P-T-X effects on equilibrium carbonate-H2O-CO2-NaCl dihedral angles; constraints on carbonate permeability and the role of deformation during fluid infiltration

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doi: 10.1007/s004100050044
Authors:Holness, M. B.; Graham, C. M.
Author Affiliations:Primary:
University of Edinburgh, Department of Geology and Geophysics, Edinburgh, United Kingdom
Volume Title:Contributions to Mineralogy and Petrology
Source:Contributions to Mineralogy and Petrology, 119(2-3), p.301-313. Publisher: Springer International, Heidelberg-New York, International. ISSN: 0010-7999
Publication Date:1995
Note:In English. 55 refs.; illus., incl. 3 tables
Summary:Fluid-solid-solid dihedral angles in the NaCl-H2O-CO2-calcite-dolomite-magnesite system have been determined at P ranging from 0.5 to 7 kbar and T from 450 to 750°C. At 1 bar and 650°C, both dolomite and magnesite exhibit a dihedral angle minimum for intermediate H2O-CO2 fluids similar to that previously determined by the authors for calcite, but the depth of the minimum is smaller, being above the critical value of 60° for both dolomite and magnesite for all fluid compositions. Calcite-calcite-brine dihedral angles at 650°C have been determined in the P range 1-5 kbar. Angles decrease with increasing salt content of the fluid, tending towards a constant value of ∼ 65° for strong brines at P >> 2 kbar. There is a general increase of angle with increasing P, which is most marked for strong brines. A positive correlation of angle with P is also observed in calcite-H2O-CO2 fluids, the position of the minimum moving towards higher angles and towards H2O-rich fluids with increasing P. The permeability window previously observed by the authors at 1 kbar and intermediate fluid compositions closes at ∼ 1.5 kbar. The results demonstrate that the permeability of carbonates at grain edge fluid flow is possible only at low P and for fluids of restricted H2O-CO2-NaCl compositions. However, geochemical evidence from metamorphic terrains suggests that pervasive infiltration does occur under conditions where impermeability is predicted. From examination of published studies of infiltrated carbonates it is concluded that deformation plays a critical role in enhancing carbonate permeability. Possible mechanisms for this include shear-enhanced dilatancy (micro-cracking), fluid inclusion drag by deformation-controlled grain boundary migration, and dynamically maintained transient grain boundary fluid films. [P.Br.]
Subjects:Carbonates; Deformation; Dolomite; Fluid phase; Infiltration; Magnesite; Mechanism; Metamorphism; P-T conditions; Permeability; Phase equilibria; Volatiles
Abstract Numbers:95M/4240
Record ID:1995049560
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from Mineralogical Abstracts, United Kingdom, Twickenham, United Kingdom
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