Evidence from Sardinian basalt geochemistry for recycling of plume heads into the Earth's mantle

Up to 10 per cent of the ocean floor consists of plateaux(1)-regions of unu sually thick oceanic crust thought to be formed by the heads of mantle plum es. Given the ubiquitous presence of recycled oceanic crust in the mantle s ource of hotspot basalts, it follows that plateau material should also be a n important mantle constituent. Here we show that the geochemistry of the P leistocene basalts from Logudoro, Sardinia, is compatible with the remeltin g of ancient ocean plateau material that has been recycled into the mantle. The Sr, Nd and Hf isotope compositions of these basalts do not show the si gnature of pelagic sediments. The basalts' low CaO/Al2O3 and Ce/Pb ratios, their unradiogenic Pb-206 and Pb-208, and their Sr, Ba, Eu and Pb excesses indicate that their mantle source contains ancient gabbros formed initially by plagioclase accumulation, typical of plateau material. Also, the high T h/U ratios of the mantle source resemble those of plume magmas. Geochemical ly, the Logudoro basalts resemble those from Pitcairn Island, which contain the controversial EM-1 component that has been interpreted as arising from a mantle source sprinkled with remains of pelagic sediments(2,3). We argue , instead, that the EM-1 source from these two localities is essentially fr ee of sedimentary material, the geochemical characteristics of these lavas being better explained by the presence of recycled oceanic plateaux. The st orage of plume heads in the deep mantle through time offers a convenient ex planation for the persistence of chemical and mineralogical layering in the mantle.