The facilities in the Department of Earth Sciences at the University of Minnesota to conduct geochemical research are second to none in the world. In general, these facilities include experimental and analytical components, as well as chemical sensors developed to better understand the in-situ chemistry of hydrothermal vent fluids at mid-ocean ridges.
- Rocking Autoclaves (the flexible gold reaction-cell system). These facilities permit experiments to be performed at a wide range of temperatures and pressures. A key aspect of this equipment, however, is that internally filtered fluids can be sampled at experimental conditions avoiding quench effects that can often obscure reactions at the conditions of the experiment. Similarly, fluid can be added to an ongoing experiment to prolong it or change bulk composition to perturb phase equilibria. The flexible gold cell system is depicted on the right, where image (a) shows an expanded view of the gold cell situated in a steel pressure vessel. The closure at the top of the gold cell is constructed of titanium, which greatly facilitates the sealing of the cell after the addition of the experimental reactants, and is described in detail by Seyfried et al. (1987, Hydrothermal Experimental Techniques in Geochemistry). Upon opening a titanium valve external to the pressure vessel, the cell partially collapses, and an aliquot of the aqueous fluid contents pass through a titanium exit tube out the valve and into a syringe or other sampling device. Simultaneously, water, under pressure, is added to the pressure vessel so that the the total pressure remains constant while a fluid sample is being obtained. As illustrated in the image marked (b), the pressure vessel and internal flexible gold cell are contained in a furnace with precise temperature control. Morever, the pressure vessel and its contents can be rotated, facilitating mass transfer reactions between minerals and coexisiting fluid. The sampling, temperature and pressure control systems are fully automated providing an unusually safe and effective way to conduct hydrothermal processes research of relevance to geothermal, environmental, and geochemical applications
- Flow-through Hydrothermal Reactors. These reactor systems have many elements in common with the flexible cell system, especially in terms of pressure and temperature control, and a strong emphasis on monitoring time series changes in the fluid as a means of assessing corresponding changes in the mineral or rock solids.
The figure immediately above displays all elements of the hydrothermal flow system that makes it ideally well suited to conduct experiments where the central aim is to investigate the response of minerals or rocks, confined in a teflon or gold (inert metal) jacket, to a range of chemical and physical processes. Indeed, a key aspect of the flow-through facilities in the hydrothermal lab at the University of Minnesota are designed to investigate changes in in-situ permeability coupled with chemical mass transfer reactions. That the minerals in the rock cores are interconnected by complex flow geometries means that time series analysis of changes in fluid chemistry and permeabilty provide insight on the role of mineral surfaces and internal surface area on rates and mechanisms of mass transfer. The recent paper by Luhmann et al. 2013, EST) describes the flow system in some detail.
In direct support of experimental and field (vent fluid) studies conducted in geochemistry at the University of Minnesota, state of the art analytical instrumentation is available. The following equipment is housed in the Geochemical lab:
- Ion Chromatography
- Gas Chromatography
- Gas Chromatography-Mass Spectrometry
- Inductively Coupled Plasma- Mass Spectrometry
- Inductively Coupled Plasma-Optical Emission Spectrometry (See image below, Thermo X2 with autosampler system)
- Automatic Titrators
- Stero microscopes
In addition to experimental and analytical equipment used in direct support of experimental research, geochemical research in the Department of Earth Sciences benefits from the routine availabliity of high-speed computers installed with state of the art software (EQ3/6, GWB, TOUGHREACT) and supporting thermodynamic databases.
These facilities are available for theoretical modeling studies. Computer simulations of phase equilibria of any chemical system provide a highly useful means to explore the effect of intensive variables on mineral solubility. The figure below depicts the effect of redox and pH on phase relations in the FeO-Fe2O3-H2S-HCl-CuO system at 350°C, 500 bars. These data are used in advance of experiments to plan strategies to determine, for example, the fractionation of non-traditional Fe and Cu isotopes between chalcopyrite and coexisting aqueous fluid. In general, theoretical modeling coupled with results of experimental and field (hydrothermal vent) data provide a highly integrated approach to investigate rates and mechanisms of mass transfer in a wide range of chemical systems. Although we have emphasized hydrothermal studies, the geochemical research group at the Universiversity of Minnesota is keenly interested in the most general aspects of environmental geochemistry, especially including contaminant transfer associated with natural weathering processes and mineral exporation activities.
Chemical Sensor Systems
Studies of the chemistry of hydrothermal vent fluids have long benefitted from the in-house development of in-situ chemical sensors (Ding and Seyfried, 2007, Chemical Reviews).
The chemical sensor systems tend to make use of the yittria stabilized zirconia (YSZ) electode, which can be used in different configurations with supporting references and target electrodes to determine the pH and/or dissolved H2 in hydrothermal vent fluids. Heretofore, these chemical sensor systems have been deployed at hydrothermal vent sites for real time and short term monitoring studies.The figure below shows a chemical sensor system recording data at EPR 9°N during a short-term deployment. Future activites in association with the NSF, Ocean Observatory Initative (OOI), and Ocean Networks Canada, offers the very real possiblity of long-term monitoring taking full advantage of power delivered to the seafloor along fiber optic cables. The geochemistry group at the University of Minnesota is very excited about the research that these rapidly developing facilities will make possibe.