Grain- to outcrop-scale transport and mineralization of CO2 and rare earth elements in a mélange vein system, northeastern Connecticut, United States

Metamorphic fluid-rock interactions make key contributions to the global carbon cycle and can transport and deposit critical elements, so it is necessary to examine these interactions quantitatively. This work focused on a network of amphibolite-facies carbonate-silicate veins in mélange in northeastern Connecticut, United States, which provides evidence for CO2 infiltration and mineralization in mélange blocks. Pseudosection modeling of mineral assemblages in veins and altered selvages indicates that they formed from a fluid with high XCO2, ~0.8, which was introduced to the mélange blocks primarily by fracturing. Infiltration of CO2 and growth of carbonate minerals in selvages were facilitated by migration along grain boundaries and mineral dissolution-reprecipitation reactions. Mass balance calculations reveal that veins and selvages underwent considerable volatile gains (+1100% on average) due to carbonate mineralization and sequestered ~580 kg CO2 per cubic meter of altered rock, indicating that they were efficient CO2 traps. Because fluids were largely channelized in veins, however, carbonated veins and selvages make up ~10% of the blocks by volume, decreasing overall carbonation efficiency. Results also indicate that heavy rare earth elements (HREEs) were mobilized during fluid-rock interaction but trapped by garnet growth in veins and selvages. We therefore suggest that garnet may trap HREEs in metamorphic systems and affect their availability for ore-forming processes. Our calculations also reveal that some elemental mass changes were large at the local level, but values were near zero for most elements when all vein-selvage systems were averaged, suggesting that these elements were primarily cycled within the vein system. Thus, individual vein-selvage systems are not necessarily representative of processes in the vein system overall.