Deformation microstructures of glaucophane and lawsonite in experimentally deformed blueschists: Implications for intermediate‐depth intraplate earthquakes

Abstract

A series of simple-shear experiments on blueschist was performed at 400-500°C and 1-2.5 GPa to understand the deformation of seismically active, subducting oceanic crust. The experiments show that brittle microstructures are mainly found at pressures of 1-2 GPa, whereas ductile microstructures form at 2.5 GPa. J-indices (a measure of fabric intensity) of glaucophane crystal preferred orientations change systematically with changing shear strain and confining pressure, and the angle between the slip plane and the shear direction of samples deformed at >2 GPa is similar to that of a strain ellipsoid. These results, together with the variable orientations of fine grains in a selected area electron diffraction image at 2 GPa, indicate that the brittle-ductile transition for glaucophane occurs at~2 GPa. In contrast to this, lawsonite in the experiments show abundant fracturing in most specimens and a poor correlation between the J-index, shear strain, and confining pressure. This demonstrates that lawsonite deformed by brittle failure under all experimental conditions. In the case of a starting material that has a strong fabric and deformed under dry experimental conditions, the brittle-ductile transition zone of glaucophane will be much shallower than 2 GPa. Therefore, our initial experimental results on the deformation behavior of blueschist indirectly support the dehydration embrittlement of subducting oceanic crust (glaucophane) as an important factor in the origin of intraplate earthquakes.

Lattice Preferred Orientation and Deformation Microstructures of Glaucophane and Epidote in Experimentally Deformed Epidote Blueschist at High Pressure

“…This result is relatively consistent with previous suggestions that glaucophane may be deformed under a ductile regime (i.e., dislocation creep), in natural epidote blueschist at the pressures of~0.5-2.0 GPa [16,17,35]. A previous experimental study on lawsonite blueschist also suggested that the brittle-ductile transition of glaucophane is likely to occur at a pressure of~2 GPa and temperatures between 400 and 500 • C [49]. One may argue that the cataclastic deformation microstructure in this study was produced by the deformation of the sample at a fast strain rate.…”

Deformation mechanisms in mafic amphibolites and granulites: record from the Semail metamorphic sole during subduction infancy

“…The brittle behavior of amphibole within a rock deforming ductilely at high temperature has already been reported (Nyman et al, 1992;Berger and Stunitz, 1996;Imon et al, 2002;Diaz Aspiroz et al, 2007;Gomez Barreiro et al, 2010;Ko and Jung, 2015;Marti et al, 2017;Giuntoli et al, 2018) but ascribed to significantly lower deformation temperatures (≤ 700 • C). Preferential microfracturing along planes including the long ([001]) crystallographic axis could explain the similar aspect ratios of fine and coarse amphibole grains (essentially ≤ 2-3, as suggested by Gomez Barreiro et al (2010).…”

“…Naturally and experimentally deformed amphibolites commonly show well-developed foliation and strong crystallographic-preferred orientation (CPO), regardless of their P -T -related water activity conditions and compositional range (e.g., Imon et al, 2004;Tatham et al, 2008;Cao et al, 2010;Getsinger et al, 2013;Ko and Jung, 2015;Marti et al, 2017Marti et al, , 2018Giuntoli et al, 2018). Foliation is usually underlined by the shape-preferred orientations (SPO) of amphibole and plagioclase.…”

“…Foliation is usually underlined by the shape-preferred orientations (SPO) of amphibole and plagioclase. Compared to plagioclase and clinopyroxene, the CPO of amphibole is generally stronger (Siegesmund et al, 1994;Imon et al, 2004;Diaz Aspiroz et al, 2007;Tatham et al, 2008;Cao et al, 2010;Gómez Barreiro et al, 2010;Getsinger et al, 2013;Getsinger and Hirth, 2014;Kim et al, 2015;Ko and Jung, 2015), hence leading several authors to consider that it accounts for seismic anisotropy in the lower and middle crust (Mainprice and Nicolas, 1989;Tatham et al, 2008;Lloyd et al, 2011;Ji et al, 2013) and in subduction zones (Ko and Jung, 2015).…”

“…However, other studies have suggested that amphibole is one of the strongest silicates, with a limited ability to deform by dislocation creep (e.g., Brodie and Rutter, 1985). Amphibole would rather behave as a rigid particle, rotating and fracturing (Hacker and Christie, 1990;Ildefonse et al, 1990;Nyman et al, 1992;Shelley, 1994;Berger and Stünitz, 1996;Imon et al, 2004;Ko and Jung, 2015), and/or dissolving and reprecipitating during deformation (Brodie, 1981;Hacker and Christie, 1990;Berger and Stünitz, 1996;Kruse et al, 1999;Imon et al, 2004;Marti et al, 2017Marti et al, , 2018Giuntoli et al, 2018). While variations in P -T conditions, water activity and grain size are known to control the activation of the different deformation mechanisms (e.g., Brodie and Rutter, 1985), their respective roles have not been clearly assessed in the specific case of amphibole.…”