Crystallographic and Seismic Anisotropies of Calcite at Different Depths: A Study Using Quantitative Texture Analysis by Neutron Diffraction

Michele Zucali,Daniel Chateigner,And Bachir Ouladdiaf

doi:10.3390/min10010026

Michele Zucali, Daniel Chateigner + Show 1 more

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https://doi.org/10.3390/min10010026

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Journal: Minerals	Publication Date: Dec 27, 2019
Citations: 4	License type: CC BY 4.0

Affiliation: University of Milan, Institut Laue-Langevin

Abstract

Eight samples of limestones and marbles were studied by neutron diffraction to collect quantitative texture (i.e., crystallographic preferred orientations or CPO) of calcite deforming at different depths in the crust. We studied the different Texture patterns developed in shear zones at different depth and their influence on seismic anisotropies. Samples were collected in the French and Italian Alps, Apennines, and Paleozoic Sardinian basement. They are characterized by isotropic to highly anisotropic (e.g., mylonite shear zone) fabrics. Mylonite limestones occur as shear zone horizons within the Cenozoic Southern Domain in Alpine thrust-and-fold belts (Italy), the Briançonnais domain of the Western Alps (Italy-France border), the Sardinian Paleozoic back-thrusts, or in the Austroalpine intermediate units. The analyzed marbles were collected in the Carrara Marble, in the Austroalpine Units in the Central (Mortirolo) and Western Alps (Valpelline). The temperature and depth of development of fabrics vary from <100 ∘ C, to 800 ∘ C and depth from <10 km to about 30 km, corresponding from upper to lower crust conditions. Quantitative Texture Analysis shows different types of patterns for calcite: random to strongly textured. Textured types may be further separated in orthorhombic and monoclinic (Types A and B), based on the angle defined with the mesoscopic fabrics. Seismic anisotropies were calculated by homogenizing the single-crystal elastic tensor, using the Orientation Distribution Function calculated by Quantitative Texture Analysis. The resulting P- and S-wave anisotropies show a wide variability due to the textural types, temperature and pressure conditions, and dip of the shear planes.

Highlights

The quantification of the seismic response of rocks is a fundamental task in understanding Earth’s structure, from the core to the surface [1]
The interpretation of seismic images strongly relies on the knowledge of the seismic response of the anisotropic aggregates of minerals composing the rocks, which in turn is intimately related to the textures of rocks [4], as well as on their extrinsic shape preferred features [5]
In the last two decades, a great effort has been made to quantify the seismic response of natural aggregates, by using the 2-Dimensional approach of the EBSD (Electron Back Scattered Diffraction) to reconstruct the Orientation Distribution Function

Summary

Introduction

The quantification of the seismic response of rocks is a fundamental task in understanding Earth’s structure, from the core to the surface [1]. The large-scale seismic experiments, with high-resolution arrays, is the most used approach to image the Earth and to resolve the distribution of natural resources in-depth, e.g., water, ore minerals, and oil & gas [2,3]. In the last two decades, a great effort has been made to quantify the seismic response of natural aggregates, by using the 2-Dimensional approach of the EBSD (Electron Back Scattered Diffraction) to reconstruct the Orientation Distribution Function. Carbonate rocks occur at the Earth’s surface and upper crust levels, as sedimentary cover, but can be found within the intermediate and lower crust, as marbles. The tectonic deformation often localizes within carbonate rocks, producing thrust-folds systems [9,10]

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