pH buffering by metastable mineral-fluid equilibria and evolution of carbon dioxide fugacity during burial diagenesis

Journal Article · Mon Mar 01 00:00:00 EST 1993 · Geochimica et Cosmochimica Acta; (United States)

DOI:https://doi.org/10.1016/0016-7037(93)90037-W· OSTI ID:6823121

Hutcheon, I; Shevalier, M ^[1]; Abercrombie, H J ^[2]

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1. Univ. of Calgary, Alberta (Canada)
2. Geological Survey of Canada, Calgary, Alberta (Canada)

Numerous potential pH buffers including reactions among aqueous organic acid and carbonate species, and carbonate and silicate minerals are typically present during burial diagenesis. Buffering of pH in natural systems is a function of mass action, mass balance, and kinetic constraints. In most sedimentary basins, carbonate and silicate minerals are present in amounts sufficient to buffer pH, and the activities of aqueous species are consistent with metastable equilibrium among observed diagenetic minerals. These observations indicate that mass balance and kinetic constraints are relatively less important than mass action constraints measured by the buffer index, [beta], here defined. The buffer index ultimately dictates which buffer reaction controls pH under diagenetic conditions; buffer reactions with high [beta] values are favored over those with low values. Buffer indices for a number of potential diagenetic buffer reactions have been calculated by reaction path modeling. Heterogeneous equilibria among carbonate and silicate minerals and an aqueous phase have greater [beta] values than those for homogeneous reactions among aqueous carbonate and organic acid species. This implies that pH, calcite dissolution, and f[sub CO[sub 2]] are strongly dependent on carbonate-silicate-fluid interactions during diagenesis. The role of carbonate-silicate reactions in controlling pH is tested by examining the evolution of CO[sub 2] fugacities. The combination of high buffer index, apparent metastable equilibrium between diagenetic minerals and waters, and the relatively accurate prediction of f[sub CO[sub 2]] trends with temperature suggest that carbonate-silicate reactions are important in determining the evolution of fluid compositions in sedimentary basins and influence the course of dissolution events in burial diagenesis. 51 refs., 11 figs., 1 tab.

• OSTI ID: 6823121
• Journal Information: Geochimica et Cosmochimica Acta; (United States), Vol. 57:5; ISSN 0016-7037
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
CARBONATE MINERALS
CHEMICAL REACTIONS
DIAGENESIS
ROCK-FLUID INTERACTIONS
ORGANIC ACIDS
SEDIMENTARY BASINS
SILICATE MINERALS
BUFFERS
CALCITE
CARBON DIOXIDE
CONSTRAINTS
DISSOLUTION
EQUILIBRIUM
INTERACTIONS
MASS BALANCE
PH VALUE
CARBON COMPOUNDS
CARBON OXIDES
CHALCOGENIDES
GEOLOGIC STRUCTURES
MINERALS
ORGANIC COMPOUNDS
OXIDES
OXYGEN COMPOUNDS
580000* - Geosciences