Syn-kinematic hydration reactions, grain size reduction, and dissolution–precipitation creep in experimentally deformed plagioclase–pyroxene mixtures

Sina Marti,Renée Heilbronner,Rüdiger Kilian,Holger Stünitz,Oliver Plümper

doi:10.5194/se-9-985-2018

Abstract

Abstract. It is widely observed that mafic rocks are able to accommodate high strains by viscous flow. Yet, a number of questions concerning the exact nature of the involved deformation mechanisms continue to be debated. In this contribution, rock deformation experiments on four different water-added plagioclase–pyroxene mixtures are presented: (i) plagioclase(An60–70)–clinopyroxene–orthopyroxene, (ii) plagioclase(An60)–diopside, (iii) plagioclase(An60)–enstatite, and (iv) plagioclase(An01)–enstatite. Samples were deformed in general shear at strain rates of 3×10−5 to 3×10−6 s−1, 800 °C, and confining pressure of 1.0 or 1.5 GPa. Results indicate that dissolution–precipitation creep (DPC) and grain boundary sliding (GBS) are the dominant deformation mechanisms and operate simultaneously. Coinciding with sample deformation, syn-kinematic mineral reactions yield abundant nucleation of new grains; the resulting intense grain size reduction is considered crucial for the activity of DPC and GBS. In high strain zones dominated by plagioclase, a weak, nonrandom, and geometrically consistent crystallographic preferred orientation (CPO) is observed. Usually, a CPO is considered a consequence of dislocation creep, but the experiments presented here demonstrate that a CPO can develop during DPC and GBS. This study provides new evidence for the importance of DPC and GBS in mid-crustal shear zones within mafic rocks, which has important implications for understanding and modeling mid-crustal rheology and flow.

Highlights

Viscous deformation of crustal rocks is usually dominated either by intracrystalline deformation or by a form of diffusion creep together with grain boundary sliding
The relative importance of these processes has been discussed by Paterson (1990, 1995), and for the sake of simplicity, we implicitly include the operation of grain boundary sliding when we speak of diffusion creep
Viscous deformation in experiments with mafic compositions at temperatures of 800 ◦C and confining pressures of 1.0 and 1.5 GPa at strain rates of ∼ 10−5 s−1 is dominantly achieved by dissolution–precipitation creep (DPC) and grain boundary sliding accompanied by syndeformational mineral reactions

Summary

Introduction

Viscous deformation of crustal rocks is usually dominated either by intracrystalline deformation (dislocation creep) or by a form of diffusion creep together with grain boundary sliding. The relative importance of these processes has been discussed by Paterson (1990, 1995), and for the sake of simplicity, we implicitly include the operation of grain boundary sliding when we speak of diffusion creep. Apart from being rate and temperature sensitive, the rheology of viscously deforming rocks is observed to be material dependent (for a comprehensive list of flow-law parameters for different rock types see, e.g., Kohlstedt et al, 1995, Shaocheng and Bin, 2002, Bürgmann and Dresen, 2008, Burov, 2011, and references therein). Flow laws for viscous creep exist for different types of rocks, with the majority of these flow laws being determined for monomineralic materials. In monomineralic aggregates at mid-crustal to lower crustal conditions, grain growth in monomineralic aggregates is assumed to be extensive and the resulting large grain size is expected to render diffusion creep less effi-

Results

Discussion

Conclusion