Dissolution and pyritization of magnetite in anoxie marine sediments
Abstract
Concentrations of magnetite were determined, with depth, in sediments of varying H 2S content taken from Long Island Sound (FOAM, NWC, and Sachem sites) and from a site in the rapidly depositing subaqueous portion of the Mississippi Delta. For the three Long Island Sound sites, dissolution of magnetite during burial is clearly demonstrated, and for all four sites the rate of magnetite dissolution is proportional to the concentration of dissolved pore water sulfide. By combining magnetite depth distributions with sedimentation rate data and measurements of dissolved sulfide, we find that magnetite dissolution is well described by the following rate law: dC mag/dt = -1.1× 10 -5C 0.5BC magA mag where Cmag is the concentration of magnetite (grams per gram of sediment), Cs is the concentration of dissolved sulfide (mM), and Amag is the specific surface area of magnetite in the sediment (cm 2 per gram of magnetite.) This equation means that, for a typical range of magnetite surface areas and dissolved sulfide concentrations, the "half-life" of magnetite in anoxic marine sediments ranges from about 50 to 1000 years. SEM observations of magnetite grains from the different sediment sites reveal that magnetite dissolution may be accompanied by extensive replacement by pyrite. This only occurs if magnetite is in contact with high concentrations of H 2S (>1 mM) for relatively long periods of time (several hundred years). With low concentrations of H 2S (<1 mM) magnetite dissolution occurs without identifiable pyrite replacement. We suggest that this feature of magnetite diagenesis may prove to be a useful paleosulfide indicator.
- Publication:
-
Geochimica et Cosmochimica Acta
- Pub Date:
- March 1987
- DOI:
- 10.1016/0016-7037(87)90076-7
- Bibcode:
- 1987GeCoA..51..645C