Alteration of magnetic mineralogy at the sulfate–methane transition: Analysis of sediments from the Argentine continental slope

Abstract

On the Argentine continental slope off the Rio de la Plata estuary, the sulfate–methane transition (SMT) has been encountered at shallow depths of a few meters below the seafloor. At around this horizon, where sulfate diffusing downward from the bottom water is met and reduced by methane rising from deeper in the sediment column, intense alteration affects the detrital magnetic mineral assemblage. Less than 10% of the dominant primary low coercivity ferrimagnetic (titano-) magnetite remains after alteration. In the upper part of the suboxic environment, underlying the iron redox boundary, which is located at a depth of ∼0.1 m, approximately 60% of the finer grained detrital fraction is already dissolved. While the high coercivity minerals are relatively unaffected in the suboxic environment, large portions (> 40%) are diagenetically dissolved in the sulfidic SMT zone. Nevertheless, the characteristics of the magnetic residue are entirely controlled by a high coercivity mineral assemblage. Unlike common observations, that diagenetic alteration produces coarser magnetic grain-sizes in suboxic milieus, a distinct overall fining is found in the sulfidic zone. Different factors should contribute to this effect. Scanning electron microscope analysis, combined with X-ray microanalysis, identified fine grained (titano-) magnetite preserved as inclusions in silicates and between high Ti titanohematite lamellae, and possibly of prime importance, a comprehensive fragmentation of larger grains in the course of maghemitization. The only secondary iron sulfide mineral detected is pyrite, which is present as clusters of euhedral crystals or directly replaces (titano-) magnetite. The thermomagnetic measurements did not provide evidence for the presence of ferrimagnetic sulfides such as greigite. Different from other studies reporting a marked magnetic enhancement at around the SMT due to the precipitation and preservation of such metastable ferrimagnetic sulfides, a complete pyritization process will cause a distinct magnetic depletion, like in the present case.