Equilibrium dihedral angles in the system H2O-CO2-NaCl-calcite, and implications for fluid flow during metamorphism

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Authors:Holness, Marian B.; Graham, Colin M.
Author Affiliations:Primary:
University of Cambridge, Department Earth Sciences, Cambridge, United Kingdom
Other:
University of Edinburgh, United Kingdom
Volume Title:Contributions to Mineralogy and Petrology
Source:Contributions to Mineralogy and Petrology, 108(3), p.368-383. Publisher: Springer International, Heidelberg-New York, International. ISSN: 0010-7999
Publication Date:1991
Note:In English. 61 refs.; illus.
Summary:Fluid-calcite-calcite dihedral angles have been measured for fluids in this system between 1 and 2 kbar, and 550-750 degrees C. It is found that the calcite-calcite-H2O dihedral angle decreases steadily with addition of NaCl from 80 degrees (pure water) to 44 degrees (60 wt.% NaCl). The CO2-H2O system displays a well-defined minimum at XCO2 = 0.5, with a dihedral angle of 50 degrees . Experiments with fluids which are immiscible at run conditions showed a bimodal distribution of dihedral angles in the CO2-H2O-NaCl system, which can be approximately correlated with the compositions of the two fluid phases. Such bimodality was only observed for immiscible fluids in the H2O-NaCl system if the quench rate exceeded approx 200 degrees C/min. The fluid phase topology in devolatilizing marbles will only be a connected network for very saline brines and fluids with XCO2 close to 0.5. Fluids trapped in fluid inclusions in calcite grains in marbles may be mainly H2O- or CO2- rich, and of low salinity. All other fluid compositions in the H2O-CO2-NaCl-calcite system will occupy isolated pores, the largest of which will grow at the expense of the smallest. Escape of fluid produced in devolatilization reactions in such conditions will occur by fluid overpressuring and hydrofracture. In contrast previous studies predict that quartz- dominated lithologies will permit pervasive flow of H2O-NaCl fluids, but not of H2O-CO2 fluids. Examples supporting these results are discussed. [R.E.S.]
Subjects:Calcite; Carbonates; Crystal chemistry; Fluid inclusions; Immiscibility; Inclusions; Isotope ratios; Isotopes; Metamorphism; O-18/O-16; Oxygen; P-T conditions; Phase equilibria; Stable isotopes; H<2`O-CO<2`-NaCl-calcite
Abstract Numbers:92M/1558
Record ID:1993010556
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from Mineralogical Abstracts, United Kingdom, Twickenham, United Kingdom
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