Effect of Cation Impurities on Steady-State Flow of Salt

  1. B.E. Hobbs and
  2. H.C. Heard
  1. H. C. Heard and
  2. F. J. Ryerson

Published Online: 18 MAR 2013

DOI: 10.1029/GM036p0099

Mineral and Rock Deformation: Laboratory Studies: The Paterson Volume

Mineral and Rock Deformation: Laboratory Studies: The Paterson Volume

How to Cite

Heard, H. C. and Ryerson, F. J. (2013) Effect of Cation Impurities on Steady-State Flow of Salt, in Mineral and Rock Deformation: Laboratory Studies: The Paterson Volume (eds B.E. Hobbs and H.C. Heard), American Geophysical Union, Washington, D. C.. doi: 10.1029/GM036p0099

Author Information

  1. Lawrence Livermore National Laboratory, University of California, Livermore, California 94550

Publication History

  1. Published Online: 18 MAR 2013
  2. Published Print: 1 JAN 1986

ISBN Information

Print ISBN: 9780875900629

Online ISBN: 9781118664353

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Keywords:

  • Rocks—Testing—Addresses, essays, lectures;
  • Rock deformation—Addresses, essays, lectures

Summary

Accurate prediction of the rbeological bebavior of balite from salt bodies in evaporite sequences at deptb requires definition of its constitutive bebavior witbin narrow limits togetber witb a validated numerical model. Direct applications of sucb predictions include elucidating tbe tectonic bistory of sedimentary basins containing evaporite sequences, engineering high level radioactive waste repositories in salt and storing of fluids in salt cavities at deptb. Published laboratory data indicate tbat most natural formations, as well as one artificial aggregate, bave closely similar mecbanical properties. However, at least two natural balites are stronger tban tbe average by ∼50%. Such variability in generic properties becomes amplified witb temperature and time, tbus making tbe estimation of site-specific bebavior using “average” parameters only of very limited usefulness.

Starting with a working bypotbesis that the high strength of tbe two salts (Paradox Basin Cycles 6,7) was caused by cation impurities in NaCl (solid solution hardening), we analyzed eight natural and artificial aggregates using the electron microprobe. Those salts, which appeared closely similar and of low strength, had K+, Mg++, and Ca++ impurities <∼0.01%. The two anomalously strong materials contained these impurities in the 0.1 to 0.5% range. Probe traverses across several samples indicated minor concentration differences in K+ and Mg+ + between grain interiors and boundaries. In contrast, Ca++ appeared concentrated in grain boundaries.

Synthetic aggregates of NaCl doped with these cations were prepared by cold pressing five different compositions containing 0.1 to 0.6 mol% of KCl, MgCl2 and CaCl2. Samples were annealed at 500°C, 200 MPa pressure for 14 hours, which resulted in starting materials having <0.5% porosity and 2 to 5 mm grain size. The rheological behavior of these materials was determined in stepped strain rate testing at 200 MPa pressure at temperatures (T) of 200 to 500°C, strain rates $$({\rm \dot \varepsilon })$$ of 10−3 to 10−7S−1, and stresses (σ) of 1 to 55 MPa. All secondary creep results were well fit by a power law flow equation of the form $${\rm \dot \varepsilon }\, = \,{\rm A}$$ σnexp(−Q/RT) where Q, A, R, and n are constants.

Comparison of the behavior of these doped materials and pure NaCl indicates the power n is slightly larger for NaCl + 0.1% KCl and NaCl + 0.3% KCl, unchanged for NaCl + 0.3% CaCl2, and slightly smaller for NaCl + 0.2% MgCl2. Similarly, comparison of the activation energies Q between that for pure NaCl and these mixtures shows that Q is larger by 30 to 80% for NaCl + KCl and 15 to 60% for NaCl + MgCl2, but 25% smaller for NaCl + CaCl2. There is good agreement between the doped NaCl bebavior and that of the Paradox cycle 6 and 7 salts at $${{\rm \dot \varepsilon }}$$, T where these may be intercompared.

Use of these data in predicting closure rates of workings in a waste repository constructed at 880 m depth indicates that impure salts containing K+, Mg+, or Ca++ in the above concentrations can be as much as 104 lower than pure NaCl at 30°C. Similarly, at 200°C rates may be as much as 104 lower when compared with pure NaCl. Equivalent viscosity contrasts between the doped and pure materials are also 109 to 104 in the 30 to 200°C temperature range.