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

Area of Science:

• Geology
• Geophysics
• Mineral Physics

Background:

• Strain localization in viscous rocks forms fine-grained shear zones with significant fluid circulation.
• Strain-induced pumping is a proposed mechanism for fluid concentration, with implications for ore deposits and rock rheology.
• The precise source of fluid concentration in deep, high-pressure shear zones remains unclear.

Purpose of the Study:

• To investigate the mechanism of fluid concentration in experimental shear zones at high pressure.
• To quantify the water (H2O) content in fine-grained olivine within a shear zone.
• To differentiate between grain size reduction and other processes as the primary source of fluid accumulation.

Main Methods:

• Experimental shear deformation of H2O-saturated olivine at 1.2 GPa and 900°C.
• Secondary Ion Mass Spectrometry (SIMS) to analyze H2O content in olivine.
• Data interpolation to map fluid concentrations across the experimental shear zone.

Main Results:

• High H2O concentrations were observed in the olivine matrix where shear strain was localized.
• Observed H2O concentrations exceeded the storage capacity of grain boundaries alone.
• H2O content increased per unit of grain boundary with increasing strain rate.

Conclusions:

• Grain size reduction is insufficient to explain the observed fluid concentrations in deep shear zones.
• Competing processes of cavitation and healing likely create increased pore volume with higher strain rates.
• This mechanism offers an alternative explanation for fluid collection in high-strain-rate deep shear zones.