Biostratigraphy and Sr isotopic composition of sediments from DSDP Legs 12, 14, 15, 17, 20-22, and 30

We present the data used to construct the Cenozoic and Cretaceous portion of the Phanerozoic curve of seawater 87Sr/86Sr that had been given in summary form by W.H. Burke co-workers. All Cenozoic samples (128) and 22 Cretaceous samples are foram-nannofossil oozes and limestones from DSDP cores distributed among 13 sites in the Atlantic, Pacific and Indian Oceans, and the Caribbean Sea. Non-DSDP Cretaceous samples (126) include limestone, anhydrite and phosphate samples from North America, Europe and Asia. Determination of the 87Sr/86Sr value of seawater at particular times in the past is based on comparison of ratios derived from coeval marine samples from widely separated geographic areas. These samples are characterized by a wide variety of diagenetic and burial histories. The large size and cosmopolitan nature of the data set decreases the likelihood that, among coeval data, systematic error has been introduced by a similar pattern of diagenetic alteration of the ratios. There is good clustering of data points throughout the Cenozoic and Cretaceous curve. The consistency of data is illustrated by Cenozoic and Cretaceous data plots that include a separate symbol for each DSDP site and non-DSDP sample location. More than 98% of the data points are enclosed by upper and lower lines that define a narrow band. For any given time, the correct seawater ratio probably lies within this band. A line drawn within the band represents our estimate of the actual seawater ratio as a function of time. The general configuration of the Cenozoic and Cretaceous curve appears to be strongly influenced by the history of plate interactions and sea-floor spreading. Specific rises and falls in the 87Sr/86Sr of seawater, however, may be caused by a variety of factors such as variation in lithologic composition of the crust exposed to weathering, configuration and topographic relief of continents, volcanic activity, rate of sea-floor spreading, extent of continental inundation by epeiric seas, and variations in both climate and paleooceanographic conditions. Many or all of these factors are probably related to global tectonic processes, yet their combined effect on the temporal variation of seawater 87Sr/86Sr can complicate a direct platetectonic interpretation for portions of the seawater curve.

Trace element abundances and Sr and Nd isotope compositions in bulk sediments from DSDP Legs 6 and 20

Rare earth element and Nd isotopic data for ten representative samples of Lower Cretaceous to Miocene pelagic sediments from the western Pacific indicate a wide range of compositions for sediments being subducted beneath the Mariana and Volcano arcs. All samples are enriched in light rare earth elements and show negative Eu and Ce anomalies. The values of e-Nd range from +0.6 to -7.3. These data are used to calculate the Bulk Western Pacific Sediment (BWPS), which is characterized by low Sr/Nd (10), Ba/La (13), and e-Nd (-5.2) and high 87Sr/86Sr (0.7078) compared to that of Mariana and Volcano arc lavas. This composite sediment is used to refine a mixing model for the origin of Mariana and Volcano arc melts. Some lavas from the northern Mariana Arc have Ba/La higher than that of BWPS, which indicates that a third component is required. The high Ba/La in the mantle source for these lavas is interpreted to result from multiple episodes of fluid fractionation. The mixing model indicates that a minor amount of sediment and a low proportion of metasomatic fluid fluxes the mantle source at a late stage when the subarc mantle is already highly metasomatized. This model also suggests that the mantle source for arc melts is affected more by metasomatic fluids than by melting or bulk mixing of sediments.