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 Core- Mantle Boundary (CMB)? from Ritsema et al

Why a CMB Origin? Size of some large igneous provinces requires an origin in the lower mantle or the CMB Mechanism of heat removal from the CMB area Large Igneous ProvincesEarth’s Heat BudgetEarth’s Geodynamo Modified from Condie (2003) CRETACEOUS SUPERCHRON 50-75% increase in rate of crustal production “Superplume” events correlate well with patterns of magnetic reversal

Outer core enriched in Os by 300x mantle values High 187 Os/ 188 Os and 186 Os/ 188 Os OSMIUM “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

OSMIUM CRUSTAL CONTAMINATION e.g. Koolau lavas, Hawaii High 187 Os/ 188 Os Low 186 Os/ 188 Os High d 18 O High Sr Low Nd

HELIUM Core enriched in primordial He during accretionHigh 3 He/ 4 He ratios MORB = 8±1 Ra Modified from van Keken et al. (2002)

FOZO: A common theme Geochemical signatures all point to a common component OIBs comprise a variety of different isotopic endmembers Material rising from the CMB incorporates deep mantle material But, FOZO is poor in Os FOZO

Ultra-Low Velocity Zones

Implications Osmium systematics suggest incorporation of outer core material at CMB Presence of recycled ancient crust suggests that slabs sink to CMB Less degassed lower mantle separated from degassed upper mantle points to a double-layer mantle system. Primordial 3 He points strongly to a CMB origin for plumes Courtesy of University of Hawaii, Institute for Astronomy

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