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

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 (alpha(co2-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 degrees C and 70 degrees C, respectively. We find that previous alpha(CO2-siderite) estimates are appropriate for GasBench-type sealed vessel digestions and propose a revised relationship between alpha(CO2-siderite) and temperature: 1000 . In (alpha(CO2-Siderite)) = 4.06 +/- 0.17 (10(3)/T) -1.93 +/- 0.5 (R-2 = 0.99) This relationship encompasses most practical laboratory digestion temperatures and has a temperature sensitivity of -0.032%./degrees C that is similar to other carbonate minerals. The open vessel digestion alpha(CO2-siderite) at 100 degrees C, on the other hand, is offset from previous estimates by 1%., with a value of 1000.In alpha = 9.79 +/- 023 (0.12 SE). This estimate is essential for the open vessel digestions that are needed for siderite clumped isotope analyses. (C) 2016 Elsevier B.V. All rights reserved.