GSA 2020 Connects Online

Paper No. 67-1
Presentation Time: 1:30 PM

THERMOCHEMICAL DATA FOR POTASSIC-CHLORO-HASTINGSITE AND DETERMINATION OF AQUEOUS CHLORIDE CONTENTS


WALTON, Monica A.1, MATTEUCCI, Jared P.1 and JENKINS, David M.2, (1)Dept. Geological Sci. Env. Studies, Binghamton University, 4400 Vestal Pkwy E, Binghamton, NY 13902, (2)Geological Sciences and Environmental Studies, Binghamton University, Binghamton, NY 13902-6000

The interaction between Cl-rich fluids and amphiboles in rocks is an area of study useful for understanding geological processes such as the formation of economic deposits, seawater-crust interaction and metasomatism in rocks. This is of interest because Cl aids in the transport of ore-forming minerals, and this data is useful for tracking these hydrothermal fluids in metamorphic rocks. Currently there is very little thermochemical data on Cl-rich amphiboles. Values for ΔHf and S° for potassic-chloro-hastingsite (KCa2(Fe4Fe3+(Al2Si6)O22Cl2)=KCH) have been derived here from the upper-thermal stability curve of a K-Cl-rich amphibole.

Synthetic amphiboles close to KCH (K0.8Ca1.9(Fe3.8Fe3+0.8Al0.4)(Al1.6Si6.4)O22OH0.4Cl1.2) were made at pressures of 3-3.5 kb and 500-800°C in gas vessels pressurized with mixed H2-Ar to achieve an fO2 near the NiNiO oxygen buffer. Reversal reactions were done using equal mixtures of the amphibole and its breakdown products in Ag50Pd50 capsules, locating the boundary at 2kb/735°, 3kb/773°, 4kb/840°, and 7kb/950°C. The end-member reaction is 4KCH = 4 An + 1 Qtz + 4 Hed + 7 Fay + 2 FeCl2+ 4 Syl + 1 O2. Phases were analyzed by electron microprobe. Ideal activity of KCH was found to be 0.169 in amphibole. A volume for KCH of 295.57 cm3/mol was obtained from Rietveld refinements of powder diffraction data for a series of related synthetic amphiboles. CP (in kJ/mol) of KCH was estimated as 1.15 + 1.04 x 10 -4(T,K) – 10643.5 / T2 – 6.40/T1/2. Combining these data with the activities (AX62, Holland, 2019) and data for the other phases in the Holland & Powell (2011, JMG) database, we are able to calculate ΔH° = -10640.6 ± 7 kJ/mol and S° = 764.6 ± 6 J/K·mol for KCH.

These thermochemical data were used to determine the aggregate Cl activities of the metamorphic fluids that formed the high-grade iron formations of the Beartooth Mtns, Montana (Henry & Daigle, 2018, AmMin). Using the reported mineral analyses, we were able to determine activities of KCH in amph (0.0062-0.021), fs (0.3-0.4), alm (0.2-0.3), and hed (0.4-0.5). Combining these with the rxn Fs + KCH = Qtz + Alm + 2 Hed + 0.5 Mt + 0.5 FeCl2 (aq) + KCl (aq) at 7 kb and 800°C, we can determine the aggregate molality of Cl as 1-9 m, indicating relatively concentrated brines were involved. A clear correlation between the aideal KCH and the mCl can also be seen in this locality.