Clumped isotope evolution during aragonite diagenesis

The clumped isotope (Δ47) proxy provides specific information on formation conditions of carbonate minerals, for example the related fluid temperature. Although the clumped isotope paleothermometer often allows the direct temperature reconstruction due to the measurements being relatively free of vital effects or mineral-specific fractionation, clumped isotope geochemistry during the diagenesis and recrystallization of sedimentary rocks is understudied. This work evaluates clumped isotope data during the alteration of bivalve shells to better understand how the proxy develops under varying diagenetic conditions. Aragonitic bivalves were experimentally altered in closed vessels with variations in durations (0-16 weeks), fluid chemistry (burial and meteoric fluids) and temperature (130 to 200°C).

Results generally show increasing calculated temperatures (lower measured Δ47 values) with increasing experimental temperature and duration. Different fluid chemistries (burial versus meteoric fluids) also resulted in a constant offset of values for modern samples following alteration. Fossil carbonates seem to be more resistant to alteration of Δ47 values, indicating a reduced alteration potential compared to modern samples. Lastly, fast re-equilibration during the alteration experiments seem to follow a relatively simple model, similar to the proposed solid-state isotope re-ordering model of Passey and Henkes (2012) and others, but at much higher rate constants and starting at lower temperatures.