Rapid healing of crystal-rich granular materials in volcanic and high-temperature upper-crustal environments: implications for fluid flow and deformation

Abstract:

Unconsolidated granular materials are produced by brittle deformation processes (e.g., shear-driven fracturing, gas-driven fragmentation, formation of damage zones ahead of propagating dikes, etc.). The sustained presence of granular materials in volcanic and tectonic environments provides long-lived conduits for fluids and regions where deformation may localize. However, granular materials can heal and be transformed into low-permeability, strong rocks.

Solid-state sintering is a process that causes crystalline clasts to heal in the absence of fluids or melt. It operates wherever granular materials are subjected to elevated pressures and temperatures for protracted periods of time, conditions that are easily satisfied within volcanic environments. However, the timescales necessary for solid-state sintering to heal crystalline geologic materials are unconstrained.

Here, we present the results of hot-pressing experiments designed to cause a natural glass-free volcanic fault gouge to undergo solid-state sintering. As a result of 4 to 60 hours of hot pressing at 700-900°C under confining pressures of 20-70 MPa, the unconsolidated gouge is transformed into more-coherent composites. Using the experimental conditions and measured physical properties of the experimental products, we develop a model that predicts time-dependent porosity loss at pressures and temperatures relevant to volcanic and some upper-crustal environments. Our modeling shows solid-state sintering causes significant porosity and permeability loss over a period of days to months in these environments. Because solid-state sintering reduces the capacity of granular materials to transmit fluids and makes them increasingly resistant to deformation, it can hinder the propagation of dikes and veins, suppress volcanic outgassingand promote cyclical explosive activity.