Chlorine Enrichment of Hydrous Minerals in Archean Granulite Facies Ironstone from the Beartooth Mountains, Montana, USA: Implications for High-Grade Metamorphic Fluids
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.
Document Availability at the Time of Submission
Release the entire work immediately for access worldwide.
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.