Noble gases, in particular helium, remain central to this discussion with three contrasting models considered here: (1) The layered mantle by Allègre et al. (1983) with the lower mantle preserving high ³He/⁴He ratios since early Earth; (2) The D" layer model by Tolstikhin & Hofmann (1995) with D" preserving high ³He/⁴He ratios since early Earth; (3) The incompletely degassing mantle model of Class and Goldstein (1995) with depleted mantle domains in the lower mantle preserving high ³He/⁴He ratios since 1-2 Ga. Here we present new constraints from our updated He-inclusive global data compilation. Ocean island basalts show a correlation between ³He/⁴He and Th contents, consistent with He isotope ratios reflecting the radiogenic production rate of the variably enriched OIB mantle sources. A high ³He/⁴He – low Th component is required to explain the global systematics. Constraints on the Th-U content in the three models place tight constraints on the absolute and relative contribution from the high ³He/⁴He reservoir to OIB mantle sources. Both the layered mantle model and the D" model requires small and extremely uniform contributions of a few percent or permil respectively; contributions outside of this small range, even if slightly higher or lower, are inconsistent with the global OIB data. Only the incompletely degassing mantle model allows for variability in the scale of contributions from old depleted mantle preserving higher ³He/⁴He ratios and from sources having a variety of ages. This model is also consistent with the requirement of dominantly peridotitic mantle sources for OIB. Allègre, C.J. (1982) Tectonophysics 81, 109-132. Allègre, C.J. et al. (1983) Nature 303, 762-766. Tolstikhin, I.N. & Hofmann, A.W. (2005) Phys. Earth Planet. Int. 148, 109-130. Class, C. & Goldstein, S.L. (2005) Nature 436, 1107-1112.
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: Union [U]
MN: 2007 Fall Meeting