Fluid production rate during the regional metamorphism of a pelitic schist

摘要

Phase equilibria modeling of the pressure-temperature (P-T) path of regional metamorphism and associated fluid expulsion, combined with constraints on the timescale of garnet growth by Sm-Nd geochronology, elucidates the fluid production rate and fluid flux during Barrovian metamorphism of pelitic rocks from Townshend Dam, VT, USA. This modeling builds on a published companion study that utilized Sm-Nd geochronology of concentric growth zones in multiple garnet grains, to constrain the duration of garnet growth in a large sample of schist at Townshend Dam to 3.8 +/- 2.2 million years (Gatewood et al., Chem Geol 401:151-168, 2015). P-T pseudosections combined with observed mineral compositions constrain garnet growth conditions, and were utilized to construct P-T path-dependent thermodynamic forward models. These models determine that garnet growth was initiated at similar to 0.6 GPa and similar to 525 degrees C, with a roughly linear loading and heating P-T trajectory to 0.8 GPa and similar to 610 degrees C. Loading and heating rates of 2.4 kmMyear(-1) (with a range of 1.6 to 5.8 km million year(-1)) and 23 degrees C million year(-1) (with a range of 14 to 54 degrees C million year(-1)), respectively, are consistent with model estimates and chronologic constraints for tectono-metamorphic rates during orogenesis. Phase equilibria modeling also constrains the amount of water release during garnet growth to be similar to 0.7 wt% (or 2 vol%), largely resulting from the complete consumption of chlorite. Coupling this estimate with calculated garnet growth durations provides a fluid production rate of 5.2 kg m(-3) million year(-1) (with a range of 3.2 to 12.2 kg m(-3) million year(-1)) and when integrated over the overlying crustal column, a regional-scale fluid flux of 0.07-0.37 kg m(-2)million year(-1). This range of values is consistent with those derived by numerical models and theory for regional-scale, pervasive fluid flow. This study signifies the first derivation of a fluid production rate and fluid flux in regional metamorphism using a direct chronology of water-producing (garnet-forming) reactions and can provide a framework for future studies on elucidating the nature and timescales of fluid release.