HR: 17:05h
AN: OS14A-05    [Abstracts]
TI: Marcasite in Sediments and Sedimentary Rocks - Conundrum and Opportunity
AU: * Schieber, J
EM: jschiebe@indiana.edu
AF: Dept. of Geological Sciences, Indiana University, 1001 E 10th Str., Bloomington, IN 47405 United States
AB: For many years marcasite in ancient sedimentary rocks was treated mainly as a curiosity. In the literature, the occurrence and distribution of pyrite, its much more abundant polymorph, dominates discussions of early sulfide mineral diagenesis in sediments. Curiously though, while marcasite has been detected in sedimentary rocks of all ages, it has yet to be reported from modern sediments. This unusual situation may present us with one of those odd instances when "the past becomes the key to the present". While both marcasite and pyrite require reducing pore-waters for their formation, marcasite is unique in that it additionally requires a low pH. In experiments, marcasite occurs below pH 6 and is predominant below pH 4, apparently due to higher growth rates of marcasite vs. pyrite under those conditions. In marine surface sediments, pH values around 8 in the overlying seawater (due to the carbonate-bicarbonate buffer) make it very difficult for pH values of pore-waters to drop low enough for predominant marcasite formation. In nature, the low pH values needed for abundant marcasite formation are for example encountered in salt marsh sediments when previously formed sedimentary sulfides are oxidized. During sequence stratigraphic studies of Devonian black shales, iron sulfides with morphological characteristics of marcasite (confirmed by EBSD) were encountered as cements in lag deposits covering erosion surfaces. By analogy with the low pH values in oxidizing salt marsh sediments, one can speculate that concentration of previously formed iron sulfides in these lags (concretions, burrow linings, framboids) laid the foundation for subsequent marcasite formation. The lags in question formed as a consequence of intermittent sea level drops, subjecting previously deposited black shales to wave erosion, and promoting formation of pyritiferous lags. Oxidation of pyrite grains should have led to abundant acid production, temporary lowering of pH in lag pore spaces, and pore waters rich in dissolved iron. Under those conditions, hydrogen sulfide production from underlying black shales should have enabled rapid marcasite formation. Rapid marcasite growth is indicated by radial fibrous aggregates that fill the pore spaces of sandy to granular lags. Thus, observations from the rock record support the hypothesis that marcasite forms where sediment layers with abundant iron sulfides undergo intermittent oxidation. Rather than indicating a true deficiency, the dearth of marcasite in modern sediments may simply be a matter of methodology. In ancient rocks at least, iron sulfide aggregates that XRD analysis identifies as pyritic, often reveal traces of residual marcasite when examined with electron backscatter diffraction (EBSD) in polished thin sections. Preliminary analyses suggest that marcasite is more widespread in ancient strata, and may have (until now) escaped detection in modern sediment because of small grain size and low abundance. Systematic study may reveal it to be a useful indicator of rapid oxidation events in sedimentary successions.
DE: 4851 Oxidation/reduction reactions
DE: 3665 Mineral occurrences and deposits
DE: 4267 Paleoceanography
DE: 4556 Sea level variations
DE: 4802 Anoxic environments
SC: Ocean Sciences [OS]
MN: 2004 AGU Fall Meeting