Abstract: Model for Trace-Element Partitioning in Limestones
Nicholas E. Pingitore, Jr.
The partitioning of strontium and magnesium into calcite (kSrcalcite = 0.14; kMecalcite = 0.05, under diagenetic conditions) yields important insight into diagenetic processes in limestones. Other trace elements, such as zinc and manganese, exhibit different behavior, for their partition coefficients are greater than unity (kZncalcite = 5.5; kMncalcite = 15). Integration of information from the distributions of zinc and manganese with that from strontium and magnesium requires a general model of trace-element behavior. The parameters of distribution coefficient, water flow relative to reaction rate (openness of chemical system), and water chemistry before entering the diagenetic site normally determine the enrichment or depletio in trace element of calcite relative to the parent aragonite (or calcite). Four distinct situations ensue. (1) Autodepletion is preferential loss of trace element from the diagenetic site due to high water flow relative to precipitation rate and k<1. Calcite depleted in trace element relative to the parent aragonite results. (2) Autoenrichment is enrichment of the calcite in trace element when k>1. (3) Alloenrichment is precipitation from a liquid enriched in trace element (relative to the parent aragonite) before it enters the diagenetic site; the enrichment of the liquid required increases with decreasing k. (4) Allodepletion occurs when a liquid rich in calcium and poor in trace element flushes through the diagenetic site. If k~1, each liquid cycle may introduce sufficient calc um that, after the liquid dissolves aragonite, its trace element/calcium ratio remains less than that of aragonite, yielding a depleted calcite.
AAPG Search and Discovery Article #90961©1978 AAPG Annual Convention and Exhibition, Oklahoma City, Oklahoma