Subduction of pelagic sediments: implications for the origin of Ce-anomalous basalts from the Mariana Islands

Abstract

Attempts to assess the significance of subducted sediments in the genesis of island arc magmas have been limited by the lack of trace element data on pelagic oozes. Accordingly, we have analysed a series of pelagic clays and nannofossil oozes from the Nazca Plate for REE and other trace elements. A calculated average––Pacific Authigenic Weighted Mean Sediment (PAWMS)––exhibits light REE-enrichment (La n /Yb n ∼4.5), high contents of Ba and Sr, but low abundances of Rb, Nb and Ta. Most significant, however, is the occurrence of large, negative Ce anomalies (Ce/Ce*~0.2). We have attempted to model the contribution of PAWMS-type material to the source of the magmas of the Mariana Island Arc, an intra-oceanic arc far removed from the effect of continent-derived detritus. Only small amounts of pelagic sediment, between 0.3 and 0.5% of the source, are required to develop the small negative Ce anomalies, high Ba/La ratios, and high LIL/HFS element ratios characteristic of these and other arc lavas. However, a small fluid contribution from the dehydrating subducted oceanic crust is required to produce the high Rb/Ba observed in several Mariana suites. The ternary mixing between sediment, mantle host and dehydrating oceanic crust also produces very low Nb and Ta abundances in the arc lava source. However, the very high abundance of Sr (>1000ppm) and the estimated high 87 Sr/ 86 Sr ratio (~0.7150) of PAWMS, results in a model 87 Sr/ 86 Sr ratio of 0.7070. This is far higher than the measured ratios in the Mariana arc lavas (0.7033–0.7040) and may suggest that the subducted sediment has a lower Sr content (<200 ppm) or a lower 87 Sr/ 86 Sr ratio, or that the carbonate-rich component is not involved in source contamination. Volumetrically it would appear that much of the sediment approaching the Mariana trench (~45 km 3 Ma −1 per km of arc) may be recycled into the deeper mantle. This will have the effect of introducing high Ba, Sr, Th, 87 Sr/ 86 Sr, 208 Pb/ 204 Pb and 207 Pb/ 204 Pb material into the mantle. However, incorporation of such material cannot alone satisfactorily account for the trace element and isotope chemistry of ocean islands; oceanic sediments have LIL/Ta, Nb ratios far too high to produce the trace element characteristics of most intraplate magmas.