| dc.description.abstract | Two types of phosphorite recovered from the continental slope off western India are described. The first type, phosphorite 1, comprises a hard, grey nodule composed of carbonate fluorapatite (CFA) and calcite as major minerals. The phosphorite consists of light-brown microcrystalline apatite containing a few skeletal fragments and planktonic foraminifera. Scanning electron microscope (SEM) studies show evidence of dissolution of skeletal calcite and filling of the resulting cavities by phosphate composed of ovoid to rod-shaped apatite microparticle. Apatite also occurs as coating on these particles. The P sub(2) O sub(5) content of the phosphorite is 29%, and the CO sub(2) content of the CFA is about 4.5%. The rare-earth element (REE) abundance (Sigma REE = 2.02 mu g g sup(-1)) is lower than in other modern phosphorites. The sup(87)Sr/ sup(86)Sr ratio and epsilon ND value of this sample are 0.70921 and -9.9 respectively. The sup(14)C age found through accelerator mass spectrometry (AMS) dating (18 720 plus or minus 120 years BP) is much younger than that determined by the U-series method (100 ka). The second type, phosphorite 2, comprises a friable, light-brown nodule consisting of CFA as the only major mineral, with a CO sub(2) content of the CFA of 4.5%. In thin section, the phosphate is light brown and homogeneous, and a few bone fragments are present. The P sub(2) O sub(5) content is 33%, and REE contents (Sigma REE = 0.18 mu g g sup(-1)) are lower than in phosphorite 1. The age of phosphorite 2 is greater than 300 ka. Phosphorite 1 appears to have formed during the late Pleistocene through replacement of carbonate by phosphate; phosphorite 2 is also of Pleistocene age but is much older than phosphorite 1. The initial substrate for phosphorite 2 was a fish coprolite, which was subsequently phosphatized during low sedimentation under low-energy conditions. Microbial mediation is evident in both phosphorites. The colour, density and P sub(2) O sub(5) content of the phosphorites are found to be dependent on the nature of the initial substrates and physico-chemical conditions during phosphatization. The CO sub(2) content of the CFA is not related to the precursor carbonate phase. The nature of sediments, rates of sedimentation and the time spent undergoing phosphogenesis at the sediment-water interface may control REE concentrations in phosphorites | |
