Crystallographic preferred orientation, magnetic and seismic anisotropy in rocks from the Finero peridotite, Ivrea-Verbano Zone, Northern Italy – Interplay of anisotropy contributions from different minerals

Abstract

Mineral alignment can provide valuable information on a rock's geological history. Many structural, tectonic, or geodynamic studies rely on determining the preferred orientation of rock-forming minerals, either by direct texture determination or using anisotropy measurements as proxies. Robust interpretations of anisotropy require detailed understanding of its relationship with texture. This study investigates the texture, magnetic and seismic anisotropies of ultramafic rocks from the Ivrea-Verbano Zone (IVZ), Northern Italy, with a special focus on the interplay between anisotropy contributions carried by different minerals. Texture was obtained from electron backscatter diffraction, magnetic anisotropy from low- and high-field anisotropy of susceptibility measurements, and seismic anisotropy from P- and S-wave velocities along the three principal directions of the macroscopic fabric, using 22 mm diameter cores. Texture-based models of magnetic anisotropy agree well with corresponding measurements. Larger variability is observed for seismic anisotropy. Possible explanations are (1) velocities were measured in three directions, insufficient to determine the full elasticity tensor, and (2) seismic anisotropy depends on grain boundaries, pores, and cracks in addition to crystallographic preferred orientation. The contributions to magnetic and seismic anisotropy of each constituent mineral can interfere positively, leading to a larger overall anisotropy, or negatively, resulting in a weaker anisotropy. Seismic anisotropy is mostly controlled by olivine, whereas magnetic anisotropy can be dominated by olivine, pyroxene, or hornblende. This study illustrates that, although the relationship between texture and anisotropy can be complex, it is possible to quantitatively predict magnetic, and to a lesser degree seismic, anisotropy from texture data. Even though both magnetic and seismic anisotropies are common proxies for texture, they are not coaxial, and no simple relationship between the degree of magnetic and seismic anisotropy was found. These results underline the importance of understanding different minerals' contributions to anisotropy in upper mantle rocks.