Fluid production rate during the regional metamorphism of a pelitic schist

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 pelit...

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Bibliographic Details
Published in:Contributions to mineralogy and petrology 2018-11, Vol.173 (11), p.1-16, Article 96
Main Authors: Dragovic, Besim, Gatewood, Matthew P., Baxter, Ethan F., Stowell, Harold H.
Format: Article
Language:English
Subjects:
Chlorite
Chronology
Clay
Computational fluid dynamics
Constraint modelling
Dams
Duration
Earth and Environmental Science
Earth Sciences
Expulsion
Fluctuations
Fluid flow
Flux
Frameworks
Garnet
Geochronology
Geochronometry
Geology
Growth conditions
Heating
Mathematical models
Metamorphic rocks
Metamorphism
Metamorphism (Geology)
Mineral composition
Mineral Resources
Mineralogy
Modelling
Moisture content
Original Paper
Petrology
Phase equilibria
Schist
Schists
Water content
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Summary: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 ~ 0.6 GPa and ~ 525 °C, with a roughly linear loading and heating P – T trajectory to > 0.8 GPa and ~ 610 °C. Loading and heating rates of 2.4 km·Myear −1 (with a range of 1.6 to 5.8 km·million year −1 ) and 23 °C·million year −1 (with a range of 14 to 54 °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 ~ 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.
ISSN:0010-7999
1432-0967
DOI:10.1007/s00410-018-1523-9