Static recrystallization of shocked calcite in Ries impact breccias

Abstract

Calcite is prone to chemical and microstructural modifications, especially after having been strained at high stresses and strain rates, as during hypervelocity impact events. These modifications include precipitation from pore fluid as well as replacement of strained volumes by recrystallization. In calcite aggregates of a metagranite breccia of the Ries Bunte Breccia, shocked calcite is partly replaced by new, undeformed grains. This breccia indicates shock conditions of 10–20 GPa by the presence of planar deformation features in quartz of the metagranite. Shocked calcite shows grain orientation spread (GOS) angles of 3–10° and contains e-, f-, and r- twins, as well as a- and f-type lamellae. In contrast, the new coarse calcite grains, which are hundreds of μm in diameter, have low GOS angles (<1°), and do not contain twins. Calcite aggregates have a chemical zonation (varying Mnn+ content), which is independent of new grains, suggestive of fast transformation. We propose that the new grains originate from sites of high crystal-plastic strain and grew by grain boundary migration driven by the reduction in strain energy, replacing previously strained grains at low stresses, that is, static recrystallization. Heating experiments on shocked calcite confirm the strain control on static recrystallization.