Subdivisions of the sediment column, carbon and calcium carbonate at DSDP Hole 64-480

We analyzed 580 integrated scrape-samples from HPC Site 480 for organic and carbonate carbon. Once precise dating is available, these will provide a high-resolution framework for understanding late Quaternary Oceanographic and climatic fluctuations in this region. Organic carbon ranges mostly within a narrow band of 1.8 to 3.5% C. Calcium carbonate varies from undetectable to over 20%, with an average of only about 5%. Source of carbonate are mostly benthic and planktonic foraminifers, although some sections are dominated by diagenetic carbonate, shelly hash, or nannofossils. Detrital sources are low in carbonate. We divided the sequence into 17 calcium carbonate (CC) zones to separate pulses, low and median values. The CC-Zones show various second-order patterns of cyclicity, asymmetry, and events. Laminated zones have lowest uniform values, but a perfect correlation between carbonate content and homogeneous or laminated facies was not found. Maximum values tend to be located near the transition of these two sediment types, showing that accumulation of carbonate is favored during times of breakdown of stable Oceanographic conditions.

Isotopic composition of rocks from the Lau/Tonga back-arc basin, data of DSDP Leg 21 and ODP Legs 91 an 135

New trace element, Sr-, Nd-, Pb- and Hf isotope data provide insights into the evolution of the Tonga-Lau Basin subduction system. The involvement of two separate mantle domains, namely Pacific MORB mantle in the pre-rift and early stages of back-arc basin formation, and Indian MORB mantle in the later stages, is confirmed by these results. Contrary to models proposed in recent studies on the basis of Pb isotope and other compositional data, this change in mantle wedge character best explains the shift in the isotopic composition, particularly 143Nd/144Nd ratios, of modern Tofua Arc magmas relative to all other arc products from this region. Nevertheless, significant changes in the slab-derived flux during the evolution of the arc system are also required to explain second order variations in magma chemistry. In this region, the slab-derived flux is dominated by fluid; however, these fluids carry Pb with sediment-influenced isotopic signatures, indicating that their source is not restricted to the subducting altered mafic oceanic crust. This has been the case from the earliest magmatic activity in the arc (Eocene) until the present time, with the exception of two periods of magmatic activity recorded in samples from the Lau Islands. Both the Lau Volcanic Group, and Korobasaga Volcanic Group lavas preserve trace element and isotope evidence for a contribution from subducted sediment that was not transported as a fluid, but possibly in the form of a melt. This component shares similarities with that influencing the chemistry of the northern Tofua Arc magmas, suggesting some caution may be required in the adoption of constraints for the latter dependent upon the involvement of sediments from the Louisville Ridge. A key outcome of this study is to demonstrate that the models proposed to explain subduction zone magmatism cannot afford to ignore the small but important contributions made by the mantle wedge to the incompatible trace element inventory of arc magmas.