HR: 16:30h
AN: V42H-03    [PDF]
TI: Geochemical Systematics of Hotspots and Mid-Ocean Ridges Arising from Melting of a Non-Layered Heterogeneous Mantle
AU: * Ito, G
EM: gito@hawaii.edu
AF: SOEST, Univ. Hawaii, Honolulu, HI
AU: Mahoney, J J
EM: jmahoney@hawaii.edu
AF: SOEST, Univ. Hawaii, Honolulu, HI
AB: Many fundamental geochemical differences between ocean island basalts (OIBs) and mid-ocean ridge basalts (MORBs) are often explained by a chemically layered mantle, with lower mantle material delivered to the surface by mantle plumes forming ocean islands, and a compositionally distinct upper mantle feeding mid-ocean ridges. A dilemma arises from geophysical evidence for whole mantle convection, which is predicted to efficiently stir the mantle and prevent any long-lasting, global chemical layering. We present models of decompression melting in which mantle heterogeneities are present as veins or small blobs, equally numerous in plume mantle as they are in the ambient mantle. Three different components are considered. Enriched mantle (EM) is highly concentrated in the most incompatible trace elements, has isotopic characteristics reflecting long-term enrichment, and begins melting deepest. Pyroxenite (PX) is relatively depleted in the most incompatible trace elements, has Pb isotope compositions reflecting a high U/Pb ratio, and begins melting at intermediate depths. Depleted mantle (DM), the most abundant (90%) component, is depleted in the most incompatible elements, has corresponding isotope signatures, and begins melting shallowest. Models predict the deeper melting, EM and PX components to be preferentially extracted at intraplate settings where thick lithosphere limits melting to large depths and low extents. In contrast, DM is more heavily sampled at mid-ocean ridges where thinner lithosphere allows for shallower and more extensive melting. Besides lithospheric thickness, differences in mantle flow also contribute to compositional contrasts between OIBs and MORBs. Plume-driven upwelling is most rapid at depth, decreases to the base of the lithosphere, and therefore enhances the extraction of EM and PX. In contrast, seafloor spreading at mid-ocean ridges allows for more uniform upwelling with depth and more even sampling of all mantle components, including DM. Models predict general systematics consistent with observations: incompatible trace-element and Sr, Nd, and Pb isotope ratios with large variability, extending to more enriched compositions at hotspots, and a smaller range of variability, with more DM-like compositions at mid-ocean ridges. Models can also explain the apparent mixing arrays defined by OIB and MORB isotope compositions. Considering melting of heterogeneous sources is thus crucial to any inferences of the bulk composition of the source material. In fact, many major geochemical systematics of MORB and OIB may arise from melting of a ubiquitously heterogeneous mantle without large-scale chemical layering.
DE: 1025 Composition of the mantle
DE: 1040 Isotopic composition/chemistry
DE: 1065 Trace elements (3670)
DE: 8121 Dynamics, convection currents and mantle plumes
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting