Carbonate modification after extreme stress loading

Carbonates, which are highly-relevant for lithosphere-biosphere-atmosphere interactions, are prone to modification - especially after being affected by high stresses and strains. Modification of carbonates after extreme stress loading during hypervelocity impact from Ries impact breccias is presented in this study. In calcite aggregates, new grains formed on the expense of partly preserved shocked host grains. The shocked host grains contain e-, f- and r- twins as well as a-type lamellae and have a high Grain Orientation Spread (GOS) of up to 8° (GOS is the mean misorientation angle between all pixels of one grain and the mean orientation of the grain). The new grains, in contrast, do not contain twins and the GOS is generally <1°. Their crystallographic orientation is close to that of the various twins or the host grain. We propose that the new grains have formed from the back-transformation of a transient high-pressure phase formed upon shock. The calcite aggregates show a zonation with variations in MnO- and MgO-contents (0-1.6 and 0-0.4, respectively), which is preserved in the new grains (i.e., the zonation is cross-cut by grain boundaries). This indicates that diffusional processes were relatively minor, which is consistent with fast transformation processes. Heating experiments of shocked carbonates indicates that temperatures after shock were <400°C, as at these temperatures the twinned microstructure is markedly modified. The study of modification processes of highly strained carbonate in impact breccias yields valuable insights into fast transformations in the carbonate system.