  We study the role of grain size in the rheological behaviour of calcite
aggregates in the semi-brittle regime. We conduct triaxial deformation tests on
three rocks, Solnhofen limestone, Carrara marble and Wombeyan marble, with
average grain sizes of 5-10 $\mu$m, 200 $\mu$m and 2 mm, respectively, at
pressures in the range 200-800 MPa and temperatures in the range 20-400
$^\circ$C. At all conditions, both strength and hardening rate increase with
decreasing grain size. Flow stress scales with the inverse of grain size to a
power between 1/3 and 2/3. Hardening rate decreases linearly with the logarithm
of grain size. In-situ ultrasonic monitoring reveals that P-wave speed tends to
decrease with increasing strain, and that this decrease is more marked at room
temperature than at 200 and 400 $^\circ$C. The decrease in wave speed is
consistent with microcracking, which is more prevalent at low temperature and
low pressure. Microstructural observations reveal high twin densities in all
deformed samples. Twin density increases with stress, consistent with previous
datasets. Spatial distributions of intragranular misorientation indicate that
twins are sometimes obstacles to dislocation motion, but this effect is not
ubiquitous. Computed slip-transfer statistics indicate that that twins are
typically weaker barriers to dislocation glide than grain boundaries, so that
their effect on dislocation accumulation and hardening rates is likely smaller
than the effect of grain size. Indeed, our data reveal that grain size exerts a
first-order control on flow stress and hardening in calcite, whereas twinning
may only have a secondary impact on these behaviours.

Geophysics