Master of Science (MS)
Geology and Geophysics
The amount of chlorine present in hydrous minerals influences mineral stability and may serve as a monitor of the evolving fluid phase during progressive metamorphism. Chlorine contents of amphibole and biotite vary as a function of temperature, pressure, crystallochemical factors, and fluid composition. The sensitivity of these minerals to serve as a monitor of Cl in aqueous fluids is particularly effective in Fe-rich amphibole and biotite such as those found in ironstones. Ironstones from the eastern Beartooth Mountains, Montana are typified by dominantly anhydrous mineral assemblages of quartz + magnetite + orthopyroxene + garnet ± clinopyroxene and have equilibrated during granulite facies conditions of ~775–800 °C at pressures of 6–6.5 kbar. These relatively anhydrous ironstones contain minor amounts of prograde metamorphic Cl-rich amphibole and biotite that occur as inclusions in orthopyroxene and garnet and as matrix minerals. The amphiboles (mostly potassic-hastingsite) and biotites contain high levels of Cl (reaching up to 2.9 wt% and 3.4 wt%, respectively) and generally show a positive correlation between Cl and XFe2+, Al[IV], K[XIII] and Ba[XIII]. The biotites also contain up to 10.5 wt% BaO and 6.9 wt% TiO2. Matrix amphiboles and biotites are more chlorine-rich than the inclusions in orthopyroxene and garnet. Calculated halogen fugacity ratios of log(XF/XCl) vs. XMg in biotite ranged from about -2.7 to -3.9, demonstrating that the metamorphic fluid coexisting with biotite was enriched in Cl. The variations in chlorine from inclusions to matrix grains suggest that the fluid evolved to be more chlorine-rich with increasing metamorphic grade. These data provide new evidence for fluid compositions of high-grade brines and high chlorine levels increasing the stability field of hydrous minerals into granulite facies conditions.
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Daigle, Nicholas Michael, "Chlorine Enrichment of Hydrous Minerals in Archean Granulite Facies Ironstone from the Beartooth Mountains, Montana, USA: Implications for High-Grade Metamorphic Fluids" (2015). LSU Master's Theses. 1188.