Siderite acid fractionation factors for sealed and open vessel digestions at 70 °C and 100 °C

•Digestion style specific phosphoric acid fractionation factors for siderite.•We report the first estimate for open vessel digestions.•At 70 °C our sealed vessel estimate agrees well with published values.•At 100 °C our open vessel estimate is approximately 1‰ larger than published values.

The oxygen isotope composition of siderites can be used to deduce the paleoenvironmental conditions under which the mineral formed. However, it is not clear whether the published fractionation factors (αCO2-siderite) for the acid digestion of siderites are appropriate for modern “on-line” sample preparation techniques. This is an important source of uncertainty that has not yet been investigated. To this aim, we obtained new siderite acid fractionation factors for open and closed vessel digestions at 100 °C and 70 °C, respectively. We find that previous αCO2-siderite estimates are appropriate for GasBench-type sealed vessel digestions and propose a revised relationship between αCO2-siderite and temperature:1000⋅lnαCO2‐Siderite=4.06±0.17103/T−1.93±0.5R2=0.99This relationship encompasses most practical laboratory digestion temperatures and has a temperature sensitivity of − 0.032‰/°C that is similar to other carbonate minerals. The open vessel digestion αCO2-siderite at 100 °C, on the other hand, is offset from previous estimates by 1‰, with a value of 1000·lnα = 9.79 ± 0.23 (0.12 SE). This estimate is essential for the open vessel digestions that are needed for siderite clumped isotope analyses.