Temporal evolution of the Mariana arc during rifting of the Mariana Trough traced through the volcaniclastic record

The sedimentary sequences that accumulate around volcanic arcs may be used to reconstruct the history of volcanism provided the degree of along-margin sediment transport is modest, and that reworking of old sedimentary or volcanic sequences does not contribute substantially to the sediment record. In the Mariana arc, the rare earth and trace element compositions of ash layers sampled. by Deep Sea Drilling Project (DSDP)) site: 451 on the West Mariana Ridge, and sites 458 and 459 on the Mariana Forearc, were used to reconstruct the evolution of the arc volcanic front during rifting of the Mariana Trough. Ion microprobe analysis of individual glass shards horn the sediments shows that the glasses have slightly light rare earth element (LREE)-enriched compositions, and trace element compositions typical of are tholeiites. The B/Be ratio is a measure of the involvement of subducted sediment in petrogenesis, and is unaffected Toy fractional crystallization, This ratio is variable over the period of rifting, increasing up-section at site 451 and reaching a maximum in sediments dated at 3-4 Ma, similar to 3-4 million years after rifting began. This may reflect increased sediment subduction during early rifting and roll-back of the Pacific lithosphere. Parallel trends are not seen in the enrichment of incompatible high field strength (HFSE), large ion lithophile (LILE) or rare earth elements (REE), suggesting that flux from the subducting slab alone does not control the degree of melting. Re-establishment of are volcanism on the trench, side of the basin at ca 3 Ma resulted in volcanism with relative enrichment in incompatible REE, HFSE and LILE, although these became more depleted with time, possibly clue to melt extraction from the mantle source as it passed under the developing back-ar-e spreading axis, prior to melting under the volcanic front.