Triple Isotope Fractionation Exponents of Elements Measured by MC-ICP-MS-An Example of Mg

摘要

In most chemical reactions, stable isotopes are fractionated in a mass-dependent manner, yielding correlated isotope ratios in elements with three or more stable isotopes. The proportionality between isotope ratios is set by the triple isotope fractionation exponent theta that can be determined precisely for, e.g., sulfur and oxygen by IRMS, but not for metal(loid) elements due to the lower precision of MC-ICP-MS analysis and smaller isotopic variations. Here, using Mg as a test case, we compute a complete metrologically robust uncertainty budget for apparent B values and, with reference to this, present a new measurement approach that reduces uncertainty on B values by 30%. This approach, namely, direct educt-product bracketing (sample-sample bracketing), allows apparent B values of metal(loid) isotopes to be determined precisely enough to distinguish slopes in three-isotope space. For the example of Mg, we assess appropriate quality control standards for interference-to-signal ratios and report apparent theta values of carbonate-seawater pairs. We determined apparent theta values for marine biogenic carbonates, where the foraminifera Globorotalia menardii yields 0.514 +/- 0.005 (2 SD), the coral Porites, 0.515 +/- 0.006 (2 SD), and two specimens of the giant clam Tridacna gigas, 0.508 +/- 0.007 (2 SD) and 0.509 +/- 0.006 (2 SD), documenting differences in the uptake pathway of Mg among marine calcifiers. The capability to measure apparent theta values more precisely adds a new dimension to metal(loid) delta values, with the potential to allow us to resolve different modes of fractionation in industrial and natural processes.