Volatile production and transport in regional metamorphism

Abstract

Calculations show that H₂O and CO₂ produced during devolatilization of an average pelite will occupy ∼12 vol. % of the rock at 500°C and 5 kb. Because the tensional strength of well foliated rock at metamorphic conditions is vanishingly small, such a volume of fluid having any vertical extent will fracture the rock and escape upward owing to its lower density.

In a simplified model of a sudden increase of heat flow from 0.8 to 2.5 H.F.U., the average pelitic rock will have a rate of fluid production averaging ∼9.4×10⁻¹⁰ g cm² s⁻¹ between 400°C and 600°C. The escape of this fluid can be accomodated by a single fracture 1 cm long and 0.2 μ wide per cm² of rock. If the fracture is reduced to 0.02 μ then 1,000 cm of fracture per cm² would be required. This width is the minimum original width as calculated from the volume of fluid observed in fluid inclusions trapped along annealed fractures within quartz in metamorphic terrains. Fluid flow will be laminar if the fracture is <0.025 cm wide. Additional calculations show that grain boundary diffusion is not an effective means of fluid transport in regional metamorphism.

The commonly observed quartz segregations in pelitic terrains appear to mark the site of major channelways for fluid escape. In this case the bulk of escaping fluid is not able to react pervasively with rocks higher in the metamorphisc pile. Regionally metamorphosed rocks will have a discrete fluid phase only when devolatization reactions are actually taking place. At other times only an absorbed surface monolayer of volatiles on the minerals will be present.