Experimental constraints on siderite clumped isotope thermometry

Abstract

We present results from chemostatic siderite precipitation experiments conducted using a modified constant composition method at temperatures between 15 and 75 °C and compare these results with previous temperature relationships to obtain a composite low temperature relationship for siderite clumped isotope ratios (Δ47) from 8.5 to 75 °C:Δ47=0.0364±0.004∗106T2+0.1672±0.046±95%CL,R2=0.708The slope of this line is statistically indistinguishable from those reported for aragonite, calcite, and dolomite clumped isotopes, but differs in its intercept. To test whether this difference results from mineralogical differences in phosphoric acid fractionation factors (AFF) between siderite and Ca-Mg carbonates, we heated siderite to 625 °C at 1 GPa, which yielded a statistically indistinguishable Δ47 value from that predicted by published clumped isotope calibrations populated by Ca-and Mg-carbonate data at higher temperatures. Thus, the apparent difference between siderite clumped isotope behavior and that of other carbonate minerals may not be well explained by differences in AFF. Despite differences in precipitation methods, our results are entirely consistent with published inorganic and microbially-mediated experimental siderite clumped isotope data, suggesting that siderite Δ47-temperature dependence differs from Ca-Mg carbonates for reasons unrelated to precipitation methods. Our isotopic results are also consistent with published isotopic measurements on natural siderites precipitated at known environmental temperatures. Our analysis leads us to conclude that supersaturation-related kinetic effects likely play an important role in clumped isotope fractionation in natural and experimentally precipitated siderite.