Crustal Imaging and Characterization Team
Ion and laser microprobes offer the possibility of studying stable isotope variation in zoned minerals and between fine-grained coexisting minerals. Stable isotope studies of zoned minerals provides 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 geothermometric 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.
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 IR laser microprobe, secondary ion mass spectrometry (SIMS ion microprobe, the newly acquired ICP-MS sector instrument laser microprobe. 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.
This year, we will continue testing in situ sulfur isotope analysis using the UV laser microprobe on the newly purchased multiple collecting magnetic sector inductively coupled plasma mass spectrometer instrument (MCMS-ICPMS). This instrument is well suited for in situ analysis of sulfide minerals with a 50-100 µm beam diameter. The instrument produces analyses quickly (several minutes per spot), has an excellent and easy to use interface, but has not yet produced high precisions (~0.5-1 per mil). We will continue to develop this technique for a number of sulfide and sulfate minerals. In particular, we will focus work on disseminated sulfide grains in feeder zones of massive sulfide deposits.
We will continue to refine laser microprobe fluorination techniques for small amounts of whole rock powders, clay minerals, zircons, amorphous precipitates, and additional minerals. In particular, we believe this technique is extremely useful for refractory minerals like magnetite, chrome spinels, olivine, clinopyroxene, garnet, kyanite etc that are difficult to analyze by coventional fluorination in externally heated Ni reaction vessels.
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