Deformation processes and rheology of pyroxenites under lithospheric mantle conditions |
| |
| Institution: | 1. Univ. Grenoble Alpes, CNRS, ISTerre, F-38000 Grenoble, France;1. UFZ Helmholtz Centre for Environmental Research, Halle, Germany;2. Helmholtz Centre for Polar and Marine Research, Alfred Wegener Institute, Bremerhaven, Germany;3. GFZ German Research Centre for Geosciences, Helmholtz Centre Potsdam, Germany;4. Geological Survey of Slovenia, Ljubljana, Slovenia |
| |
| Abstract: | We combined microstructural observations and high-resolution crystallographic preferred orientation (CPO) mapping to unravel the active deformation mechanisms in garnet clinopyroxenites, garnet–spinel websterites, and spinel websterites from the Beni Bousera peridotite massif. All pyroxenites display microstructures recording plastic deformation by dislocation creep. Pyroxene CPOs are consistent with dominant slip on 001]{110} in clinopyroxene and on 001](100) or 001](010) in orthopyroxene. Garnet clinopyroxenites have however high recrystallized fractions and finer grain sizes than spinel websterites. Recrystallization mechanisms also differ: subgrain rotation dominates in garnet clinopyroxenites, whereas in spinel websterites nucleation and growth also contribute. Elongated shapes and strong intracrystalline misorientations suggest plastic deformation of garnet, but CPOs are weak. Clinopyroxene porphyroclasts in spinel websterites show deformation twins underlined by orthopyroxene exsolutions. Thermodynamic calculations indicate that garnet clinopyroxenites deformed at 2.0 GPa and 950–1000 °C and spinel pyroxenites at 1.8 GPa and 1100–1150 °C. The lower temperatures may explain the faster work rates implied by the finer grained microstructures in garnet clinopyroxenites. Greater stresses may have also reduced the competence contrast between garnet and pyroxene in the garnet pyroxenites and, at the outcrop scale, lowered the competence contrast between pyroxenites and peridotites, favoring mechanical dispersion of pyroxenites in the cooler lithospheric mantle. |
| |
| Keywords: | |
| 本文献已被 ScienceDirect 等数据库收录！ |
|
