Seismic anisotropy of serpentinite from Val Malenco, Italy

Kern, H.; Lokajı́ček, T.; Svítek, T.; Wenk, Hans‐Rudolf

doi:10.1002/2015jb012030

Cited by 14 publications

(5 citation statements)

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“…10, V S values obtained in X, Y, Z directions on the cube and on the sphere at a pressure of 100 MPa are equal within experimental error margins. This is a major improvement compared to earlier work of Kern et al (2015) who detected significant differences between shear wave velocities measured on cubic and spherical samples.…”

Section: Comparison Of Two Ultrasonic Experimentsmentioning

confidence: 49%

“…Here we follow up on previous work where elastic wave velocities have been measured on spheres and cubes (e.g. Lokajíček et al 2014;Kern et al 2015), with the possibility to derive complete elastic properties of the rock, estimate pore distribution and compare these data with models based on microstructural information. Much emphasis has been on studying anisotropy in shales (e.g.…”

Section: Introductionmentioning

confidence: 99%

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Elastic anisotropy of Tambo gneiss from Promontogno, Switzerland: a comparison of crystal orientation and microstructure-based modeling and experimental measurements

et al. 2017

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Felsic and mafic gneisses constitute large proportions of the upper and lower continental crust. Gneisses often display high anisotropy of elastic properties associated with preferred orientations of sheet silicates. Here we study the elastic anisotropy of a sample of Tambo gneiss from Promontogno in the Central Alps. We apply optical microscopy, time-of-flight neutron diffraction, neutron and X-ray tomography to quantify mineral composition and microstructures and use them to construct self-consistent models of elastic properties. They are compared to results of ultrasonic measurements on a cube sample in a multi-anvil apparatus and on a spherical sample in an apparatus that can measure velocities in multiple directions. Both methods provide similar results. It is shown that models of microstructure-derived elastic properties provide a good match with ultrasonic experiment results at pressures above 100 MPa. At a pressure of 0.1 MPa the correspondence between the model and the experiment is worse. This may be caused by an oversimplification of the model with respect to microfractures or uncertainties in the experimental determination of S-wave velocities and elastic tensor inversion. The study provides a basis to determine anisotropic elastic properties of rocks either by ultrasonic experiments or quantitative models based on microstructures. This information can then be used for interpretation of seismic data of the crust.

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Section: Comparison Of Two Ultrasonic Experimentsmentioning

confidence: 49%

Section: Introductionmentioning

confidence: 99%

Elastic anisotropy of Tambo gneiss from Promontogno, Switzerland: a comparison of crystal orientation and microstructure-based modeling and experimental measurements

et al. 2017

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“…However, most often both [100] Atg and [010] Atg are dispersed in a girdle in the foliation plane of the serpentinites, with no strong point maximum of [010] Atg ( Fig. 10; Padró n-Navarta et al, 2012; Soda & Wenk, 2014;Kern et al, 2015;Dilissen et al, 2018). Furthermore, when SPO data for magnetite are available (Dilissen et al, 2018) there is no clear correlation between the concentrations of [100] Atg or [010] Atg and the lineation defined by magnetite aggregates.…”

Section: Implications For Dehydration Reactions and Fluid Transport Imentioning

confidence: 99%

Interplay between Fluid Extraction Mechanisms and Antigorite Dehydration Reactions (Val Malenco, Italian Alps)

Clément

Padrón‐Navarta

Tommasi

2019

Journal of Petrology

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We investigate the feedback between antigorite dehydration reactions and fluid extraction mechanisms through the analysis of field-scale structures and microtextures formed by antigorite dehydration reactions producing olivine and talc in the Bergell intrusion contact aureole at Alpe Zocca (Malenco Unit, Northern Italy). The reactions, which resulted in the replacement of serpentinites by metaperidotites, occurred under quasi-static conditions. The main reaction front, which defines the equilibrium isograd, is an ∼150 m wide zone composed of variably reacted rocks with an irregular distribution at scales ranging from a few centimeters to a few meters. Veins composed of the prograde mineral assemblage occur downstream (<100 m) of this front. They are often surrounded by centimeter- to decimeter-scale dehydration reaction zones that propagate into the serpentinite wall-rock. Olivine in the metaperidotites and partially reacted serpentinites has a crystallographic preferred orientation (CPO) correlated with the antigorite CPO, with [010]Ol axes parallel to [001]Atg. This CPO is accompanied by a shape-preferred orientation (SPO) that marks the foliation in both rock types. Olivine crystals in Ol–Tlc dehydration veins also have SPO and CPO that define a jackstraw texture within the plane of the vein. They are elongated parallel to [001] within the vein plane and have their [010]Ol axes normal to the vein. We interpret the olivine–talc assemblage in the veins as resulting from dehydration reactions at lower temperature than the equilibrium conditions owing to fluid extraction from the wall-rock into the veins. The jackstraw texture indicates fast kinetics, with the crystal orientation controlled by anisotropic growth under a fluid pressure gradient. We interpret the foliated metaperidotites as formed at near equilibrium conditions, with pervasive fluid extraction from the metaperidotite by viscous metamorphic compaction. Olivine CPO in these rocks may result from topotaxy, oriented growth in the presence of an anisotropic (foliation-controlled) fluid flow, and/or solid reorientation of the anisometric olivine crystals accommodated by the deformation of the weaker talc (± chlorite) matrix during compaction.

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“…L‐type CPO is featured by [010] axes pointing toward the lineation and [001] and [100] axes forming girdles perpendicular to the lineation (Figure 1d). While LS‐b‐type (Hirauchi, Michibayashi, et al, 2010; Horn et al, 2020; Jung, 2011; Nishii et al, 2011; Shao et al, 2014; Soda & Wenk, 2014; Watanabe et al, 2011), S‐type (Brownlee et al, 2013; Horn et al, 2020; Jung, 2011; Kern et al, 2015; Morales et al, 2013; Shao et al, 2014; Vogler, 1987), and L‐type CPOs (Brownlee et al, 2013; Horn et al, 2020; Kern et al, 2015; Nishii et al, 2011; Shao et al, 2014; Soda & Takagi, 2010; Watanabe et al, 2011, 2014) were frequently found in natural antigorite‐bearing rocks, the LS‐a‐type CPO of antigorite has only been observed by a handful of deformation experiments (Katayama et al, 2009) and natural rock studies (Bezacier, Reynard, Bass, Sanchez‐Valle, et al, 2010; Dilissen et al, 2018; Morales et al, 2018; Padrón‐Navarta et al, 2012; Van de Moortèle et al, 2010).…”

Section: Introductionmentioning

confidence: 99%

Geneses of Two Contrasting Antigorite Crystal Preferred Orientations and Their Implications for Seismic Anisotropy in the Forearc Mantle

et al. 2020

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Crystal preferred orientation (CPO) of antigorite is an important parameter for the interpretation of seismic anisotropies and related geodynamic processes in subduction zones. However, mechanisms for the development of various antigorite CPOs and their effects on seismic anisotropies in the forearc mantle still remain to be constrained. In this study, we investigated olivine and antigorite CPOs and calculated seismic anisotropies of the serpentinized peridotites from Val Malenco, Central Alps. The results show that antigorite in the olivine-rich (weakly serpentinized) layer displays an L-type CPO (i.e., (h0l) plane//foliation and [010] axis//lineation), whereas antigorite in the antigorite-rich (highly serpentinized) layer develops an LS-b-type CPO (i.e., (001) plane//foliation and [010] axis//lineation). The antigorite L-type CPO is most likely formed by diffusion and dislocation creep accommodated phase boundary sliding and substantial grain rotation at a higher temperature and smaller strain regime. In contrast, the development of an antigorite LS-b-type CPO requires diffusion and dislocation creep accommodated grain boundary sliding, smaller contribution of grain rotation, and significant role of dissolution-precipitation creep at a lower temperature and larger strain regime. In this context, we propose an antigorite CPO distribution model in the forearc mantle. Based on this model and under vertically incident teleseismic waves, if significant serpentinization and asymmetric shearing do indeed occur within serpentinized layers, then strong and weak trench-parallel seismic anisotropies can be expected for cold and warm moderate-angle subduction zones, respectively. This model may provide an alternative interpretation on the seismic anisotropy observations in some modern subduction systems. Plain Language Summary Antigorite is an important hydrous silicate mineral commonly present in deep subduction zone and thus contains important information concerning subduction dynamics. Previous studies have suggested that antigorite aggregates can be deformed ductilely and exhibit various types of crystal preferred orientations (CPOs)-alignments of crystallographic axes and planes along preferential directions in a structural reference frame-generally via the mechanism of dislocation creep. However, based on our natural sample study, we discovered that at least two types of antigorite CPO (i.e., L and LS-b types) can develop dominantly by the mechanisms such as grain and phase boundary sliding, grain rotation, and dissolution-precipitation creep. Combined with their hydration, temperature, and strain conditions, we conceived a model of antigorite CPO distributions in the forearc mantle at subduction zones with different thermal states and subduction angles. This model may provide an alternative interpretation on the observed strong and weak trench-parallel seismic anisotropies (i.e., fast shear wave polarizing parallel to the strike of trench) in some modern cold (e.g., Tonga, Aleutians, Izu-Bonin, and Ryukyu) and warm ...

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Seismic anisotropy of serpentinite from Val Malenco, Italy

Cited by 14 publications

References 62 publications

Elastic anisotropy of Tambo gneiss from Promontogno, Switzerland: a comparison of crystal orientation and microstructure-based modeling and experimental measurements

Elastic anisotropy of Tambo gneiss from Promontogno, Switzerland: a comparison of crystal orientation and microstructure-based modeling and experimental measurements

Interplay between Fluid Extraction Mechanisms and Antigorite Dehydration Reactions (Val Malenco, Italian Alps)

Geneses of Two Contrasting Antigorite Crystal Preferred Orientations and Their Implications for Seismic Anisotropy in the Forearc Mantle

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