HR: 13:55h
AN: V23D-02    [Abstracts]
TI: Origins of Enriched and Depleted Mantle Reservoirs
AU: * Langmuir, C H
EM: langmuir@eps.harvard.edu
AF: Harvard University, Earth & Planetary Sciences 20 Oxford Street, Cambridge, MA 02138 United States
AU: Goldstein, S L
EM: STEVEG@LDEO.COLUMBIA.EDU
AF: Lamont-Doherty Earth Observatory, P.O. Box 1000 61 Route 9W, Palisades, NY 10964 United States
AU: Donnelly, K
EM: kdonnelly@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory, P.O. Box 1000 61 Route 9W, Palisades, NY 10964 United States
AU: Su, Y J
EM: ysu@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory, P.O. Box 1000 61 Route 9W, Palisades, NY 10964 United States
AB: The relationships among plate tectonic circulation, continent formation and mantle heterogeneity have long been addressed through investigations of trace elements and isotopes in oceanic basalts. Various models, some involving recycled ocean crust, others involving mantle metasomatism by low degree peridotite melts, have been called upon to create the enriched signatures observed on ridges and ocean islands. The depleted reservoir, reflected by depleted MORB, is usually considered to complement extraction of continental crust. We propose a scenario that accounts for both enriched and depleted mantle sources, whereby mantle heterogeneity results from secondary processes subsequent to continent removal, caused by redistribution of low degree melts within the mantle by plate recirculation and mantle convection. First, we constructed an average upper mantle (UM) composition based on all data from the global ocean ridge system. The UM chemical composition, which is significantly more enriched than depleted mantle, successfully complements continental crust, in contrast to ``depleted'' mantle, which is too depleted for some elements by at least 50%. More than two thirds of the mantle is implicated by the mass balance. Second, we analyzed mantle heterogeneity in trace element space. Compositional vectors lie at high angles to lines of continent extraction. Trace element systematics are inconsistent with a recycled ocean crust source, but can be quantitatively accounted for by low degree melts of UM compositions. For peridotites, the extent of melting required is lower than 0.5%. We propose an important role for recycled eclogite, which can melt up to 5% and still create the trace element signature. Third, we evaluated isotope variations. Slopes on isotope- isotope plots define the relative parent-daughter fractionations (e.g. [(Rb/Sr)<sub>1</sub>- (Rb/Sr)<sub>2</sub>]/ [(Sm/Nd)<sub>1</sub>-(Sm/Nd)<sub>2</sub>]). The fractionation factors are constrained to very small ranges to account for the Sr-Nd and Hf-Nd isotope correlations. Very low degree melting in the presence of garnet is required to create mantle heterogeneity. For N. Atlantic and Pacific samples this model accounts roughly for Pb isotopes as well as Sr, Nd and Hf. Both very low F peridotite melting and low F eclogite melting can produce the isotope variations. In contrast, the conventional model of a marble cake mantle consisting of enriched, recycled eclogite accounts for neither the isotope nor the trace element systematics. Low degree melts, created in various tectonic locations at various times, can reflect partial melting of depleted, average, or enriched recycled eclogite, of diverse ages. Particular ocean island compositions are well accounted for by slight variations in the low degree melt component. At very low degree melting, accessory phases may also play an important role in fractionating highly incompatible elements. The ``depleted reservoirs" are created by the extraction of low degree melts from the UM composition. The eclogite residue to low F melt removal is an appealing depleted source, because it combines major element fertility with incompatible element depletion. Therefore we propose a generic three component mantle. The whole of the interactive mantle has an average UM composition. Enriched sources are generated by low degree melt metasomatism; N-MORB sources are created by the complement of low degree melt extraction, and incompatible elements are contained largely in recycled eclogite; exceptionally depleted sources result from residual mantle created by melting at ocean ridges.
DE: 1009 Geochemical modeling (3610, 8410)
DE: 1025 Composition of the mantle
DE: 1031 Subduction zone processes (3060, 3613, 8170, 8413)
DE: 1037 Magma genesis and partial melting (3619)
DE: 1038 Mantle processes (3621)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005