Chemical Diagenesis of a Multicomponent Carbonate System--1: Trace Elements

Abstract

ABSTRACT Theoretical considerations (i.e., partition coefficients, water/rock ratio, chemistry of interstitial meteoric water) of elemental behaviour during diagenetic stabilization with meteoric waters suggests that it leads to a decrease in strontium, sodium, and possibly magnesium and an increase in manganese, iron, and zinc in progressively altered carbonates. Such elemental behaviour is exhibited by the different carbonate components of the Mississippian Burlington Limestone of Iowa and Missouri and the Silurian Read Bay Formation of Arctic Canada. In the Burlington Limestone the rock matrix (e.g., biosparite), the enclosed crinoids, and to some degree the rugose corals are chemically similar. The crinoid ossicles have average strontium content of 160 ppm, rugose corals 180 ppm, and the enclosing biosparite 120 ppm. In contrast, in the Read Bay Formation each of the above mentioned components has a specific chemistry, with 210 ppm strontium for crinoids, 780 ppm for rugose corals, and 360 ppm for their enclosing micrite matrix. These chemical trends are accompanied by textural changes of the host carbonate sediments. In the Burlington Lime tone this involves the presence of depositional sparite, whereas in the Read Bay Formation this increase in textural maturity involves the transition from micrite to microspar to minor pseudospar and sparite. The combination of these textural trends with the elemental patterns shows that the degree with which a particular carbonate component approaches either the open system or the partly closed system equilibrium is dictated by its respective mineralogical stability and the water/rock ratio. While the results show that the carbonate assemblage may act as a completely open diagenetic system (e.g., Burlington Limestone), available data for the majority of studied sequences (e.g., Read Bay Formation) suggest that diagenetic equilibration ceases while some original depositional differences in chemical composition are still preserved. This implies that diagenetic stabilization proceeds through partly closed reaction zones on solid-liquid interfaces. Transfer of the chemical and textural inform tion from the dissolving phase (original sedimentary carbonate particle) to the precipitating phase (diagenetic carbonate component) proceeds via a Messenger Film water in the reaction zone, which is in disequilibrium with the meteoric bulk aquifer water. Thus the chemical composition of carbonate components of ancient limestones may serve as a potential tool for evaluating the degree of diagenesis and for deducing the original mineralogy of the different stabilized carbonate phases. Application of this trace element model suggests that Paleozoic crinoids were composed originally of metastable high-magnesium calcite, rugose corals were composed originally of stable low-magnesium calcite or high-magnesium calcite with low Mg2+ content, and micrite was origially aragonite lime mud.