Mathematics – Logic
Scientific paper
Dec 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002agufm.v71c..13n&link_type=abstract
American Geophysical Union, Fall Meeting 2002, abstract #V71C-13
Mathematics
Logic
1025 Composition Of The Mantle, 3040 Plate Tectonics (8150, 8155, 8157, 8158), 3655 Major Element Composition, 3670 Minor And Trace Element Composition, 8125 Evolution Of The Earth
Scientific paper
One of the advances in modern geochemistry is the recognition of compositional heterogeneities in the Earth's mantle through studies of oceanic basalts. Ocean island basalts (OIB) are particularly variable in composition such that several isotopically distinct mantle source end-members are required to explain the variability. It is generally considered that the mantle compositional heterogeneity is a consequence of plate tectonics by means of crust-mantle recycling. Among many contributions endeavoring to understand the origin of mantle compositional heterogeneity is the classic model by Hofmann and White [Mantle plumes from ancient oceanic crust, Earth Planet. Sci. Lett., 57, 421-436, 1982.]. While some details are considered conjectural, the principal idea of the model has been widely accepted by the solid-Earth geochemical community as being, to a first order, correct. Here we offer arguments based on well-understood petrological processes, geochemical observations, and recent experimental data on mineral physics that ancient subducted oceanic crusts cannot be the source materials supplying OIB. Melting of oceanic crusts with basaltic/picritic compositions cannot produce high magnesian (> 15 wt. % MgO) melts parental to most OIB. Ancient oceanic crusts (> 1 Ga) are isotopically too depleted to produce the isotopic signatures (e.g., for the simple Sr, Nd and Hf isotopic systems) of most OIB. Subducted oceanic crusts that have passed through subduction-zone dehydration reactions should be depleted in water-soluble incompatible elements (e.g., Ba, Rb, Cs, U, K, Sr, Pb and, to a lesser extent, light rare earth elements) relative to water-insoluble incompatible elements (e.g., Nb, Ta, Zr, Hf, Ti and heavy rare earth elements). Melting of residual crusts with such trace element composition cannot produce OIB. OIB Sr-Nd-Hf isotopes preserve no signature that indicates previous subduction-zone histories. Oceanic crusts subducted into the lower mantle will be > 2% denser than the ambient mantle at shallow lower-mantle depths. This negative buoyancy will impede return of the subducted oceanic crusts into the upper mantle. If subducted oceanic crusts melt at the base of the mantle, the resultant melts are even denser, by up to ~ 15%, than the ambient peridotitic mantle. Neither in the solid state nor in the melt form can subducted bulk oceanic crusts return to upper mantle source regions of oceanic basalts. Small fragmented components of subducted oceanic crusts could be returned to the upper mantle source regions of oceanic basalts provided they were carried along with streams of ascending buoyant material, but there is no convincing evidence for the presence of bulk subducted crust in the source regions of oceanic basalts. This irreversible process requires a "hidden component" deep in the mantle unsampled by known volcanism, and would also lead to chemical stratification of the mantle with the mean composition of the lower mantle becoming progressively enriched in residual ocean crust lithologies (i.e., compositionally lower in Ca/Al, and higher in Fe/Mg, Si/Mg, Al, and water-insoluble incompatible elements such as Ti, Nb, Ta, Zr and Hf etc.). Deep portions of recycled oceanic lithosphere (refertilized previously depleted peridotites) are the most likely candidates for OIB sources [Niu et al., Geochemistry of near-EPR seamounts: importance of source vs. process and the origin of enriched mantle component, Earth Planet. Sci. Lett., 199, 327-345, 2002] in terms of petrology, geochemistry and mineral physics.
Niu Yuezhen
O'Hara Michael James
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