Application of Electron Microprobe Scanning Techniques to Resolve Trace Element Variations in Biogenic Aragonite.

Extract

Aragonite (CaCO₃) is one of the principal carbonate minerals utilized by metazoans to form shells, tests and other skeletal elements. It may also be inorganically precipitated under certain geological conditions. Aragonite is unstable in meteoric diagenetic environments (relative to its polymorph calcite), and is prone to dissolution and/or replacement by other minerals (usually calcite). The mobilization of Ca²⁺ and CO₃²⁻ during aragonite alteration is largely responsible for early diagenetic cementation and the formation of limestone from unconsolidated carbonate sediment. Our study has been investigating the very earliest stages of aragonite alteration. To date, we have limited our work to bivalves of the genera Donax, Chione and Austrovenus collected from, respectively, southern Alabama, southwestern Florida and eastern North Island, New Zealand. Chione and Austrovenus are closely related genera which should permit direct comparison of results. The Florida and New Zealand study locations were chosen because they were near fossil localities containing identical Tertiary-aged specimens that had been subjected to more prolonged alteration. Our results confirm that initial aragonite alteration in meteoric environments is driven by dissolution. Outer surfaces begin to dissolve very rapidly (laboratory experiments suggest that this may occur after only a few months of exposure to meteoric water); leading to a chalky consistency along the outer extremities of the shell. Simultaneous selective leeching of some trace elements (notably Sr²⁺) also occurs. More prolonged dissolution results in progressive loss of mass and Sr²⁺ as well as penetration of the chalky “zone” into the interior of shells. Highly altered shells consist almost entirely of chalk.