SOURCE OF CARBONATE AND ORIGIN OF ZONATION IN PYRITIFEROUS CARBONATE CONCRETIONS - EVALUATION OF A DYNAMIC-MODEL

Spheroidal, diagenetic concretions in the Toarcian black shales of the Jet Rock (Yorkshire, United Kingdom) are predominantly calcite with a microspar texture. Pyrite occurs within the concretions, but its concentration increases to the margins to form a sharply-defined massive pyrite rim. This striking zonation can be described by a dynamic model with the following characteristics. In an initial, pre-concretionary stage, framboidal pyrite resulted from pervasive sulfate reduction in porewaters containing dissolved iron (produced by local iron-reduction), but diffusive loss of the alkalinity generated to overlying seawater precluded carbonate precipitation. Subsequently, conditions for concretionary carbonate precipitation were created by localized sulfate reduction which caused direct precipitation of euhedral pyrite from surrounding pore-fluids with lower concentrations of reduced iron. During concretion growth the rate of generation of sulfide in the core of the concretion exceeded the rate of inward diffusion of reduced iron. The resultant centrifugal diffusion of sulfide allowed precipitation of carbonate and defined the growth zone for euhedral pyrite, which formed the rim. Thus, the mineral zonation of the concretions was spatially rather than temporally controlled, and successive zones do not represent changes in porewater composition during burial. The model is consistent with observations of the chemical and isotopic compositions, size, shape, and stratigraphic distribution of concretions. However, the model requires very specific porewater environments not found in present-day sediments and relies on reactions that have not been described in specific detail.