Field Testing Raman Microspectroscopic Therobarometers in Garnet

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This project will use a novel analytical technique - confocal Raman microspectroscopy - to investigate the physical character of natural mineral inclusions inside their host minerals. This information is important for understanding the physical conditions under which rocks form, and the degree to which rocks retain that information during cooling. This work will also identify whether measurements are biased by inclusion geometry or high temperatures of rock formation. If successful, Raman microspectroscopy could revolutionize investigations of the pressures and temperatures of rock formation, which are important to a variety of fields including igneous, metamorphic, and ore-forming processes. We may also identify what holds rocks together, which would have broad applications to materials synthesis science. Funding will support the research and training of a new PhD student. This research will be used to leverage educational opportunities in undergraduate and graduate courses at Boise State University, which reaches a different student demographic than other American universities. The central focus of this project is to critically test the Quartz-in-Garnet (QuiG) barometer and Zircon-in-Garnet (ZiG) thermometer. Three main questions include: (1) Is there a threshold temperature above which thermoba-Raman-try fails in garnet? (2) Are inclusion underpressures reliably preserved? and (3) Does inclusion shape matter? Five basic sets of data will address these questions: (1) Raman spectra will be collected over a range of pressure-temperature (P-T) conditions to see whether garnet reliably encodes entrapment P-T conditions over all P-T space. Application of ZiG is particularly important in identifying whether entrapment T's are preserved. (2) Chemical zoning will be investigated around inclusions to see whether pressures induced by expanding or contracting inclusions induce chemical modification to garnet molar volumes. (3) Raman spectra will be collected for low-P, high-T rocks, where QuiG underpressures are expected, and compared against expected Pincl. (4) Pincl (Raman peak shifts) in quartz and zircon inclusions with a range of aspect ratios will be spatially mapped at a µm scale. (5) Infrared spectra will be collected on sub-mm spots in garnet to identify OH contents and possible impact of hydrolytic weakening on garnet strength. These data will test assumptions that underpin applications of thermoba-Raman-try and empirically identify the region of P-T space over which the method can be applied to inclusions in garnet. Data will also permit expanded research into the P-T conditions of igneous and ore deposit formation, independent of chemical or thermodynamic models.