Clumped Isotope Paleothermometry in Bahamian Dolomites

Abstract

The clumped isotope paleothemometer offers an important advance in understanding carbonate diagenesis in that for the first time the roles of temperature and the isotopic composition of the fluid can be separated. For the process of dolomitization this is particularly important as there have been at least nine different equations which have linked the δ18O of the dolomite to temperatures and the δ18O of the fluid and consequently there are opportunities to better understand the formation of this enigmatic mineral. We have explored the use of the clumped isotope proxy using dolomites from well constrained diagenetic settings in the Bahamas and applied equations and corrections developed for the analysis of aragonites and calcites. Our studies have shown that dolomites produce erroneously high temperatures and fluids using equations and corrections designed for the analysis of clumped isotopes in calcite and aragonite. A correction for this problem was found by measuring the acid digestion fractionation on a suite of different dolomite samples which resulted in a value of 0.153 ‰ compared to a value of 0.092‰ previously used. This correction resulted in the calculation of temperatures and fluids that were within reason for the majority of our study sites and revealed unique features that were used to re-visualize the models of dolomitization at these sites. Applying the updated correction to previously published research produces temperatures that agree better with other paleo-temperature proxies. Dolomites formed associated with bacterial sulfate reduction have also been investigated. These analyses were found to yield anomalously high temperatures even with adjusted acid digested fractionation suggesting the possibility of a non-equilibrium processes perhaps associated with localized pH changes. While this effect is small and will not hinder the separation of dolomites formed at high temperatures from those formed at lower temperatures, it may help us distinguish the influence of bacterial sulfate reduction in dolomite formation.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016