GEOCHEMISTRY OF IRON-CALCIUM HYDROXOPHOSPHATES IN PELAGIC SEDIMENTS: ORIGIN AND COMPOSITIONAL EVOLUTION IN THE COURSE OF DIAGENESIS

Abstract

A phase consisting of iron, phosphorus, and calcium was found in pelagic sediments for the first time. This phase was referred to as iron–calcium hydroxophosphates. They were isolated from the >50 mm fraction of eupelagic clays sampled at Station 35, which was located in the South Pacific Depression. The iron– calcium hydroxophosphates are encountered both as clastics or nodules and as inclusions in phillipsites. The bulk composition of the >50 mm phillipsite fraction from four eupelagic clay horizons (burial depths 37– 40, 105–110, 165–175, and 189–190 cm) showed that the abundances of iron, calcium, and tervalent rare-earth elements (REE) correlate with phosphorus: they increase with increasing hydrothermal matter fraction in the sediment. Therefore, presumably, iron–calcium hydroxophosphates are hydrothermal in origin. Electron probe microanalysis showed a variable composition of iron–calcium hydroxophosphates from the upper two sampled horizons (burial depths 37–40 and 105–110 cm). The molar ratio P : Ca approximates 0.6, as in apatite, and iron becomes more abundant with decreasing in-grain calcium and phosphorus concentrations. X-ray powder diffraction showed that iron–calcium hydroxophosphates were fine, submicron intergrowths of apatite and goethite aggregates (at burial depths 105–110 cm) or of high-phosphorus goethite (at burial depths 37–40 cm). The absence of iron–calcium hydroxophosphates in the two ancient sediment horizons, in spite of a tight association between iron, calcium, and phosphorus and of their increasing abundances, suggests that iron–calcium hydroxophosphates are unstable during diagenesis and that they decompose to goethite and apatite. The analysis of structural alterations experienced by iron–calcium hydroxophosphates showed that they changed with time from globular through cellular to massive structures. The iron is lost, and variations in the in-grain calcium and phosphorus abundances are reduced during this process. Based on secondary-ion mass spectrometry (SIMS) evidence from two iron–calcium hydroxophosphate particles, REE accumulation (up to 400 ppm La), and cerium deficit were discovered. This suggests that the hydroxophosphates are generated in deep and/or near-floor oceanic water, that is, in the localities of submarine hydrothermal vents.