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| Titel |
Experimental Deformation of Olivine Crystals at Mantle P and T: Evidences for a Pressure-Induced Slip Transition and Implications for Upper-Mantle Seismic Anisotropy and Low Viscosity Zone |
| VerfasserIn |
P. Raterron, J. Chen, T. Geenen, J. Girard |
| Konferenz |
EGU General Assembly 2009
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 11 (2009) |
| Datensatznummer |
250027189
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| Zusammenfassung |
| Recent developments in high-pressure deformation devices coupled with synchrotron
radiation allow investigating the rheology of mantle minerals and aggregates at the extreme
pressure (P) and temperature (T) of their natural occurrence in the Earth. This is particularly
true in the case of olivine, which rheology has been recently investigated in the
Deformation-DIA apparatus (D-DIA, see Wang et al., 2003, Rev. Scientific Instr., 74, 3002)
at upper-mantle P and T conditions.
Olivine deforms by dislocation creep in the shallow upper-mantle, as revealed by the
seismic velocity anisotropy observed in this region. The attenuation of seismic
anisotropy at depth greater than 200 km is interpreted as a pressure-induced change in
olivine main deformation mechanism. It was first attributed to a transition from
dislocation creep to diffusion creep (Karato and Wu, 1993, Science, 260, 771). This
interpretation has been challenged by deformation data obtained at high pressure
(P > 3 GPa) in the dislocation creep regime (Couvy et al., 2004, EJM, 16, 877;
Raterron et al., 2007, Am. Miner., 92, 1436; Raterron et al., 2009, PEPI, 72, 74), which
support a second interpretation: a transition in olivine dominant dislocation slip, from
[100] slip at low P to [001] slip at high P (e.g., Mainprice et al., 2005, Nature,
433, 731). Such a P -induced [100]/[001] slip transition is also supported by recent
theoretical studies based on first-principle calculations of olivine dislocation slips
(Durinck et al., 2005, PCM, 32, 646; Durinck et al., 2007, Eur. J. Mineral., 19,
631).
In order to further constrain the effect of pressure on olivine slip system activities,
deformation experiments were carried out in poor water condition at P > 5 GPa and
T =1400Ë C, on pure forsterite (Fo100) and San Carlos olivine crystals, using the D-DIA at
the X17B2 beamline of the NSLS (Upton, NY, USA). Crystals were oriented in
order to active either [100] slip alone or [001] slip alone in (010) plane, or both
[100](001) and [001](100) systems together. Constant applied stress Ï < 300 MPa and
specimen strain rates were monitored in situ using time-resolved X-ray diffraction and
radiography, respectively. Run products were investigated by transmission electron
microscopy (TEM) in order to verify the actual activation of the tested dislocation
slip systems. The obtained data were compared with rheological data previously
obtained at comparable T and Ï conditions, but at room P (Darot and Gueguen,
1981, JGR, 86, 6219; Bai et al., 1991, JGR, 96, 2441), resulting in creep power
laws which quantify the effect of P on olivine rheology. The new data confirm the
occurrence of a P -induced [100]/[001] slip transition, and suggest that [001](010) system
dominates olivine deformation in the deep upper mantle. Extrapolation of the obtained
rheological laws to natural Ï condition along upper-mantle geotherms, shows that
the [100] / [001] slip transition should occur in the Earth at ~ 200 km depth, thus
can explain the attenuation of seismic anisotropy in the deep upper mantle. The
obtained rheological laws were also integrated into a straightforward olivine aggregate
model, then extrapolated to mantle condition using a 2-D geodynamic modeling
application (Van den Berg et al., 1993, Geophys. J. International, 115, 62), which is
the simplest approach to investigate upper-mantle steady-state deformation. In the
application, the velocity of the lower boundary (the transition-zone boundary at 410-km
depth) was set to 0, while that at the Earth’s surface was set to 2 cm/year. Results
from this modeling suggest that the combine activity of [100] and [001] slips in
olivine aggregates may significantly decrease mantle viscosity below the oceanic
lithosphere, thus, may contribute to the low viscosity zone (LVZ) required in plate
tectonics to decouple rigid plates from the more ductile asthenophere underneath. |
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