Petrographic and Geochemical Changes in Biogenic Aragonite During Dissolution

QUINN, LAURA C. and HAYWICK, DOUGLAS W., Department of Geology & Geography, University of South Alabama, Mobile, AL

Diagenesis of aragonite is a fundamental control on cementation and porosity generation in carbonate rocks and is therefore of interest to petroleum geologists studying hydrocarbon migration and reservoir development. While stages of aragonite alteration are well documented, the specific petrographic and geochemical changes that occur to aragonite during dissolution are not well understood. Previous work has suggested that aragonite preferentially loses Sr²⁺ and O¹⁸ during the dissolution stage. We are testing this hypothesis by comparing and contrasting the geochemistry of pristine and altered bivalves (Chione spp.) from southwest Florida and New Zealand. In Florida, pristine samples were collected from beach environments near Fort Meyers. Variably altered (chalky and re-crystallized) specimens were collected from the Tertiary Tamiami Formation near Naples. Trace element geochemistry (Sr²⁺, Mn²⁺, Mg²⁺, Fe²⁺) was established through electron microprobe and ICPS analysis.
Our results demonstrate that distinct petrographic changes occur during aragonite dissolution. Initially only surface areas of shells dissolve, but eventually, void spaces form along first and second order foliations within the shell fabric. Initial geochemical data support the contention that Sr²⁺ content of shells decreases with progressive dissolution of aragonite; chalky aragonite contains less Sr²⁺ (and occasionally more Fe²⁺) than does pristine aragonite. Stable isotopic geochemistry of the shells and their crystallography will be established through isotopic analyses and x-ray diffractometry.

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