Excess water storage induced by viscous strain localization during high-pressure shear experiment

Jacques Précigout,Holger Stünitz,Johan Villeneuve

doi:10.1038/s41598-019-40020-y

Abstract

Strain localization in viscously deformed rocks commonly results in fine-grained shear zones where massive fluid circulation is regularly observed. Recently attributed to strain-induced pumping, this phenomenon may have major implications for the distribution of ores deposits and rock rheology. However, although grain size reduction and/or creep cavitation have been proposed as important processes, the source mechanism of fluid concentration remains unresolved, particularly at high pressure. Here we use secondary ion mass spectrometry to document the H2O content of fine-grained olivine across an experimental shear zone, which developed with grain size reduction during a H2O-saturated shear experiment at 1.2 GPa and 900 °C. Through data interpolation, the olivine matrix reveals high fluid concentrations where shear strain is localized. These concentrations far exceed the predicted amount of H2O that grain boundaries can contain, excluding grain size reduction as a unique source of water storage. Instead, we show that H2O increases per unit of grain boundary across the shear zone, suggesting that cavitation and “healing” processes compete with each other to produce a larger pore volume with increasing strain rate. This provides an alternative process for fluids to be collected where strain rate is the highest in deep shear zones.

Highlights

On Earth, both the crust and mantle incorporate aqueous fluids that interact with solid rock materials in many ways
Referred to as creep cavitation, the production of these micro-pores results either from grain boundary sliding (GBS) or, in a minor extent, from Zener-Stroh cracking if dislocations interact with grain boundaries[17]
As creep cavitation is a process producing pressure-dependent dilatancy[21], could we expect cavitation to occur if lithostatic pressure increases and limits dilatancy? The role of creep cavitation in fluid infiltration within ductile shear zones remains uncertain, at high pressure

Summary

Introduction

On Earth, both the crust and mantle incorporate aqueous fluids that interact with solid rock materials in many ways. Referred to as fluid-rock interactions, they strongly affect rock deformation and petrogenetic processes, giving rise, for instance, to hydrolytic weakening, pressure-solution creep or metamorphic reactions, including partial melting During strain localization, this type of interactions may involve one or several chemo-physical processes that help to channelize fluid flow in ductile shear zones where grain size is substantially reduced. Referred to as creep cavitation, the production of these micro-pores results either from grain boundary sliding (GBS) or, in a minor extent, from Zener-Stroh cracking if dislocations interact with grain boundaries[17] In both cases, the micro-cavities arise from limitations of the material to flow, when diffusive mass transfer is slow at low temperature. As creep cavitation is a process producing pressure-dependent dilatancy[21], could we expect cavitation to occur if lithostatic pressure increases and limits dilatancy? The role of creep cavitation in fluid infiltration within ductile shear zones remains uncertain, at high pressure

Methods

Results

Conclusion