A quantitative method for determination of trace metal concentrations in sedimentary pyrite

Abstract

Trace metals commonly undergo chemical diagenesis in anoxic sediments from association with metal oxides, organic matter, and other reactive components to coprecipitates with authigenic pyrite. Whereas this process has long been recognized as a major aspect of trace metal diagenesis, no satisfactory method for quantifying trace metal concentrations in sedimentary pyrite has been available. We present a new method, derived from the Lord technique (as described in C.J. Lord, J. Sediment. Petrol., 52: 664–666), for the quantitative extraction of trace metals from sedimentary pyrite. The new method involves the sequential leaching of sediments using 1 M HCl (reactive fraction), 10 M HF (silicate fraction) and concentrated HNO 3 (pyrite fraction). Examples are presented of the application of this new method to trace metal diagenesis in two different sedimentary marine environments: Baffin Bay, TX, and the Mississippi Delta. To establish the relation between the transfer of Fe and the trace metal of interest to the pyrite phase, the degree of pyritization (DOP) was compared with the newly introduced degree of trace metal pyritization (DTMP). Results obtained by combining these two operationally defined parameters indicate that transition metals exhibit a trend of increasing DTMP with increasing DOP, whereas DTMP values for class B metals remain essentially constant with increasing DOP levels.