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The Role of the Core in Mantle Plumes Graham Smith University of Durham Dept. of Earth Sciences How is the Core Involved? Source of HEAT Origin of plumes at the CoreMantle Boundary (CMB)? from Ritsema et al. 1999. Why a CMB Origin? Large Earth’s Earth’sIgneous Geodynamo Heat Budget Provinces “Superplume” events correlate well requires Size of some large igneous provinces with patterns oflower magnetic reversal Mechanism removal from theCMB CMB area an origin in of theheat mantle or the 50-75% increase in rate of crustal production CRETACEOUS SUPERCHRON Modified from Condie (2003) www.creaso.com OSMIUM Outer core enriched in Os by 300x mantle values High 187Os/188Os and 186Os/188Os “Plume-related” basalts carry this signal Addition of ≤1wt.% outer core material BUT, High Os ratios can also be generated in the crust Why Not the Upper Mantle? PUM = Primitive Upper Mantle From Brandon et al. 1999. OSMIUM CRUSTAL CONTAMINATION e.g. Koolau lavas, Hawaii High 187Os/188Os Low 186Os/188Os High d18O High Sr Low Nd HELIUM Core enriched in primordial He during accretion Modified from van Keken et al. (2002) High 3He/4He ratios MORB = 8±1 Ra FOZO: A common theme OIBs comprise a variety of different isotopic endmembers Geochemical signatures all point to a common component FOZO Material rising from the CMB incorporates deep mantle material But, FOZO is poor in Os Ultra-Low Velocity Zones Implications Osmium systematics suggest incorporation of outer core material at CMB Presence of recycled ancient crust to suggests that slabs sink to CMB 3He points Primordial strongly a CMB origin for plumes Less degassed lower mantle separated from degassed upper mantle points to a double-layer mantle system. Courtesy of University of Hawaii, Institute for Astronomy References Brandon et al. (1998). Nature 280 1570-73. Rost & Revenaugh (2001). Science 294 1911-14. Brandon et al. (1999). Earth & Planetary Science Letters 174 25-42. Schubert et al. (2001). Mantle Convection in the Earth & Planets. Campbell & Griffiths(1992). J.Geol. 92 497-523. Condie. (2003) Plate Tectonics and Crustal Evolution, 4th ed.. Butterworth-Heinemann. Dubrovinsky et al. (2001). Nature 412 527-29. Tschauner et al. (1999). Nature 398 604-7. Van Keken et al. (2002). Annu. Rev. Earth Planet. Sci. 30 493-525 Gurnis et al. (1998). The Core-Mantle Boundary Region. AGU Geodynamics Series 28. Walker et al. (1995). Science 269 819-22. Jacobs (1993). Deep Interior of the Earth. Chapman & Hall. Widom & Shirey (1996). Earth & Planetary Science Letters 142 451-65. Jephcoat (1998). Nature 393 355-58. Knittle & Jeanloz (1991). Science 251 1438-43. Larson (1991). Geology 19 547-50. Li & Agee (1996). Nature 381 686-89. Macpherson et al. (2000). Earth & Planetary Science Letters 176 171-83. Marty et al. (1996). Earth & Planetary Science Letters 144 223-37. McElhinny (1979). The Earth: Its Origin, Structure & Evolution. Peltier (1989). Fluid Mechanics of Astrophysics & Geophysics 4.