The geochemistry of marine sediments, island arc magma genesis, and crust-mantle recycling

Abstract

To assess the role of sediment subduction and recycling in island arc magma genesis and mantle evolution, we have determined Sr, Nd, and Pb isotope ratios and the concentrations of K, Rb, Cs, Ba, Sr, U, Th, Pb and rare earth elements in 36 modern marine sediments, including Mn nodules, biogenic oozes, and pelagic and hemipelagic clays from the Pacific, Atlantic and Indian Oceans. From these data we draw the following conclusions. Sr and Nd isotope ratios and the Sr/Nd concentration ratios in sediments are such that mixing between subducted sediment on the one hand and depleted mantle or subducted oceanic crust on the other can produce mixing arrays which may pass either through or outside of the oceanic basalt Sr Nd isotope “mantle array”. Thus whether isotope compositions of island arc volcanics (IAV) plot inside or outside of the mantle array is not a good indication of whether or not their sources contain a subducted sediment component. The presence of subducted sediment in the sources of IAV should lead to Cs/Rb and Pb/Ce ratios which are higher than those in oceanic basalts, and Ba/Rb ratios which may be either higher or lower than oceanic basalts. Simple mixing calculations suggest that as little as a percent or so sediment in island arc magma sources can account for the observed Cs/Rb, Pb/Ce, and Ba/Rb ratios in IAV. However, it does not appear that high Ba/La ratios and negative Ce anomalies in IAV are inherited from sediment in IAV magma sources. It is more likely these features reflect fractionation of alkalis and alkaline earths from rare earths during slab dehydration and metasomatism. Pb isotope ratios in sediments from the Warton Basin south of the Sunda Arc are collinear in 208Pb/ 204Pb- 207Pb/ 204Pb/ 206Pb/ 204Pb space with volcanics from West Sunda, but not with volcanics from the East Sunda. This collinearity is consistent with the hypothesis that sediments similar to these are being subducted to the magma genesis zone of the West Sunda Arc. Sediment recycling to the deep mantle appears capable of explaining much of the Sr and Nd isotopic variation in oceanic basalt magma sources. However, because of the low 238U/ 204Pb ratios in sediments, anciently recycled sediment should have lower 206Pb/ 204Pb ratios than most oceanic basalts, though this effect will be in part balanced by high 238U/ 204Pb in hydrothermally altered oceanic crust. Unless 238U/ 204Pb ratios in ancient sediments were different than in modern ones, it would appear that deep sediment recycling cannot account for the Pb isotopic compositions of most oceanic basalts. In addition, deep sediment recycling should lead to higher Pb/Ce and Cs/Rb ratios and more variable Ba/Rb ratios in oceanic basalts than is observed. High Pb/Ce ratios have recently been observed in young seamounts from the Society Island chain, suggesting their source contains a recycled component. However, on the whole, the limited variation of Pb/Ce, Ba/Rb, and Ca/Rb in oceanic basalts suggests recycling of sediment of the deep mantle is limited.