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| Titel |
A new barometer from stress fields around inclusions |
| VerfasserIn |
Diana Avadanii, Lars Hansen, David Wallis, David Waters |
| Konferenz |
EGU General Assembly 2017
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 19 (2017) |
| Datensatznummer |
250138145
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| Publikation (Nr.) |
 EGU/EGU2017-1073.pdf |
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| Zusammenfassung |
| A key step in understanding geological and geodynamic processes is modelling the
pressure-temperature paths of metamorphic rocks. Traditional thermobarometry relies on
mineral assemblage equilibria and thermodynamic modelling to infer the pressures and
temperatures of chemical equilibration. This approach requires the presence of specific
mineral assemblages and compositions, which narrows its applicability. In this study we aim
to develop a geobarometer based on mechanical interactions between inclusions and
their host grains. Exhumation of minerals with inclusions causes heterogeneous
residual stress fields due to the different, and often anisotropic, elastic properties
of the inclusion and host. Recent studies measure residual mean stresses within
inclusions using Raman spectroscopy and use those stresses as a barometer. In
contrast, we map each component of the stress tensor around inclusions using high
angular-resolution electron backscatter diffraction (HR-EBSD). This technique provides
both higher spatial resolution and increased sensitivity to elastic strains relative to
Raman spectroscopy. We focus on quartz inclusions in garnet, a common feature
in metamorphic rocks. This assemblage also provides an opportunity to test our
results with compositional thermobarometry. We analyse samples metamorphosed at
pressures ranging from ∼ 300 MPa to ∼ 1600 MPa, as recorded by independent
geobarometers. HR-EBSD reveals symmetric and lobate signals around inclusions,
with elastic strains and residual stresses of the order 10−3 and ±102 −−103 MPa,
respectively. We solve Eshelby’s problem for the ‘inhomogeneous inclusion’ case to
simulate the elastic strain/stress field around an anisotropic ellipsoidal inclusion
surrounded by an isotropic, homogeneous, infinite matrix. This model calculates
the stress disturbances caused by differential expansion of an inclusion and host
subjected to decompression. We additionally account for differential expansion
related to cooling by imposing an eigenstrain in the inclusion, according to the
thermal expansivity of quartz. Thermal contraction in the host garnet is accounted for
by modifying the macroscopic pressure. The simulations reproduce the general
pattern of the elastic fields that we observe from HR-EBSD and account for different
geometries of the inclusion. The simulations provide the basis for quantitatively
relating the stress fields measured by HR-EBSD to the entrapment pressures of
inclusions. |
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