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Partitioning of chlorine between NaCl brines and ferro-pargasite; implications for the formation of chlorine-rich amphiboles in mafic rocks

Albert Chan, David M. Jenkins and M. Darby Dyar
Partitioning of chlorine between NaCl brines and ferro-pargasite; implications for the formation of chlorine-rich amphiboles in mafic rocks (in A tribute to Frank Christopher Hawthorne, Lee A. Groat (prefacer))
The Canadian Mineralogist (January 2016) 54 (1): 337-351

Abstract

Determining the partitioning of chloride between fluid and rock has relevance to a wide range of geological processes, including high-grade metamorphism, economic deposit formation, and seawater-oceanic crust interactions. Calcium amphiboles, particularly those that are iron-rich, are a common host for chlorine and have the potential to reveal information about the ambient fluid composition, if the crystal-chemical controls on chlorine incorporation are known. In this study we investigated the incorporation of chlorine into synthetic ferro-pargasite [NaCa (sub 2) (Fe (sub 4) Al)(Al (sub 2) Si (sub 6) )O (sub 22) (Cl,OH) (sub 2) ] formed mostly at 700 degrees C, 0.2 GPa, and oxygen fugacities (f (sub O2) ) between Co-CoO and wustite-magnetite (-20.2 < log(f (sub O2) ) < -17.5) for durations of 3-28 days. Starting mixtures consisted of reagent oxides and metallic iron to which chlorine was added as either a stock solution of NaCl brine up to 5 molal (m) (equivalent to mole fraction of NaCl, X (sub NaCl) , up to 0.082), or by mixing NaCl + or - water directly with the starting mixture to achieve higher concentration brines by homogenization at elevated pressures and temperatures. Good yields of amphibole were obtained, with the most common additional phases being plagioclase, hedenbergite, and halite. At low salinities the Cl content of the amphibole, determined by electron microprobe analysis (EMPA) of individual grains, increases linearly to about 0.045 anions per formula unit (apfu) as the brine concentration increases to about 1.7 m NaCl (X (sub NaCl) = 0.03). Thermodynamic modeling of the partitioning of Cl (super -) between brine and the amphibole yields a constant value for the equilibrium constant (K (sub a) ) of 0.78 + or - 0.09 at 700 degrees C and 0.2 GPa over the range 0.1-2.0 m NaCl with the empirically derived relationship for the activity of Cl (super -) in amphibole (a (sub Cl-) (super amph) ) being a (sub Cl) (super amph) = (X (sub Cl) (super O(3)) ). This activity-composition relationship suggests much less random mixing of Cl (super -) , OH (super -) , and O (super 2-) at the O(3) site. At salinities above 1.7 m NaCl, the Cl content of the amphibole is fairly insensitive to the brine concentration. Only at halite saturation (83 m NaCl, or X (sub NaCl) = 0.60, at 700 degrees C and 0.2 GPa) is there a noticeable increase in the Cl content of the amphibole, with a bimodal distribution centered at 0.2 and 0.7 Cl apfu observed in the highest salinity synthesis. The implication of this study is that (1) amphiboles with Cl contents above about 0.1 apfu ( approximately 0.4 wt.% Cl) require ambient brines at halite saturation, and (2) oceanic crust reacting with homogeneous seawater could incorporate 50-200 ppm Cl via the presence of 8-31 wt.% ferro-pargasite in the oceanic crust, respectively.


ISSN: 0008-4476
EISSN: 1499-1276
Coden: CAMIA6
Serial Title: The Canadian Mineralogist
Serial Volume: 54
Serial Issue: 1
Title: Partitioning of chlorine between NaCl brines and ferro-pargasite; implications for the formation of chlorine-rich amphiboles in mafic rocks
Title: A tribute to Frank Christopher Hawthorne
Author(s): Chan, AlbertJenkins, David M.Dyar, M. Darby
Author(s): Groat, Lee A.prefacer
Affiliation: Binghamton University, Department of Geological Sciences and Environmental Studies, Binghamton, NY, United States
Affiliation: University of British Columbia, Department of Earth, Ocean and Atmospheric Sciences, Vancouver, BC, Canada
Pages: 337-351
Published: 201601
Text Language: English
Publisher: Mineralogical Association of Canada, Ottawa, ON, Canada
References: 66
Accession Number: 2017-006103
Categories: General geochemistryMineralogy of silicates
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 2 tables
Secondary Affiliation: California Institute of Technology, USA, United StatesBinghamton University, USA, United StatesMount Holyoke College, USA, United States
Country of Publication: Canada
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Abstract, copyright, Mineralogical Association of Canada. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 201705
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