Diagenesis and pyritization of crinoid ossicles

Abstract

Diagenetic alteration of original high-Mg calcite crinoid skeletal material was achieved by reaction of the calcium carbonate with isotopically light meteoric water. The light 61s0 values (-10.57 + 0.74%,, PDB) are a reflection of the water source, whereas the 613C values (-3.49 + 2.63%,, PDB) are a consequence of the high reactive-organic-matter content in the sediments of the Brush Creek at Sewickley, Pennsylvania. High Fe (22 970 + 30 440 ppm) and Mn (3760 + 1450 ppm) contents suggest further that the diagenetic fluids were reducing waters that reacted with the crinoid ossicles in the shallow-burial marine and meteoric environments.Four different types of pyrite are found within crinoid ossicles. Framboidal pyrite, aggregate pyrite, and nanopyrite are generally found in the pores of the stereom. Micropyrite, which is present in the form of octahedral, dodecahedral, and pyritohedral crystals, and nanopyrite and aggregate pyrite replace the calcitic stereom. The degree of pyritization and the pore filling increase toward the outer periphery of the crinoid ossicles. Pyrite formation of the crinoid ossicles took place in two stages. In stage I, upon death, the crinoids disarticulated and were quickly buried. In this shallow sediment layer (1 -10 cm) the pore water was both undersaturated with respect to calcite and oxidizing. This brought about oxidation of the organic tissue in the stroma of the crinoids and selective dissolution of the high-Mg calcite skeletons. With further burial in stage 11, in the presence of reactive organic matter local iron was solubilized, marine sulphate was reduced to sulphide, and isotopically light carbon was produced by bacterial action. Reactive iron combined with sulphur to form framboidal and nanopyrite in the pores of the stereoms. With further burial the micropyrite formed in the crinoid ossicles. Termination of the pyritization process came about with depletion of the iron and (or) sulphur, and this process proceeded very rapidly under shallow-burial conditions while the crinoids resided in the microbial sulphate-reduction zone of the marine-phreatic environment.Fossils are preserved in different stages, with brachiopod valves preserved in their original low-Mg calcite mineralogy, whereas molluscs and crinoids show the complete trend from preserved aragonite and high-Mg calcite, respectively, to diagenetic low-Mg calcite. The preservation potential of fossils is closely linked to the thermodynamic stability of the skeletal carbonate in the presence of diagenetic fluids. Carbon/sulphur ratios support the assertion that Brush Creek sediments were deposited in normal marine waters and favour generally oxic redox conditions for Pennsylvanian seawater.L'altkration diagknktique du matCriel squelettique des Crinoides, form6 initialement de calcite riche en Mg, a kt6 accomplie par la kaction du carbonate de calcium avec une eau mtttorique isotopiquement pauvre. Les valeurs faibles de 6180 (-10,57 + 0.74%,, PDB) reflbtent la nature m&me de la masse d'eau, tandis que les valeurs de 613C...

Secular variation of calcium carbonate mineralogy; an evaluation of ooid and micrite chemistries

“…Carboniferous crinoid samples that plot closest to this HMC field are the diagenetically least-altered. This is based on microstructural, trace elemental, and stable isotopic evidence (BRAND & MORRISON, 1987). These crinoids are from units that are associated with preserved aragonitic molluscan faunas (Boggy, Breathitt, and Brush Creek Formations;MORRISON et al, 1985;BRAND, 1987BRAND, a, b, 1989 Chemistry of Kohlenkalk ooids is superimposed in Fig.…”

Chemical diagenesis and dolomitization of paleozoic high-Mg calcite crinoids

“…Skeletons of modern echinoderms consist of the calcitic stereom and the stroma filled by organic tissue and body fluids. These skeletal structures are preserved in some of the crinoids from the Brush Creek ( Figure 5, plate A: Brand and Morrison, 1987). But, this open meshwork is not preserved in specimens from the Kiewitz ( Figure 5, plate B), although they have geochemical signatures similar to those from the Brush Creek.…”

“…The trend for crinoid material from shales is more complex; some populations show a decrease of both Mn and Fe with progressive alteraMicrostructures are useful indicators of the degree of diagenetic alteration in biogenic carbonate grains (cf. Blake et al, 1982: Brand andMorrison, 1987;Brand, 1989 a.b). Skeletons of modern echinoderms consist of the calcitic stereom and the stroma filled by organic tissue and body fluids.…”

“…Additional material was procured from the studies of Veizer (1980, 1981), Milinkovitch and Brand (1984). Brand (1987a,b), Brand and Morrison (1987), Popp ( 1981 ) and Popp et at. (1986) for a total trace-element data base of 368 samples.…”

“…Land, 1967;Pingitore, 1976;Brand and Veizer, 1980;Buchardt and Weiner, 1981;Morse, 1983: Popp et aI., 1986Brand and Morrison, 1987;Brand, 1989a,b). The sum of these studies permits the Carbonates and Evaporites.…”