Combining clumped isotope and trace element analysis to constrain potential kinetic effects in calcite

Abstract

The field of clumped isotope paleothermometry is over a decade old, but the influence of precipitation rate on the fractionation of clumped isotopes between natural carbonates and their environmental solutions remains unclear. Here we apply two different proxies, carbonate clumped isotope paleothermometry and trace element analysis, to investigate whether or not precipitation rates play a major control on the fractionation of clumped isotopes. Twenty-one transect points from three calcite samples coming from veins along fractures (Jebel Madar, Oman) were investigated. The samples selected based on two different types of calcite cements were recognized: (i) Lower temperature macro-columnar calcite, and (ii) Higher temperature calcites within thin vein (“thin-vein calcites”). The Δ47 values vary between 0.513 ± 0.00‰ and 0.667 ± 0.01‰ corresponding to calculated clumped isotope temperatures of 29–59 °C for macro-columnar calcite samples, and 59 ± 9 to 109 ± 1 °C for the thin-vein calcite sample. The calculated δ18OFluid [VSMOW] composition ranges between −11.2 ± 0.8 to +13.4 ± 0.0‰. Trace element results reveal a strong correlation between Fe and Mn and the clumped isotope temperatures. We argue based on published work on the incorporation of Mn and Fe into calcite minerals that this is indicative of a change in crystal growth rates. Thus for the first time we can independently show potential kinetic fractionation in ancient natural samples that resulted in apparent clumped isotope temperatures higher than environmental temperatures. The new combined clumped isotopes/trace elements approach is a critical step to future applications of the clumped isotope proxy to natural systems as it can reveal potential isotopic disequilibrium independent of the calculated temperature.