Pore water evolution during sediment burial from isotopic and mineral chemistry of calcite, dolomite and siderite concretions

Details Pore water evolution during sediment burial from isotopic and mineral chemistry of calcite, dolomite and siderite concretions Pore water evolution during sediment burial from isotopic and mineral chemistry of calcite, dolomite and siderite concretions

Oct 1986

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1986gecoa..50.2321c&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 50, Issue 10, pp.2321-2334

Computer Science

20

Scientific paper

Coal measures often contain concretions; segregations of diagenetic minerals originally formed within unconsolidated sediments. Three different types (calcite/pyrite, dolomite/pyrite and siderite) occurring spatially quite close together in the Central Pennine Region of England vary widely in carbon isotope composition (+10.35%. > 13 C > -21.49%.) and in major cation chemistry (Ca, Mg, Fe, Mn). Within some siderite concretions, very high Mn / Fe ratios were found in central subsamples; these were also most enriched in 13 C. The Fe / Mg ratio decreases systematically from centre to edge (early, shallow to deeper, later precipitation). The calcite/pyrite and dolomite/pyrite concretions developed completely prior to significant burial. Both have high Mn / Fe ratios but negative 13 C values (calcite -21.49%., dolomite -8.67 to -10.48%.). All of these patterns can be equated precisely with theories of pore water evolution developed on the basis of geochemical investigations of modem sediments. Microbial processes (sulphate reduction, methanogenesis) contributed significantly, as did thermal decarboxylation (to siderite precipitated at considerable burial depth). Mn(IV) and Fe(III) acted differentially as oxidants; producing CO 2 and increasing alkalinity. The interplay of fresh and marine depositional waters is seen most obviously in the presence or absence of sulphate reduction. This controlled mineral type (iron sulphide or carbonate) as well as isotopic and mineral chemistry.

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