Recent advances in analytical geochemistry have demonstrated that stable isotopes of iron and sulfur can provide clues to the physical and chemical conditions deep in the earth that are responsible for the chemistry of hydrothermal vent fluids and that help to control the chemistry of seawater. Until now, how these isotopes partition between minerals and fluids at different temperatures and oxidation-reduction conditions has been speculated upon but not experimentally verified. This research involves controlled laboratory experiments at temperatures from 300 to 450 Celsius and a pressure of 500 bars to determine fractionation of non-traditional stable isotopes (Fe, Cu, S) between minerals and fluids. Experiments are designed to examine the effect of fluid chemistry on the rates and processes of isotope exchange and determine the kinetic rates of formation of pyrite and chalcopyrite, two important sulfide minerals that precipitate from hydrothermal vent waters. Results of the research will be used to interpret iron, copper, and sulfur isotope ratios in terms of mineral formation mechanisms and to constrain conditions of interaction between minerals and fluids in subseafloor hydrothermal systems. Broader impacts of the work include training undergraduate students and enhancing science infrastructure with the development of new experimental approaches that can be used for research outside the fields of geochemistry and ocean sciences.
Introduction: This project provided the first experimental determination of the fractionation of non-traditional isotopes of iron and sulfur between minerals and fluids at a wide range of temperatures and pressures. Non-traditional isotopes are present in all earth materials, but with abundances that have long been recognized to be below detection by available analytical instruments. Recently, however, new instruments with improved sensitivity and enhanced resolution have been developed, providing, for the first time, the opportunity to measure these isotopes in natural materials. The first measurements of the trace isotopes for a number of chemical systems demonstrated surprisingly large variations in both minerals and aqueous fluids. But what does the isotope variability actually mean? It is not possible to answer this question in the absence of experimental data examining the effect of temperature, pressure and reaction rates on isotopic fractionation processes. Two isotope systems of particular interest to earth scientists involve iron (56Fe, 54Fe) and sulfur (32S, 33S, and 34S). Not only are iron and sulfur widespread in the earth and ocean, but coexist in the mineral pyrite (FeS2), which is the most abundant sulfide mineral in and on earth, and which is found in particularly high concentrations at deep sea vents (Fig. 1). Although our research is primarily focused on the use of non-traditional isotopes of iron and sulfur to unravel inorganic processes controlling the chemical evolution of marine hydrothermal systems, these same elements serve as "nutrients" for microbial metabolism, which, in turn, sustain communities of larger animals, such as the blind shrimp shown in Figure 1, broadening the implications of the research. Intellectual Merit: A major objective of the project was to calibrate how the non-traditional isotopes of iron and sulfur partition between minerals, like pyrite, and coexisting aqueous fluid at temperatures and pressures relevant to the formation of hydrothermal vent fluids at mid-ocean ridges. Experiments were conducted with pyrite synthesized at experimental conditions (dynamic recrystallization experiments) as well as with pyrite crystals that remained intact throughout (static exchange experiments). In both cases, fluid and pyrite crystals were periodically sampled using a novel experimental strategy and analyzed for changes in the composition of iron and sulfur isotopes. The experiments were conducted at different temperatures, with different fluid compositions, and for different lengths of time. These data represent the first experimental calibration of the effect of key chemical and physical variables on the magnitude of non-traditional iron and sulfur isotopic fractionation between pyrite and a coexisting aqueous fluid. With these data, we can now use the multiple isotopic compositions of both iron and sulfur in pyrite recovered from deep sea vents to constrain the conditions of formation (e.g., formation rate, temperature, and redox conditions), with implications for the sources of iron and sulfur at depth within the ocean crust from which hydrothermal vent fluids are derived. Broader Impacts: In the course of the project the following outcomes were achieved with important implications for science education, ways of learning, and physical and institutional infrastructure development, as follows: Undergraduate and graduate students played key roles in all aspects of the study. Accordingly they received training in experimental geochemistry, material science, and electrical and computer engineering. These skills go far beyond the core research area in the earth sciences and will provide them with an expanded array of career opportunities. The development of state of the art experimental facilities needed to fulfill research objectives provided data on the stability of metal alloys (titanium and stainless steel) in aqueous fluids at challenging chemical and physical conditions. These data can be used by others in other disciplinary areas, such as chemical and nuclear engineering, for the construction of experimental reactors to meet the needs of these communities and provide strategies applicable to nuclear and chemical waste disposal and carbon sequestration. The data acquired in the course of the study has both fundamental and applied applications. The fundamental data can be used to supplement data in computer programs that attempt to simulate mass transfer and isotopic fractionation processes in marine geothermal systems. These same data can be used as well in a more applied sense to show the way forward for those using molecular dynamic models to predict isotope fractionation in aqueous fluids at elevated temperatures and pressures.
