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Titel |
Seismicity triggered by the olivine-spinel transition: New insights from combined XRD and acoustic emission monitoring during deformation experiments in Mg2GeO4 |
VerfasserIn |
Alexandre Schubnel, Nadège Hilairet, Julien Gasc, Eva Heripre, Fabrice Brunet, Yanbin Wang |
Konferenz |
EGU General Assembly 2011
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Medientyp |
Artikel
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Sprache |
Englisch
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Digitales Dokument |
PDF |
Erschienen |
In: GRA - Volume 13 (2011) |
Datensatznummer |
250054616
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Zusammenfassung |
Polycrystalline Mg2GeO4-olivine has been deformed (strain rates from 2.10-4-s to 10-5-s)
in the deformation-DIA in 13-BM-D at GSECARS (Advanced Photon Source) at ca.
2 GPa confining pressure for temperatures between 973 and 1573 K (i.e., in the
Mg2GeO4-ringwoodite field). Stress, advancement of transformation, and strain were
measured in-situ using X-ray diffraction (XRD) and imaging, and acoustic emissions (AE)
full waveform were recorded simultaneously. When differential stress is applied (ca. 1- to 2
GPa GPa) and temperature is increased, the very beginning of the transformation to the
ringwoodite structure (as evidenced by in situ XRD) is accompanied by AE bursts which
locate within the sample. At high strain rates (> 10-4-s) and low temperatures (800-900
degrees C), the number of AEs is comparable, if not larger, to that observed during the cold
compression of quartz grains. The largest events always occur at a temperature slightly
below that of appearance of the ringwoodite-structure phase on the XRD images
patterns. This suggests that AEs are generated while the transition is still nucleation
controlled (pseudo-martensitic stage). During stress-relaxation periods, the rate
of AE triggering decreases, but does not completely vanish. The AE production
rate increases again as soon as deformation is started again. Importantly, we still
observed very large AEs at strain rates as low as approx. 10-5-s. At these early
stages of the transformation, the samples did not show any macroscopic rheological
weakening.
Focal mechanism analysis of the largest AEs showed that they are all of shear type, some
being even pure double couple. They radiate about the same amount of energy as
typically recorded during fast crack propagation in amorphous glass material. This
suggests that they cannot only originate from the martensintic nucleation of oriented
spinel-lamellae within a single germanium olivine crystal. Microstructural analysis (SEM,
EBSD and TEM) highlights the presence of thin transformation bands made of
incoherent spinel micro-grains which, possibly, run across germanium-olivine grain
boundaries. These bands are all oriented near perpendicular to the principal compressive
stress.
Our observations point out that under high deviatoric stress, the olivine – spinel transition
is a source of instability which produces micro-seismicity (no AEs are were recorded when in
a similar experiment are performed hydrostatically). These instabilities might eventually be
precursor to brittle fracturing as observed by Burnley et al. (1990) in their deformation
experiments on very similar samples. Both types of study emphasize the potential of phase
transitions (with negative volume variations?) in radiating acoustic energy and triggering
brittle failure. Obviously, this has important consequences for the understanding of
deep-focus earthquakes occurring in cold and metastable olivine within the transition zone. |
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