Reaction-induced grain boundary cracking and anisotropic fluid flow during prograde devolatilization reactions within subduction zones

Atsushi Okamoto, Hiroyuki Shimizu, Jun ichi Fukuda, Jun Muto, Takamoto Okudaira

Research output: Contribution to journalArticlepeer-review

4 Citations (Scopus)

Abstract

Devolatilization reactions during prograde metamorphism are a key control on the fluid distribution within subduction zones. Garnets in Mn-rich quartz schist within the Sanbagawa metamorphic belt of Japan are characterized by skeletal structures containing abundant quartz inclusions. Each quartz inclusion was angular-shaped, and showed random crystallographic orientations, suggesting that these quartz inclusions were trapped via grain boundary cracking during garnet growth. Such skeletal garnet within the quartz schist formed related to decarbonation reactions with a positive total volume change (∆Vt > 0), whereas the euhedral garnet within the pelitic schists formed as a result of dehydration reaction with negative ∆Vt values. Coupled hydrological–chemical–mechanical processes during metamorphic devolatilization reactions were investigated by a distinct element method (DEM) numerical simulation on a foliated rock that contained reactive minerals and non-reactive matrix minerals. Negative ∆Vt reactions cause a decrease in fluid pressure and do not produce fractures within the matrix. In contrast, a fluid pressure increase by positive ∆Vt reactions results in hydrofracturing of the matrix. This fracturing preferentially occurs along grain boundaries and causes episodic fluid pulses associated with the development of the fracture network. The precipitation of garnet within grain boundary fractures could explain the formation of the skeletal garnet. Our DEM model also suggests a strong influence of reaction-induced fracturing on anisotropic fluid flow, meaning that dominant fluid flow directions could easily change in response to changes in stress configuration and the magnitude of differential stress during prograde metamorphism within a subduction zone.

Original languageEnglish
Article number75
JournalContributions to Mineralogy and Petrology
Volume172
Issue number9
DOIs
Publication statusPublished - 2017 Sep 1

Keywords

  • Devolatilization reactions
  • Distinct element method (DEM)
  • Permeability anisotropy
  • Prograde metamorphism
  • Reaction-induced cracking
  • Skeletal garnet

ASJC Scopus subject areas

  • Geophysics
  • Geochemistry and Petrology

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