The magnetic properties of natural and synthetic (Fe_x, Mg_{1 - x})₂ SiO₄ olivines

Olivine is an abundant orthosilicate in the solar system, in the upper mantle of rocky planets, in meteorites and interstellar dust. The magnetic properties of (Fe_x, Mg_{1 - x})₂ SiO₄ olivines result from the silicate matrix and its iron-rich inclusions and have not always been separated in previous studies. The properties of the matrix are important to understand mantle rocks' anisotropy and their deformation, both in xenoliths and peridotite massifs, while the inclusions are potential paleomagnetic and paleointensity recorders. In this study, we performed new measurements on 7 natural and 23 synthetic ferromagnesian olivines, covering the whole range from forsterite Mg₂SiO₄ (Fo₁₀₀) to fayalite Fe₂SiO₄ (Fo₀) and from 4 to 310 K. Many of our specimens contain ferromagnetic inclusions (magnetite or maghemite), with magnetic sizes ranging from superparamagnetic to multidomain. The respective contributions of the matrix and the inclusions are systematically isolated using magnetic fields large enough to saturate the ferromagnetic component due to inclusions. At room temperature, as predicted by molecular field theory, while the forsterite end-member is diamagnetic (X_HF = - 6.8 10^{- 10} m³/kg) and the fayalite end-member is paramagnetic (X_HF = 1.10 10^{- 6} m³/kg), their magnetic properties do not vary linearly with iron content. These olivines also exhibit one or two magnetic transitions at composition-dependent low temperatures. At the Néel temperature (T_N), olivines exhibit a first magnetic transition from paramagnetic to antiferromagnetic behavior, indicated by negative paramagnetic Curie temperatures (θ). A second transition (T_t) occurs at lower temperatures for specimens with x ≥ 0.1, and could be attributed to a change from collinear to canted antiferromagnetic state. In the range Fo₈₃-Fo₉₃, corresponding to the Earth's upper mantle, the anisotropy of magnetic susceptibility (AMS) is directionally consistent, with the AMS principal axes K₁, K₂ and K₃ respectively corresponding to the a, c and b crystallographic axes. At room temperature, the degree of anisotropy increases from 1.028 to 1.302 with decreasing iron content. The presence of small and submicroscopic iron-rich inclusions significantly complicates the investigation of olivine physical properties but also constitutes opportunities to record paleomagnetic field intensities.