Primary mineralogy of ODP Leg 115 basalts

Basement rocks were recovered at four sites on Leg 115 along the Reunion hotspot track in the western Indian Ocean. Plate tectonic reconstructions indicate that the drilled structures formed in three different volcanic environments. Sites 706 and 713 from the eastern side of the Saya de Malha Bank and the northern end of the Chagos Bank, respectively, are on a large volcanic platform analogous to Iceland on the Mid-Atlantic Ridge. Lavas at Site 707 on the northwestern side of the Saya de Malha Bank erupted during the early stages of rifting of the Seychelles from India. Basalts from Site 715 were erupted onto an isolated oceanic island that was distant from ocean ridges and continents much as Reunion Island is today. Many of the rocks were examined in thin section and found to be primarily augite-plagioclase basalts with minor olivine and rare opaque oxides. Site 715 is unusual in that it contains a variety of basalts including olivine-rich and aphyric Fe-Ti basalts. At each of the four sites the rocks were grouped into chemical types (units) on the basis of ship- board bulk-rock analyses and at least one thin section from each chemical unit was analyzed by electron microprobe. The plagioclase and augite chemistry reflects the bulk-rock chemistry and, in general, these minerals were in equilibrium with their host magmas at the time the basalts were quenched. Olivine was rarely preserved, but where it is still present it also appears to have crystallized in equilibrium with the host magma. At three of the drill sites plagioclase phenocrysts or megacrysts that crystallized from a primitive magma are also present. The one site (715) that does not contain these primitive plagioclase phenocrysts is also the site that appears to have been influenced the least by ocean- ridge or Deccan-type magmas. Site 715, furthermore, has a mineralogy that is dominated by olivine as compared with the plagioclase-rich lavas of the other sites.

(Table 2) Actual and synthetic mass accumulation rate data and their absolute differences for ODP Site 115-707

Synthetic mass accumulation rates have been calculated for ODP Site 707 using depth-density and depth-porosity functions to estimate values for these parameters with increasing sediment thickness, at 1 Ma time intervals determined on the basis of published microfossil datums. These datums were the basis of the age model used by Peterson and Backman (1990, doi:10.2973/odp.proc.sr.115.163.1990) to calculate actual mass accumulation rate data using density and porosity measurements. A comparison is made between the synthetic and actual mass accumulation rate values for the time interval 37 Ma to the Recent for 1 Myr time intervals. There is a correlation coefficient of 0.993 between the two data sets, with an absolute difference generally less than 0.1 g/cm**2/kyr. We have used the method to extend the mass accumulation rate analysis back to the Late Paleocene (60 Ma) for Site 707. Providing age datums (e.g. fossil or magnetic anomaly data) are available the generation of synthetic mass accumulation rates can be calculated for any sediment sequence.

Miocene benthic foraminifer oxygen and carbon isotope record of ODP Site 115-709

The benthic isotopic record of Miocene Cibicidoides from Site 709 provides a record of conditions in the Indian Ocean at a depth of about 3200 mbsf. As expected, the record qualitatively resembles those of other Deep Sea Drilling Project and Ocean Drilling Program sites. The data are consistent with the scenario for the evolution of thermohaline circulation in the Miocene Indian Ocean proposed by Woodruff and Savin (1989, doi:10.1029/PA004i001p00087). Further testing of that scenario, however, requires isotopic data for Cibicidoides from other Indian Ocean sites. There is a correlation between d13C values of Cibicidoides and planktonic:benthic (P:B)ratios of Site 709 sediments, implying a causal relationship between the corrosiveness of deep waters and concentration of CO2 derived from oxidation of organic matter.

Quaternary carbonate record of ODP Hole 115-709A

The CaCO3 content in Quaternary deep-sea sediments from Pacific and Atlantic oceans have been suggested to respond differently to glacial/interglacial cycles; CaCO3 contents are highest during glacials in the Pacific but highest during interglacials in the Atlantic Ocean. It is not yet clear as to whether a Pacific or an Atlantic pattern of CaCO3 fluctuations dominates the Indian Ocean. We have analyzed the Ocean Drilling Program (ODP) Site 709A from the western equatorial Indian Ocean for the last 1370 ka to determine the relationships between percentages and fluxes of CaCO3 and Quaternary paleoclimatic changes. We also analyzed the coarse (>25 µm) and fine (<25 µm) fractions of CaCO3 in an attempt at estimating the influence of differences in productivity of foraminifera and calcareous nannofossils in shaping the CaCO3 record. Carbon isotopes and Ba/Al ratios were used as indices of productivity. Percentages and fluxes of CaCO3 in the total sediment and <25 µm fraction do not show any clear relationships to glacial/interglacial cycles derived from d18O of the planktonic foraminifera Globigerinoides ruber. This indicates that CaCO3 fluctuations at this site do not show either a Pacific or an Atlantic pattern of CaCO3 fluctuations. Fluxes of CaCO3 (0.38 to 2.46 g/cm**2/ ka) in total sediment and Ba/Al ratios (0.58 to 3.93 g/cm**2/ka) show six-fold variability through the last 1370 ka, which points out that productivity changes are significant at this site. Fluxes of the fine CaCO3 component demonstrate a 26-fold change (0.02 to 0.52 g/cm**2/ka), whereas the coarse CaCO3 component exhibit eight-fold change (0.13 to 1.07 g/cm**2/ka). This suggests that productivity variations of calcareous nannofossils are greater in comparison with the foraminifera. On the other hand, mean values of coarse CaCO3 fluxes are higher compared to those of fine CaCO3, which reveals that the foraminifera contribute more to the bulk CaCO3 flux than the calcareous nannofossils in the equatorial Indian Ocean.