Simple models of CO2 release from metacarbonates with implications for interpretation of directions and magnitudes of fluid flow in the deep crust

Simple one-dimensional models of coupled advection-hydrodynamic dispersion- reaction are used to investigate processes of CO2 release from metacarbonat e beds during deep crustal (similar to 8 kbar) Acadian prograde metamorphis m in New England, USA. Two broad models in which reaction progress is contr olled by gradients in H2O-CO2 fluid composition between different rock type s are presented. In the first, diffusional exchange of volatiles across lit hologic contacts is significant. CO2 generated during prograde temperature (T) rise is transported away from metacarbonate layers to surrounding (1) m etapelitic layers which generate H2O by dehydration and/or (2) flow conduit s (e.g. permeable layers or fractures) for externally derived, elevated X-H 2O/X-CO2 fluids. H2O is transported from the surroundings into the metacarb onate layers and drives further mineral reaction. In the second model, reac tion in metacarbonate layers is driven mostly by layer-parallel flow of ext ernal fluids with elevated X-H2O/X-CO2 derived from, for example, dehydrati ng schists or outgassing magmas. For both models, the X-CO2 of the fluid wi thin metacarbonate layers is generally predicted to increase with increasin g grade from the Ankerite-Oligoclase to the Amphibole zones, and then decre ase in the Diopside zone-key relationships that are commonly observed in th e field. The slow reaction progress in metacarbonates driven by progressive dehydration of surrounding metapelite from greenschist to amphibolite faci es probably requires time scales of fluid-rock interaction comparable to th e duration of the Acadian orogeny (similar to 10(6)-10(7) my). Intense epis odes of fluid flow through conduits such as fractures may produce veins and alteration selvages over fluid-rock interaction times as short as 10(3)-10 (4) years. Model results emphasize that the accuracy of field-based fluid f lux estimates depends critically on correct identification of mass transpor t processes. The modeling suggests that reactive transport of volatiles bet ween metacarbonate layers and their lithologically heterogeneous surroundin gs can account for basic T-fluid composition-reaction progress relationship s observed in much of the Acadian orogen of New England. The results provid e an alternative to up-T flow scenarios that account for these relationship s by large, pervasive, horizontal fluid fluxes up regional T gradients.