Compositional and Temperature Effects of Phosphoric Acid Fractionation on Δ47 Analysis and Implications for Discrepant Calibrations

Abstract:

An essential procedure to increase the analytical efficiency of Δ47 measurements requires raising the temperature of phosphoric acid digestion for carbonate materials. This temperature change introduces a fractionation offset in Δ47 that must be accounted for prior to calculation of temperatures of carbonate formation and to allow interlaboratory comparison of results. We measured the phosphoric acid fractionation factor relative to reaction at 25 °C for calcite, aragonite, and dolomite across a temperature range from 25-90 °C. Significantly, all three minerals behave similarly during phosphoric acid digestion, allowing for a single temperature dependent acid fractionation relationship:

1000ln∝𝐶𝑂2(𝐴𝑐𝑖𝑑)−𝛥47=(0.022434±0.001490)∗10^6 𝑇2 −(0.2524±0.0168)

where α is the phosphoric acid fractionation factor, and T is in degrees Kelvin. Mineralogical or isotopic compositional effects on the fractionation factor were not observed, suggesting that this acid fractionation factor may be valid for all carbonate minerals.

 We also present inorganic temperature calibrations for both calcite and aragonite at low temperatures (5-70 °C) and find them to agree with prior published data. Using the new acid fractionation factor, published Δ47-temperature calibrations are recalculated. This analysis confirms a statistically significant Δ47-temperature calibration difference between data analyzed at 25 °C versus higher temperatures. The origin of the discrepancy remains unknown, but it appears that the acid fractionation factor is not the cause.