The role of deformation-reaction interactions to localize strain in polymineralic rocks: Insights from experimentally deformed plagioclase-pyroxene assemblages

Highlights

• Deformation considerably enhances the kinetics of mineral reactions.

• Diffusion creep is dominant in fine grained aggregates.

• The interplay between deformation and reaction is of great importance for strain localization.

• The degree of connectivity of the reacted material controls the bulk rheology.

Abstract

In order to study the mutual effect of deformation and mineral reactions, we have conducted shear experiments on fine-grained plagioclase-pyroxene assemblages in a Griggs-type solid-medium deformation apparatus. Experiments were performed at a constant shear strain rate of 10⁻⁵ s⁻¹, a confining pressure of 1 GPa and temperatures of 800, 850 and 900 °C. While the peak stress of plagioclase + orthopyroxene assemblages reaches values between those of the end-member phases, the strength of polymineralic materials strongly decreases after peak stress and reaches flow stresses that stabilize far below those of the weaker phase (plagioclase). This weakening correlates with the coeval development of high-strain shear zones where new phases are preferentially produced, including new pyroxene, plagioclase and amphibole. The reaction products mostly occur as intimately mixed phases within fine-grained and interconnected shear bands, together with different compositions with respect to the starting material. This indicates that deformation significantly enhances the kinetics of mineral reactions, which in turn strongly weaken the deforming sample, here attributed to a switch to grain-size-sensitive diffusion creep through phase nucleation and grain size reduction. Such an interplay between deformation and mineral reactions may have strong implications for the initiation, development, and durability of shear zones in the lower crust.

Introduction

Strain localization and resulting shear zones are considered to be fundamental features of plate tectonics on Earth (e.g. Tackley, 1998; Bercovici and Ricard, 2012). They accommodate a large amount of strain and have a direct control on rock rheology, so their development is critical to understanding the dynamics of the lithosphere. The formation of viscous shear zones has been considered to result from one or several processes of strain-induced weakening, which expresses as a stress drop at constant strain rate or an increase of strain rate at constant stress (e.g. Paterson, 2013). Possible weakening mechanisms include: (1) Geometric and/or fabric softening, (2) a change in deformation mechanism, commonly due to grain size reduction, (3) fracturing, (4) metamorphic mineral reactions, (5) shear heating (6) water- or (7) melt-induced weakening (e.g. Poirier, 1980; White et al., 1980; Burlini and Bruhn, 2005). While some of these mechanisms apply to monophase rocks, others are more typical of polyphase materials.

As a starting point, laboratory-derived flow laws have been determined for the deformation of monophase materials to understand the rheology of important rock-forming minerals in the viscous regime of the lithosphere, including olivine (e.g., Chopra and Paterson, 1981), quartz (Paterson and Luan, 1990), plagioclase (Rybacki and Dresen, 2000; Dimanov et al., 1999) and pyroxene (Bystricky and Mackwell, 2001). For instance, the experiments of, e.g., Rybacki and Dresen (2000) and Chen et al. (2006), have shown that water may considerably reduce the strength of feldspar and pyroxene aggregates. Plastic flow and grain size reduction by dynamic recrystallization are also best described in deformation experiments of monophase materials, giving rise to state-of-the-art flow laws (e.g. White et al., 1980; Schmid, 1982; Rutter and Brodie, 1988). These experiments highlighted a potential source of weakening induced by a transition from grain-size-insensitive to grain-size sensitive creep, but this transition is not expected to cause long-term weakening in monophase aggregates, because the efficient grain growth at elevated temperature is expected to counteract the weakening effect of grain size reduction (e.g. Rutter and Brodie, 1988; De Bresser et al., 2001).

Except for rare cases, the lithosphere consists of polyphase rocks. Thus, a growing body of literature has also addressed the rheology of polyphase material, given the fact that such a rheology is likely to be different from that of monophase ones (e.g. Bürgmann and Dresen, 2008). For instance, the presence of additional phases in polyphase aggregates inhibits grain growth and controls microstructures through pinning (e.g. Olgaard, 1990; Herwegh et al., 2011). The dominant deformation mechanism can thus be expected to differ as a consequence of phase interactions.

In this context, the rheology of polyphase aggregates for gabbroic composition has a fundamental importance for understanding the mechanical behaviour of the oceanic and lower crusts; mafics are the most abundant rock types in these crustal layers (e.g. Weaver and Tarney, 1984; Christensen and Mooney, 1995). However, experimental studies of high-temperature deformation of gabbroic composition are still comparatively rare (e.g. Dimanov et al., 2003, 2007; Dimanov and Dresen, 2005), and these studies have not considered mineral reactions. They focused indeed on the role of secondary phases, grain size, water content, stress and spatial distribution of grains to account for changing flow stress and dominant deformation mechanism in polyphase feldspar-pyroxene aggregates. Yet, other studies suggest that the occurrence of mineral reactions during viscous flow may have the potential to weaken rocks and consequently localize strain (e.g. Stünitz and Tullis, 2001; De Ronde et al., 2004, 2005; Getsinger and Hirth, 2014; Marti et al., 2017, 2018).

Strain localization and weakening in viscous shear zones as a result of mineral reactions are mostly achieved through changes in P-T conditions (e.g. Gapais, 1989; Newman et al., 1999) or through fluid-rock interactions (e.g. Austrheim, 1987; Menegon et al., 2015). Changes in P-T conditions commonly occur during oceanic and continental subduction and exhumation along crustal-scale shear zones, where the thermodynamic disequilibrium promotes the growth of new stable minerals (e.g. Gerya et al., 2002; Jamtveit et al., 2016). In addition, shear zones represent permeable pathways for fluids that enhance diffusion and strongly catalyse mineral reactions under both, low (e.g. Fitz Gerald and Stünitz, 1993; Newman and Mitra, 1993; Mansard et al., 2018) and high metamorphic grades (e.g. Brodie, 1980; Boundy et al., 1992; Glodny et al., 2003). The importance of mineral reactions lies in the possible grain size reduction and change in deformation mechanism leading to a switch from grain-size-insensitive to grain-size sensitive creep, giving rise to substantial weakening (e.g. Etheridge and Wilkie, 1979; Olgaard, 1990; Stünitz and Fitz Gerald, 1993; Fliervoet et al., 1997; De Bresser et al., 1998; Kruse and Stünitz, 1999; De Bresser et al., 2001; Kenkmann and Dresen, 2002; Precigout et al., 2007; Raimbourg et al., 2008; Linckens et al., 2011; Kilian et al., 2011; Viegas et al., 2016). Mineral reactions also contribute to form mixing zones that play an important role of weakening because the pinning of grain boundaries impedes grain growth and keep the grain size small (e.g. Etheridge and Wilkie, 1979; Herwegh et al., 2011).

In this contribution, rock deformation experiments on H₂O-added plagioclase-orthopyroxene samples were performed to investigate the rheology and evolution of microstructures with increasing strain. The experiments provide insight into weakening mechanisms and localization of deformation during shear zone development. This study presents a simplified model example of how deformation can facilitate metamorphic reactions, heterogeneous nucleation, formation of fine-grained phase mixtures, and how conversely such an evolution in microstructures eventually results in strain localization and weakening of polyphase aggregates. The study also demonstrates the importance of interaction of individual phases in polymineralic assemblages when dealing with the behaviour of the lithosphere.