Volcanic and hydrothermal history of ridge segments near the Rodrigues Triple Junction (Central Indian Ocean) deduced from sediment geochemistry

Sediments from the Central Indian Ridge (CIR) and Southeast Indian Ridge (SEIR) near the Rodrigues Triple Junction (RTJ) were investigated in order to relate the accumulation rates of sediment components to the volcanic and hydrothermal history of the respective ridge segments. The sediments contain 50-90% biogenic carbonates and have been deposited over at least the last 2.5 Myr without large hiatuses. Since there is no early diagenetic element mobilization the main primacy geochemical signals are preserved in the sediments. A three-step multivariate analysis of carbonate-free geochemical data showed that the sediments consist of two main endmembers with respect to Si, Ti, Al, Fe, Mg, K, Mn, Cu, Ni, and Zn, which are mixed in different ratios in the samples. The first endmember comprises volcanic detrital components, mainly basalt fragments, basaltic glass and their weathering products (smectites). The second endmember is made up of hydrothermal Fe-Mn oxides. The volcanic detritus is distributed via bottom currents, the Fe-Mn oxides are formed as particle fallout from hydrothermal plumes. The first component dominates in the rift valley, the second on the ridge Ranks. Along the SEIR varying accumulation rates were found to reflect the cyclic magmatic-volcanic activity of the intermediate-spreading rift axis. In cores from the CIR the accumulation of both endmembers reflects the evolution of a non-transform fault separating the first and second CIR segments rather than the spreading axis history. (C) 2000 Elsevier Science B.V. All rights reserved.