The microstructural record of porphyroclasts and matrix of partly serpentinized peridotite mylonites – from brittle and crystal-plastic deformation to dissolution–precipitation creep

Abstract

Abstract. We present microfabrics in high-pressure, metamorphic, partly serpentinized peridotite mylonites from the Voltri Massif, in which porphyroclasts and matrix record independent deformation events. The microfabrics are analysed using polarization microscopy and electron microscopy (SEM/EBSD, EMP). The mylonites contain diopside and olivine porphyroclasts originating from the mantle protolith embedded in a fine-grained matrix consisting mainly of antigorite and minor olivine and pyroxene. The porphyroclasts record brittle and crystal-plastic deformation of the peridotite at upper-mantle conditions and differential stresses of a few hundred MPa. After the peridotites became serpentinized, deformation occurred mainly by dissolutionprecipitation creep resulting in a pronounced foliation of the antigorite matrix, crenulation cleavages and newly precipitated olivine and pyroxene from the pore fluid at sites of dilation, i.e. in strain shadows next to porphyroclasts and folded fine-grained antigorite layers. Antigorite reveals a pronounced associated shape preferred orientation (SPO) and crystallographic preferred orientation (CPO) with the basal (001) cleavage plane oriented in the foliation plane. In monomineralic antigorite aggregates at sites of stress concentration around porphyroclasts, a characteristically reduced grain size and deflecting CPO as well as sutured grain boundaries indicate also some contribution of crystal-plastic deformation and grain-boundary migration of antigorite. In contrast, the absence of any intragranular deformation features in newly precipitated olivine in strain shadows reveals that stresses were not sufficiently high to allow for significant dislocation creep of olivine at conditions at which antigorite is stable. The porphyroclast microstructures are not associated with the microstructures of the mylonitic matrix, but are inherited from an independent earlier deformation. The porphyroclasts record a high-stress deformation of the peridotite with dislocation creep of olivine in the upper mantle probably related to rifting processes, whereas the serpentinite matrix records dominantly dissolution-precipitation creep and low stresses during subduction and exhumation.

Mineral Dissolution and Precipitation Under Stress: Model Formulation and Application to Metamorphic Reactions

“…The acquisition of a GSPO during dissolution-precipitation requires the presence of fluid serving as a transport medium for the dissolved minerals. GSPO is observed here in volatile-rich rocks (Bestmann et al, 2004;Bial & Trepmann, 2013;Imon et al, 2004;Stokes et al, 2012;Wassmann et al, 2011;Wassmann & Stöckhert, 2013a) and could be used as a marker for fluid presence during metamorphism. In addition, our numerical simulations clearly show that GSPO can only be formed near thermodynamic equilibrium.…”

“…The acquisition of GSPO during dissolution‐precipitation requires mass redistribution from highly to low stressed surfaces. It was recognized in a variety of rocks including marble (Bestmann et al., 2004), amphibolite (Giuntoli et al., 2018; Imon et al., 2004; Stokes et al., 2012), serpentinite (Bial & Trepmann, 2013; Wassmann et al., 2011) and micaschist (Wassmann & Stöckhert, 2013a). However, other processes can lead to the acquisition of GSPO (Awad et al., 2000; Brady, 1983; Kretz, 1966; Means & Paterson, 1966; Willis, 1977).…”