Towards Understanding Artifacts in the Clumped Isotope System

Abstract:

The clumped isotope system in carbonates (Δ₄₇) relies on the extraction of CO₂ from the carbonate minerals using phosphoric acid. Despite the fact that this method dates back to the original stable isotopic work in the 1950s, there are significant aspects of the fractionation of the ¹⁸O/¹⁶O (and by inference the ratio of mass 47 to 44) which are not understood. We believe that subtle variations in the isotopic fractionation as a function of temperature, acid density (and acid preparation method), and extraction line design cause variation between the clumped isotope data produced by different laboratories. One of the most obvious of these is difference in reaction temperatures. While most laboratories employ temperatures of between 75 and 90^oC, the original method employed a temperature of 25^oC. Although various estimate of the difference in fractionation of Δ₄₇ between 25 and 90^oC have been made, we have measured significantly different values for dolomites compared to published data. In order to understand this we have performed experiments in sealed Pyrex vessels to measure the exchange between CO₂ and 103% phosphoric acid. We have determined there to be significant and measurable changes in the Δ₄₇ of CO₂ when exposed to phosphoric acid. This exchange is a function of temperature, time, acid strength, and the surface area of the acid exposed to the CO₂. We postulate that, perhaps as a result of the lower reaction rate of dolomite, compared to calcite, that there is greater opportunity for CO₂ to exchange with the phosphoric acid as bubbles of CO₂ are retained within the acid for longer periods of time. Such a mechanism would predict that well-ordered dolomites will have different fractionation compared to protodolomite. Similar differences might account for different fractionation for other carbonate minerals.