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| Titel |
Experimental Deformation of Antigorite at Mantle Wedge Conditions |
| VerfasserIn |
Linda Chernak, Greg Hirth |
| 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 |
250045094
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| Zusammenfassung |
| The rheology of antigorite can control intraslab seismicity, the strength of the oceanic
lithosphere and mantle wedge, and the slip stability of creeping continental faults. We have
conducted high pressure and temperature axial compression experiments on antigorite
serpentinite within the antigorite stability field and where dehydration to forsterite and talc is
expected to investigate how dehydration affects rheology. Samples were deformed in a
Griggs-type apparatus at P = 0.5 - 1.5 GPa, T = 300 - 700-C and at strain rates of 10-5 s-1
and 10-6 s-1. As detailed in Chernak and Hirth (in review), samples experienced localized
deformation at temperatures from 300 to 625-C when deformed to high enough
strain. Surprisingly, sample ductility decreases with increasing temperature. Samples
deformed at 300-C strain harden until ~20% strain when faulting initiates; there is
no evidence for localization at lower strain. At 400-C, strain hardening ceases at
~10-15% strain and samples are characterized by a broad zone of deformation
surrounding one or more fractures. Samples deformed at 550-C and 625-C strain weaken
abruptly after ~5-10% strain and contain one distinct fault. Strain rate stepping
experiments indicate that antigorite has velocity strengthening behavior from 300 -
700-C.
Our results have important implications for interpreting laboratory stress measurements
obtained in situ using the deformation-DIA (D-DIA) apparatus, where strength
is estimated by averaging stress measured by X-ray diffraction on three different
lattice planes. At low strain, our results are in good agreement with the results of
Hilairet et al. (2007), who conducted D-DIA experiments on antigorite at the same
conditions as in our study. However, at higher strain, our samples experienced localized
deformation whereas Hilairet et al. (2007) concluded that antigorite deformed by
ductile processes. Despite differences in deformation behavior, we find that our
stress measurements are in close agreement with stresses measured on the ‘hardest’
lattice plane, suggesting that direct comparisons between the apparatuses may be
possible.
Samples deformed above the thermal stability of antigorite at high pressure (1.5 GPa) are
extremely weak and show evidence for dehydration along grain boundaries. However,
microstructural observations indicate the weakness is not associated with localization;
samples are homogeneously deformed to ~25% strain. Thus, in contrast to conventional
wisdom, our results suggest that antigorite dehydration at high pressure causes a
transition from localized to distributed deformation and that antigorite dehydration
may not be directly responsible for intermediate depth seismicity. As hypothesized
by Rutter et al. (2008) it is possible that earthquakes are generated when water
that is released during dehydration is expelled into surrounding – less ductile –
rock. |
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