Validation of subgrain-size piezometry as a tool for measuring stress in
polymineralic rocks
Abstract
We deformed samples with varied proportions of olivine and orthopyroxene
in a deformation-DIA apparatus to test the applicability of
subgrain-size piezometry to polymineralic rocks. We measured the stress
within each phase in situ via X-ray diffraction during deformation at a
synchrotron beamline. Subgrain-size piezometry was subsequently applied
to the recovered samples to estimate the stress that each phase
supported during deformation. For olivine, the final in-situ stresses
are consistent with the stresses estimated via subgrain-size piezometry,
both in monomineralic and polymineralic samples, despite non-steady
state conditions. However, stress estimates from subgrain-size
piezometry do not reliably record the in-situ stress in samples with
grain sizes that are too small for extensive subgrain-boundary
formation. For orthopyroxene, subgrain boundaries are typically sparse
due to the low strains attained by orthopyroxene in
olivine-orthopyroxene mixtures. Where sufficient substructure does
exist, our data supports the use of the subgrain-size piezometer on
orthopyroxene. These results do, however, suggest that care should be
taken when applying subgrain-size piezometry to strong minerals that may
have experienced little strain. Stresses estimated by X-ray diffraction
also offer insight into stress partitioning between phases. In mixtures
deformed at mean stresses > 5 GPa, orthopyroxene supports
stresses greater than those supported by olivine. This stress
partitioning is consistent with established theory that predicts a
slightly higher stress within a ‘strong’ phase contained in a material
consisting of interconnected weak layers. Overall, these results
demonstrate that subgrain-size piezometry is a valuable tool for
quantifying the stress state of polymineralic rocks.