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Titel |
Experimental Deformation of Dehydrating Antigorite: Challenging Models of Dehydration Embrittlement |
VerfasserIn |
Greg Hirth, Linda Chernak |
Konferenz |
EGU General Assembly 2010
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Medientyp |
Artikel
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Sprache |
Englisch
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Digitales Dokument |
PDF |
Erschienen |
In: GRA - Volume 12 (2010) |
Datensatznummer |
250045095
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Zusammenfassung |
To test the hypothesis that intermediate depth earthquakes in subduction zones are caused
by the dehydration of hydrous phases, we conducted temperature-ramping experiments on
antigorite serpentinite. Cold-pressed powdered samples of antigorite were deformed to a high
differential stress at 400-C and 1.0 GPa, within the antigorite stability field, where we have
shown that deformation localizes. Temperature was then increased at different rates,
1800-C/hr and 180-C/hr, to cross the reaction boundary while the sample continued to
deform; samples were deformed at strain rates of 10-4 s-1, 10-5 s-1 and 10-6 s-1. Two
additional experiments were conducted in a similar manner at 300-C, 1.5 GPa and
10-5 s-1 but samples remained “statically” at high stress during the temperature
increase.
Our results show that although the decrease in stress during temperature ramping is
large, stress relaxes stably, even after dehydration. We find that the slopes of the
unloading curves are approximately the same for constant values of the ratio (strain
rate/ramp rate) and that the unloading slope is greater for higher values of this ratio. In
addition, we find that the unloading curves with the greatest slopes are similar to the
apparatus compliance, suggesting that we are generating “slow earthquakes” in our
experiments over the course 5 to 10s of minutes. A strain rate stepping experiment
indicates that antigorite has velocity strengthening behavior at 700-C and 1.5 GPa
suggesting that as soon as an instability develops in the antigorite, the material
strengthens sufficiently to not go unstable. Our results thus suggest that antigorite
dehydration does not result in “dehydration embrittlement” but that it may promote slow
earthquakes.
We have also conducted a preliminary experiment to study the role of effective pressure on
deformation behavior after dehydration. A cold-pressed powdered sample of antigorite with a
small core of coarse-grained olivine at one end was deformed at 700-C, 1.5 GPa and a strain
rate of 10-5 s-1. This sample had a strength of 300 MPa, which is significantly higher than
samples deformed at the same conditions without olivine present; strengths were
approximately 100 MPa for these samples. We hypothesize that the highly porous and
permeable olivine layer provided a reservoir for the water released by the dehydration
reaction and suggests that the presence of water causes the strength of antigorite to
decrease. |
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