Mantle Plumes are NOT From Ancient Oceanic Crust

Abstract

Basaltic volcanism mainly occurs in three tectonic settings on the Earth. Volcanism along sea-floor spreading centers produces Mid-Ocean Ridge basalts(MORB) that are depleted in incompatible elements. Volcanism above intra-oceanic subduction zones produces island arc basalts (IAB) that are enriched in water-soluble incompatible elements (e.g., Ba, Rb, Cs, Th, U, K, Pb, Sr), but depleted in water-insoluble incompatible elements (e.g., Nb, Ta, Zr, Hf, Ti). MORB and IAB are products of plate tectonics, and their geochemical differences result from differences in their respective sources and physical mechanisms through which they form. MORB are formed by plate-separation-induced passive mantle upwelling and decompression melting, thus sampling the uppermost mantle that is depleted in incompatible elements. Depletion of the MORB mantle is widely accepted as resulting from the extraction of incompatible element-enriched continental crust during the Earth’s early history (Armstrong 1968; Gast 1968; O’Nions and Hamilton 1979; Jacobsen and Wasserburg 1979; DePaolo 1980; Allegre et al. 1983; Hofmann 1998). IAB are widely accepted as resulting from subducting slab-dehydration-induced melting of mantle wedge peridotites, giving rise to the characteristic geochemical signatures of slab “component”, which is rich in water and water-soluble elements (e.g., Gill 1981; Tatsumi et al. 1986; McCulloch and Gamble 1991; Stolper and Newman 1994; Hawkins 1995; Pearce and Peate 1995; Davidson 1996).