Affiliation: | aNational Institute of Oceanography, Dona Paula 403 004, Goa, India bDepartment of Earth and Environmental Sciences, Ludwig-Maximilians-Universität, Luisenstraße 37, D-80333 München, Germany cNational Geophysical Research Institute, Uppal Road, Hyderabad, India dOil and Natural Gas Corporation, Dehradun, Uttaranchal, India |
Abstract: | Phosphorites from the Murray Ridge, NW Arabian Sea comprise nodules, bioclasts, and bone fragments. The nodules are made up of a homogeneous, light-colored phosphate nucleus consisting of Rivulariacean filamentous cyanobacteria and a thin dark-grey colored phosphate cortex showing abundant microbial filaments and microborings. The bioclasts comprise of 14–14.5 Ma old planktonic foraminifers, accepted as the time of deposition. Spherical to ovoid-shaped apatite microparticles resembling fossil bacteria are distinct components in the bioclasts. Bone fragments exhibit apatite fillings. The nodules and bone fragments consist entirely of carbonate fluorapatite (CFA) with low Al, K, and Th concentrations suggesting absence of continental detritus. Shale-normalized REE patterns of the samples support a seawater-derived composition. The highly uniform initial Nd values of − 4.8 to − 5.1 are interpreted as the seawater value at the onset of phosphatization 14 Ma ago. In contrast, 87Sr/86Sr ratios show a large range of 0.709055 to 0.709124 corresponding to unusually young stratigraphic ages of 1 to 3 Ma. The data are interpreted as evidence for post-depositional Sr exchange of the recrystallizing phosphorites with fluids isotopically not much different from modern seawater. It is concluded that the phosphorites formed under oxic, shallow-water conditions where microbial populations assimilated phosphorus primarily from seawater and mediated precipitation of CFA during early diagenesis at the sediment–water interface on different substrates. |