Constraining the pressure threshold of impact induced calcite twinning: Implications for the deformation history of aqueously altered carbonaceous chondrite parent bodies

Calcite twinning in carbonaceous chondrite meteorites can be used to reconstruct the deformation history and the parent body environment during and/or after aqueous alteration, but the shock pressure threshold at which the twins develop is unknown. Accordingly, the aim of this study is to determine the magnitude of shock pressure that is needed to generate calcite twins. This was done by measuring the depths of twinning beneath the resulting craters from experimental impacts in six calcite targets, combined with hydrocode modelling of the peak pressures at the corresponding depths within the targets. Brecciation, fracturing and calcite e-twinning occur below the floors of all the craters and results from the hydrocode modelling show that the twins start to form at shock pressures of ∼110 to 480 MPa, which is at least a factor of ten higher than the 10 MPa that is considered to be required to produce calcite twins in low strain rate terrestrial settings. These pressures are equivalent to shock stage S1 as determined by olivine microstructures and consistent with calcite twinning in carbonaceous chondrites being a result of impact gardening in shallow levels of their asteroidal parent bodies.