Isotopic Composition and Diagenetic Alteration of Lower Carboniferous Brachiopod Shells: Constraints from Proton Microprobe (PIXE) Trace Element Analysis

Abstract

The chemical and isotopic evolution of ocean water mirrors the dynamics of the earth system. Information concerning seawater isotopic composition and its chemistry is derived from rock components that have precipitated in equilibrium with ambient seawater. For a correct interpretation of isotope data, samples that have suffered a l tera t ion by diagenetic processes must be avoided. Trace element analysis is frequently utilized as a diagnostic tool. Unfortunately, conventional techniques, such as AAS or ICP, frequently require more sample than is available or yield only a bulk mean composition (Veizer et aL, 1987). Furthermore, for most ancient calcitic materials, the concentrations for the elements of interest (e.g. Mn, Fe, Sr) are generally below the detection limit of standard electron microprobes (cf. Riciputti et al., 1994). A microanalytical tool is therefore required that combines low detection limits with high lateral resolution. In this study, t r a c e e l e m e n t c o m p o s i t i o n s o f L o w e r Carboniferous brachiopod shell fragments were analysed using the Bochum proton microprobe. The data are compared to the ICP 'bulk' analyses and the 8180 data of a number of fragments prepared from the same shells.