Strain localization and fabric development in polycrystalline anorthite + melt by water diffusion in an axial deformation experiment

Abstract

We performed two axial deformation experiments on synthetic polycrystalline anorthite samples with a grain size of ~ 3 μm and 5 vol% Si-Al-rich glass at 900 °C, a confining pressure of 1.0 GPa, and a strain rate of 10 −4.8 s −1. One sample was deformed as-is (dry); in the other sample, two half-cut samples (two cores) with 0.15 wt% water at the boundary were put together in the apparatus. The mechanical data for both samples were essentially identical with a yield strength of ~ 700 MPa and strain weakening of ~ 500 MPa by 20% strain. The dry sample appears to have been deformed by distributed fracturing. Meanwhile, the water-added sample shows plastic strain localization in addition to fracturing and reaction products composed of zoisite grains and SiO 2 materials along the boundary between the two sample cores. Infrared spectra of the water-added sample showed dominant water bands of zoisite. The maximum water content was 1500 wt ppm H 2 O at the two-core boundary, which is the same as the added amount. The water contents gradually decreased from the boundaries to the sample interior, and the gradient fitted well with the solution of the one-dimensional diffusion equation. The determined diffusion coefficient was 7.4 × 10 −13 m 2 /s, which agrees with previous data for the grain boundary diffusion of water. The anorthite grains in the water-added sample showed no crystallographic preferred orientation. Textural observations and water diffusion indicate that water promotes the plastic deformation of polycrystalline anorthite by grain-size-sensitive creep as well as simultaneous reactions. We calculated the strain rate evolution controlled by water diffusion in feldspar aggregates surrounded by a water source. We assumed water diffusion in a dry rock mass with variable sizes. Diffused water weakens a rock mass with time under compressive stress. The calculated strain rate decreased from 10 −10 to 10 −15 s −1 with an increase in the rock mass size to which water is supplied from < 1 m to 1 km and an increase in the time of water diffusion from < 1 to ~ 10,000 years. This indicates a decrease in the strain rate in a rock mass with increasing deformation via water diffusion.Keywords: Anhydrous polycrystalline anorthite, Water diffusion, Griggs-type deformation apparatus, Plastic deformation, Reaction © The Author(s) 2018. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.

Crustal dynamics: unified understanding of geodynamic processes at different time and length scales

“…Consequently, its progress is crucial for our understanding of crustal dynamics. Fukuda et al (2018) showed how the addition of small amounts of water to polycrystalline anorthite under high temperature induces a change from distributed fracturing to plastic flow promoting grain-size-sensitive creep in the lower middle crust. Kameda et al (2017) investigated the alteration and dehydration of subducting oceanic crust, specifically pillow basalts within the Shimanto belt showing how the saponite-chlorite conversion within mixed layer C/S minerals may contribute fluid to plate boundary fault systems with consequent mechanical effects.…”

Partial melting and reaction along deformation features in plagioclase

“…Therefore, the deformation behaviour of these rocks, but also the chemical reactions taking place in them, will depend on the physical and chemical properties of plagioclase. Besides studies investigating the mechanical behaviour of plagioclase‐rich rocks over a broad pressure ( P ) temperature ( T )–strain rate ( ) range (Fukuda et al, 2018; Fukuda et al, 2022; Rybacki et al, 2006; Rybacki & Dresen, 2004; Tullis, 2018; Tullis & Yund, 1992, 1987), there are also studies focusing on the impact of plagioclase breakdown at high pressure (H P ) conditions. This decomposition of plagioclase at H P is considered as one of the most important metamorphic reactions in the crust, and the modalities of its breakdown directly control the mechanical behaviour of the reacting rock (Stünitz & Tullis, 2001).…”

Dynamic Mechanical Behavior of Dry and Water Saturated Igneous Rock with Acoustic Emission Monitoring

“…The water content is known as one of the most notable factors for lowering the strength of rocks [31]. A large number of studies [32][33][34] consistently show that the water-rock interaction undermines the mechanical property of rock, which is mainly found expressed in strength decrease, the change of the deformation, and failure characteristics. Li et al [35] suggested that moisture content has a significant effect on shear properties reduction of both sandstone and mudstone, which must be considered in mining or excavation processes.…”