Abstract

A comparison of the trace-element abundances and radiogenic isotope ratios in oceanic basalts and the primordial mantle suggests that the present suboceanic upper mantle has undergone depletion and, in some places, recent re-enrichment events. The following model is proposed: Incipient partial melts were removed from much of the Archean mantle; this caused a depletion in the hygromagmatophile (incompatible) trace elements. At the same time, higher levels of the mantle were veined with these melts and were thus enriched. Trace-elements and radiogenic isotope data indicate that mantle sources of basalts which erupted at oceanic hot spots have had a complex, multistage evolution from the primordial mantle and consist of more than one component. A mixing process involving the veining of parts of the mantle on a centimetre scale by an undersaturated mafic magma is preferred to the mantle-plume model. Some quantitative mixing models show that the trace-element chemistry of hot-spot mantle sources can be generated by introducing about 5% of an undersaturated mafic magma into a mantle source for a normal mid-ocean ridge.

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