Relation between depositional environment and the elemental composition of early diagenetic siderite

Abstract

Early diagenetic siderites from marine and fresh-water depositional environments are characterized by distinctive compositional trends. Siderite from fresh-water environments is often relatively pure (i.e., greater than 90 mol% FeCO{sub 3}) and commonly attains end-member composition. Siderite from marine environments, however, is always extremely impure and has extensive substitution of Mg (up to 41 mol%) and, to a lesser extent, Ca (up to 15 mol%) for Fe in the siderite lattice. In addition, marine siderite generally contains less Mn and has a higher Mg/Ca ratio than fresh-water siderite. This compositional variation appears to result from differences in the chemistry of early marine and meteoric pore waters, inasmuch as early marine pore waters generally have a higher Mg{sup 2+}/Ca{sup 2+} ratio and contain less Mn{sup 2+} and Fe{sup 2+} and more Ca{sup 2+} and Mg{sup 2+} than meteoric waters.