2015 GSA Annual Meeting in Baltimore, Maryland, USA (1-4 November 2015)

Paper No. 168-13
Presentation Time: 4:45 PM


HENRY, Darrell J., Dept. of Geology and Geophysics, Louisiana State University, Baton Rouge, LA 70803, DAIGLE, Nicholas M., Dept. of Geology & Geophysics, Louisiana State University, Baton Rouge, LA 70803, MUELLER, Paul A., Department of Geological Sciences, University of Florida, 241 Williamson Hall, Gainesville, FL 32611 and MOGK, David W., Dept. of Earth Sciences, Montana State University, PO Box 173480, Bozeman, MT 59717, glhenr@lsu.edu

The amount of chlorine (Cl) present in provisionally “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 capability for these minerals to serve as monitors of Cl in coexisting aqueous fluids is particularly effective in Fe-rich amphibole and biotite such as those found in high-grade iron formations.

Archean (2.8-2.9 Ga) iron formations from the eastern Beartooth Mountains, Montana are typified by dominantly anhydrous mineral assemblages of quartz + magnetite + orthopyroxene + garnet ± clinopyroxene ± plagioclase that have equilibrated during granulite facies conditions of ~775–800°C and 6–6.5 kbar. These relatively anhydrous iron formations contain minor amounts of prograde metamorphic Cl-rich amphibole and biotite that occur as inclusions in orthopyroxene and garnet as well as matrix minerals. The high-grade Fe-rich amphiboles (mostly Cl-rich potassic-hastingsite and magnesio-ferri-hornblende) and biotites contain concentrations of Cl reaching up to 2.9 wt% and 3.4 wt%, respectively. The biotites also contain up to 10.5 wt% BaO and 6.9 wt% TiO2. The substitution of Cl into amphibole and biotite is more probable if the anion sites are enlarged – generally due to a decrease in tetrahedral rotation and/or an increase in the overall unit cell dimensions. These features are reflected by the strong positive correlation between Cl and XFe, IVAl and K (in amphibole). The influence of progressively more saline aqueous fluid compositions with progressive metamorphism is reflected by matrix amphiboles and biotites being more chlorine-rich than the inclusions in orthopyroxene and garnet. Based on calculations using the anion contents in biotite, the coexisting aqueous fluids were highly saline with XH2O < 0.3, fHCl/fH2O = 0.4 to 1.1 and log(fHF/fHCl) = -2.7 to -3.9. In addition, the fluid was relatively oxidizing (~NNO) and produced Fe3+-rich amphiboles. These data provide evidence for fluid compositions of high-grade brines and high chlorine levels that increase the stability field of “hydrous” minerals into granulite facies conditions.