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Abstract We present a flow law for dislocation‐dominated creep in wet quartz derived from compiled experimental and field‐based rheological data. By integrating the field‐based data, including independently calculated strain rates, deformation temperatures, pressures, and differential stresses, we add constraints for dislocation‐dominated creep at conditions unattainable in quartz deformation experiments. A Markov Chain Monte Carlo (MCMC) statistical analysis computes internally consistent parameters for the generalized flow law: = Aσne−(Q+VP)/RT. From this initial analysis, we identify differenteffectivestress exponents for quartz deformed at confining pressures above and below ∼700 MPa. To minimize the possible effect of confining pressure, compiled data are separated into “low‐pressure” (<560 MPa) and “high‐pressure” (700–1,600 MPa) groups and reanalyzed using the MCMC approach. The “low‐pressure” data set, which is most applicable at midcrustal to lower‐crustal confining pressures, yields the following parameters: log(A) = −9.30 ± 0.66 MPa−n−r s−1;n = 3.5 ± 0.2;r = 0.49 ± 0.13;Q = 118 ± 5 kJ mol−1; andV = 2.59 ± 2.45 cm3 mol−1. The “high‐pressure” data set produces a different set of parameters: log(A) = −7.90 ± 0.34 MPa−n−r s−1;n = 2.0 ± 0.1;r = 0.49 ± 0.13;Q = 77 ± 8 kJ mol−1; andV = 2.59 ± 2.45 cm3 mol−1. Predicted quartz rheology is compared to other flow laws for dislocation creep; the calibrations presented in this study predict faster strain rates under geological conditions by more than 1 order of magnitude. The change innat high confining pressure may result from an increase in the activity of grain size sensitive creep.
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High-temperature creep of magnetite and ilmenite single crystals
Abstract We performed deformation experiments on dry natural single crystals of magnetite and ilmenite to determine the rheological behavior of these oxide minerals as a function of temperature, orientation, and oxygen fugacity. Samples were deformed at temperatures of 825–1150 $$\,^{\circ }$$ ∘ C to axial strains of up to 15–24% under approximately constant stress conditions up to 120 MPa in a dead-load-type creep rig at ambient pressure in a controlled gas atmosphere. Oxygen fugacity ranged from 10 $$^{-9.4}$$ - 9.4 to 10 $$^{-4}$$ - 4 atm. Ilmenite creep was insensitive to oxygen fugacity, while magnetite displayed a strong, non-monotonic oxygen fugacity dependence, with creep rates varying as $$f_{O_{2}}^{-0.7}$$ f O 2 - 0.7 and $$f_{O_{2}}^{0.4}$$ f O 2 0.4 at more reducing and more oxidizing conditions, respectively. Dislocation creep rates of magnetite single crystals were weakly dependent on crystallographic orientation with stress exponents that varied between 2.8 and 4.3 (mean 3.5 ± 0.4). Magnetite compressed parallel to <100>, <110>, and <111> axes exhibited apparent activation energies of 315±5, 345±30, and 290±5 kJ/mol, respectively. We estimated $${f_O}_2$$ f O 2 -independent magnetite activation energies of 715 ± 150, 725 ± 145, and 690 ± 150 kJ/mol for <100>, <110>, and <111> orientations, respectively, in the region of negative $${f_O}_2$$ f O 2 -dependence. Ilmenite single crystals were compressed parallel, normal, and inclined to the c-axis. Stress exponents of 3.4, 4.3, and 3.9 indicate dislocation creep with activation energies of 420 ± 35, 345 ± 30, and 360 ± 40 kJ/mol, respectively, for these orientations. Mechanical anisotropy in ilmenite is notably higher than in magnetite, as expected from its lower crystal symmetry. Constitutive equations were formulated for ilmenite and magnetite creep.
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- Award ID(s):
- 1642268
- PAR ID:
- 10209702
- Date Published:
- Journal Name:
- Physics and Chemistry of Minerals
- Volume:
- 47
- Issue:
- 12
- ISSN:
- 0342-1791
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1007/s00269-020-01122-6
