Trace-Element Geochemistry and Diagenesis in Capitan Reef (Permian), West Texas: ABSTRACT

Abstract

The lattice substitution of doubly charged cations into calcite during aqueous precipitation depends on solution composition and the appropriate partition coefficients. Thus, the trace-element geochemistry of a limestone encodes the chemical composition of the solution in which mineralogic stabilization occurred. Because different environments are characterized by specific solution chemistries, we can use the trace-element composition of a limestone to infer the diagenetic environment in which it stabilized. A suite of calcite samples was collected in the reef core (massive) and upper fore-reef facies of the Permian reef complex (Guadalupian), from the cliffs above the entrance to McKittrick Canyon. These rocks averaged 375 ppm Sr2+, 13,900 ppm Mg2+, 9.4 ppm Zn2+, and 40.1 ppm Mn2+ (AA analyses). Petrographic examination of these wackestones and packstones provided no conclusive evidence of the environment in which mineralogic stabilization occurred. However, the trace-element values, when compared to probable starting (sedimentary) compositions, indicate equilibration in an open chemical system with insignificant introduction of cations from an external source. Autodepletion of strontium and magnesium (kSrcal = 0.14; kcal = 0.02) accompanied autoenrichment of zinc and manganese (kZncal = 5; kMncal = 15). The degree of autoenrichment and autodepletion of these chemical species is characteristic of an open chemical system, that is, one in which fresh waters flush rapidly through the diagenetic site. This combination of fresh water and open system is diagnostic of a freshwater phreatic zone. We infer, then, that this part of the reef stabilized in a End_Page 509------------------------------ freshwater phreatic environment. This interpretation is consistent with the vadose origin ascribed to features observed within other parts of the complex. End_of_Article - Last_Page 510------------