Crustal Geophysics and Geochemistry Science Center

Isotope and chemical methods for mineral and geoenvironmental assessments

Development of micro-beam stable isotope techniques

Task Objectives

Laser and ion microprobes offer the possibility of studying stable isotope variation in zoned minerals and between fine-grained coexisting minerals. Stable isotope studies of zoned minerals can provide important information on temporal and spatial processes of mineral formation, including changing conditions, secondary overgrowths, and replacements. Stable isotope microanalysis of fine-grained coexisting minerals offers the potential of accurate geothermometry data gathered in a textural context. Both of these types of studies offer significant protential for advancing the science of stable isotope geochemistry, but both require high precision and accuracy and a very small spot size.

Secondary ion mass spectrometry (SIMS) on the ion microprobe offers the best combination of spatial resolution (20 µm beam diameter) and precision (0.2 per mil for sulfur isotopes), but analyses are expensive (~$1000 per day) and not widely available. Sulfur isotope analysis techniques by SIMS have been developed to relatively high precision, but other stable isotope studies of oxygen, carbon, and hydrogen have not been as successful in achieving high precision and accuracy.

We have developed a laser fluorination system that is capable of analyzing d18O on 2-3 mg samples (but not on spots in samples) and we have developed techniques for d34S analysis on about 50 µm spots on sulfides and sulfates using the laser ICP-MS system. A goal of this task is to make these techniques widely available to geochemists and others interested in these kind of analyses.

The objectives of this task are to continue development of ion microprobe and laser microprobe techniques for in situ or small sample analysis of minerals, amorphous materials, and zones in mineral grains so that these techniques can be utilized by a broad spectrum of scientists in the USGS.

We will continue to develop and prove techniques of sulfur and oxygen isotope analysis using the laser microprobe on the ICP-MS sector instrument and we will continue to refine our laser fluorination methods. Our goal is to develop systematic methods, with appropriate standards and reliable techniques, for analyzing sulfur and oxygen isotopes to high precision in small (<30 µm) areas of thin or polished sections, or using small amounts of separated minerals.

Statement of Work

We will pursue Laser ICP-MS analyses on the Nu-Plasma sector instrument which has been demonstrated to produce analyses accurate to better than 1 per mil. Slack et al (2008, Goldschmidt Abstract) reported good concordance between d34S analyses by laser and ion probe over a range of 70 per mil on sulfide minerals. Laser analyses were on spot sizes of 25-40 µm, which makes this a very useful tool. In addition, we intend to pursue analyses of other isotopes in sulfide minerals, such as Fe, Cu, and Zn, by this technique.

This year we will begin a collaborative project with the University of Minnesota to experimentally measure Fe, Cu, and 33S fractionation between sulfide minerals and aqueous species at hydrothermal temperatures. We will use laser techniques to measure isotopic values and look for zoning or metastability in our experimental products.

We will continue to refine and develop oxygen isotope methods for analyses of small amounts of silicates, oxides, and sulfates (1-2 mg) by BrF5 reaction with samples by laser heating using our 50 watt CO2 laser system. We have developed working standards of quartz, garnet, ferrihydrite, goethite, hematite, and schwertmannite (the last 4 being important materials in studies of abandoned mine lands and acid mine drainage) and are refining laser fluorination techniques. This technique is advantageous for analyzing iron minerals and refractory minerals that are difficult to analyze by bulk fluorination methods.

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