HR: 09:30h
AN: GP21C-07    [Abstracts]
TI: Multiple mechanisms of remagnetization involving sedimentary greigite (Fe3S4)
AU: * Roberts, A P
EM: arob@soc.soton.ac.uk
AF: Southampton Oceanography Centre, European Way, Southampton, SO143ZH United Kingdom
AU: Weaver, R
AF: Southampton Oceanography Centre, European Way, Southampton, SO143ZH United Kingdom
AB: Sedimentary greigite (Fe3S4) is being increasingly implicated as the carrier of late diagenetic remagnetizations in fine-grained marine sediments. Scanning electron microscopy (SEM) coupled with energy-dispersive X-ray spectrometry is a powerful tool for discriminating between different iron sulphide phases. Determining authigenic growth relationships using SEM is an important supplemental tool to "remote sensing" using rock magnetic techniques. We have conducted detailed SEM observations and elemental microanalysis on polished sections from a wide range of sediments to identify greigite and to characterize the mode of occurrence and the relationships between the observed greigite and other authigenic and detrital mineral phases. This detailed observational work, in conjunction with recently published work, has enabled identification of several mechanisms for remagnetization involving greigite. (1) Neoformation of greigite on the surfaces of early diagenetic framboidal and nodular pyrite. (2) Greigite growth within cleavages of detrital sheet silicate grains. (3) Neoformed authigenic clays (smectite, illite) that have been corroded to provide iron for diagenetic neoformation of greigite. (4) Greigite nucleation on the surfaces of siderite, where the abundant reactive iron in the siderite has come into contact with sulfidic pore waters. (5) Oxidation of pyrite nodules to form gypsum, which has supplied sulphate for bacterial metabolism under later anoxic conditions, which, in the presence of dissolved iron, provided a site for nucleation of greigite. Many other mechanisms for greigite neoformation are conceivable. The key variables are appropriate redox conditions for generation of sulfide and availability of iron from the dissolution of a wide range of possible reactive detrital and authigenic iron-bearing minerals. Documentation of a wide range of mechanisms for neoformation of greigite provides compelling evidence that sediments containing greigite should be routinely suspected of remagnetization, which will complicate or compromise studies of environmental magnetism and geomagnetic field behavior.
DE: 1512 Environmental magnetism
DE: 1519 Magnetic mineralogy and petrology
DE: 1533 Remagnetization
DE: 1540 Rock and mineral magnetism
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2004 AGU Fall Meeting