CO2 outgassing during collisional orogeny is facilitated by the generation of immiscible fluids

Chiara Groppo,Maria Luce Frezzotti,Franco Rolfo

doi:10.1038/s43247-022-00340-w

Chiara Groppo, Maria Luce Frezzotti + Show 1 more

Open Access

https://doi.org/10.1038/s43247-022-00340-w

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Abstract

Orogenic degassing is emerging as a potentially relevant source of carbon dioxide (CO2) from the continental crust. However, the processes of carbon mobilization are still poorly explored. Here, we use thermodynamic modeling to investigate the decarbonation of sediments metamorphosed under high geothermal gradients. Our modeling shows that immiscible CO2-rich vapors and hydrosaline brines are generated at these conditions, with different properties and mobility through the crust. The CO2-rich fluid fraction could rapidly rise toward the surface without interacting with the host rocks by carbo-fracturing the host rocks or through deep faults. The denser hydrosaline brines likely permeate the source rocks. When applied to the active Himalayan orogen, these observations reconcile measured CO2 fluxes at the surface and positive conductivity anomalies associated with micro-seismicity at depth. Our modeling shows that the continental crust represents a relevant reservoir of CO2 that can be efficiently degassed during hot collisions.

Highlights

Orogenic degassing is emerging as a potentially relevant source of carbon dioxide (CO2) from the continental crust
It is well known that metamorphism of crustal rocks in collisional orogens produces carbon-bearing fluids over geologic time scales, primarily through decarbonation reactions[2,3,4,5,6,7]
If metamorphic CO2-bearing fluids do not react with the host rocks, orogenic decarbonation would represent an important source of CO2 at the global scale

Summary

Introduction

Orogenic degassing is emerging as a potentially relevant source of carbon dioxide (CO2) from the continental crust. It is well known that metamorphism of crustal rocks in collisional orogens produces carbon-bearing fluids over geologic time scales, primarily through decarbonation reactions[2,3,4,5,6,7]. The efficiency of metamorphic CO2 degassing in orogenic settings is primarily controlled by the ease at which the CO2-bearing aqueous fluids produced at depth are transported upward without interacting with the host rocks. We study decarbonation reactions occurring in carbonate-bearing sediments along high geothermal gradients to define the physical and chemical nature of fluids that transport carbon in large hot orogens. Our results demonstrate a crucial role of fluid immiscibility in driving CO2 transport from the deep crust, explaining how and why significant amounts of CO2 could be effectively degassed at the surface from orogenic belts. Results further highlight the role of hydrosaline brines as metasomatizing and/or granulitizing agents in the lower crust

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