EXPANSION BRECCIAS IN CARBONATE ROCKS
The origin of the expansion breccias appears to require an expanding fluid to break up the carbonate rock and a fluid to deposit the sparry calcite. Both tasks could have been achieved by water carrying dissolved CO2 as HCO3– together with Ca2+. Chemical balance would require an input of CO2 from outside the system to dissolve nearby carbonate rock and source the sparry calcite; both Wyoming and the Apennines are regions of known CO2 accumulations. Stable (δ18O, δ13C) and radiogenic (87Sr/86Sr) isotopes are compatible with derivation of the ingredients of the calcite cement from nearby parts of the carbonate stratigraphy. Expansion of CO2to gas from a liquid or supercritical-fluid phase would take place at a depth of a few hundred meters, suggesting that this occurred during erosional unroofing of the anticlines; breccia formation would thus be a late event, which agrees with the lack of younger structures cutting the breccias.
These breccias may provide insights relevant to geological carbon sequestration, particularly the risk of CO2 escape from underground traps and its permanent storage by mineralization to CaCO3.