Higgins Lab construction and installation of the Thermo Neptune multi-collector inductively coupled plasma mass spectrometer (MC ICP-MS) was completed in February 2013. Since that time, we have established protocols for a number of metal isotope systems—magnesium, calcium, and most recently, potassium. The development of stable potassium isotope measurements is significant as our achieved precision is a factor of 3-5 better than previously reported, allowing us to demonstrate stable K isotope variation in low temperature environments for the first time. This work may have a range of applications given the importance of potassium in many geological, environmental and biological systems. In the last year we have also pursued automation of sample processing for isotope analysis. In cooperation with Dionex Corporation we have developed a method for using an ion chromatography instrument connected to a fraction collector. This setup permits rapid (~30 minute) sample throughput and opens the possibility of collecting multiple cation fractions (e.g. Mg, Ca, and K) on a single injection. Using this system we are able to produce data sets that are roughly 10x larger than previous studies.
By leveraging the high-throughput capacity in ICP systems with rapid automated sample processing, we have been able to tackle a number of geological questions which require large data sets. Projects currently in progress include a high-resolution study of Ca isotope variability in Wonoka Formation rocks of Ediacaran age—host of Earth history’s largest carbon isotope perturbation—as well as a systematic study of Mg isotope variability in Phanerozoic dolomites, a survey of Mg and Ca isotope variability in modern shallow-water carbonate depositional environments, and a reconstruction of seawater Ca/SO4 ratios using measured Ca isotope values in marine sulfate evaporites. This latter study is in press in Geology and authored by postdoctoral fellow Dr. Clara Blättler. In the coming year I anticipate further progress on these and additional projects, with a greater focus on K isotopes and their utility in studying K cycling in both low temperature (i.e. Earth surface) and high temperature (i.e. subduction zones) environments.
Accomplishments include the publication of a theoretical paper on the history of the carbon cycle as recorded in the carbon isotopic composition of carbonates in Science (Schrag, Higgins et al., 2013), and the publication of manuscripts reconstructing the Mg isotopic composition of seawater over the Cenozoic in pelagic carbonates in Earth and Planetary Science Letters (Higgins & Schrag, 2012).
Automated cation separation
Isotope ratio measurement by multicollector (MC) ICP-MS requires clean sample separation from matrix ions to avoid matrix effects and isobaric interferences. While we can process ~40 samples per day on our Thermo Neptune+ MC-ICP-MS, traditional methods of sample separation by gravity-driven column chromatography cannot come close to keeping pace and are also prone to elution time drift and human error. The Higgins Group has instead coupled a Dionex ICS5000 automated ion chromatography system with an AS-AP autosampler capable of sample collection. A chromatograph for every sample shows a peak in conductivity when each cation system elutes from the resin-filled column. We program the autosampler to collect the target cation by elution time, excluding the peaks of other matrix ions. This system allows us to visually check for elution time drift or contamination by other matrix ions and to process ~40 samples per day, the same rate at which we measure them on our MC-ICP-MS. So far we have successfully applied our methods to Ca, Mg, Sr, K, and Li in a variety of inorganic and organic, natural and synthetic matrices. Accurate measurement of international standards proves the efficacy of our approach.
