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

Sina Marti Holger Stünitz 1, 2, 3 Renée Heilbronner Oliver Plümper Rüdiger Kilian 
3 Géodynamique - UMR7327
BRGM - Bureau de Recherches Géologiques et Minières (BRGM), ISTO - Institut des Sciences de la Terre d'Orléans - UMR7327 : UMR7327, INSU - CNRS - Institut national des sciences de l'Univers, UO - Université d'Orléans : UMR7327
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.
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Sina Marti, Holger Stünitz, Renée Heilbronner, Oliver Plümper, Rüdiger Kilian. Syn-kinematic hydration reactions, grain size reduction, and dissolution-precipitation creep in experimentally deformed plagioclase-pyroxene mixtures. Solid Earth, European Geosciences Union, 2018, 9, pp.985-1009. ⟨10.5194/se-9-985-2018⟩. ⟨insu-03595903⟩

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