MECHANISMS OF CO2-BEARING FLUID INFILTRATION INTO DRY ROCKS IN MÉLANGE, NORTHEASTERN CONNECTICUT, USA

The infiltration of aqueous fluids plays a major role in the global cycling of carbon and influences metamorphic CO₂ fluxes. Aqueous fluids infiltrating metacarbonate rocks may lead to the release of CO₂ during metamorphism, whereas CO₂-bearing fluids infiltrating silicate rocks may deposit carbonate minerals and sequester CO₂ at depth. It is therefore necessary to understand the mechanisms by which these fluids infiltrate and interact with rocks along their flow paths, in particular initially dry rocks lacking a preexisting fluid-filled porosity. Here, we focus on Acadian–Neoacadian age mélange in northeastern Connecticut, USA¹, containing initially dry mafic–ultramafic blocks that have been variably hydrated and carbonated by a large-scale vein forming event that introduced matrix fluids to the blocks. The veins contain carbonate minerals (primarily dolomite–ankerite and magnesite–siderite), indicating the presence of CO₂ in the fluid, and silicate minerals including quartz, garnet, and kyanite. The blocks lack carbonate phases, so the CO₂ must have been introduced by the fluids. The growth of euhedral crystals into open spaces suggests fluid flow through cracks, but carbonated selvages surrounding the veins indicate that pervasive infiltration into the blocks also occurred. Petrographic observations reveal the growth of small kyanite and sometimes carbonate and staurolite crystals along plagioclase grain boundaries in the selvages, indicating that pervasive infiltration of water and CO₂ likely initiated along these grain boundaries. Extensive interface-coupled dissolution-reprecipitation (ICDR) textures are observed in plagioclase. ICDR is a process by which a disequilibrium mineral phase is epitaxially replaced by a more stable phase due to fluid-mineral interaction. This process produces an extensive micro–nanoporosity network, which could facilitate further fluid ingress and subsequent CO₂ mineralization. Thus, we posit that channelized flow introduced matrix fluids to the blocks, whereas migration of fluid along grain boundaries and nanoporosity resulting from ICDR processes allowed for CO₂ and H₂O ingress at the grain scale. Together, these processes facilitated CO₂-bearing fluid infiltration and associated carbonate mineralization in these mélange blocks.

1. Tassara, S., et al. Geology 49, 168-173 (2021).