Effect of redox on Fe–Mg–Mn exchange between olivine and melt and an oxybarometer for basalts

The Fe–Mg exchange coefficient between olivine (ol) and melt (m), defined as $${\text{Kd}}_{{{\text{Fe}}^{T} {-} {\text{Mg}}}}$$ Kd Fe T - Mg = (Feol/Fem)·(Mgm/Mgol), with all FeT expressed as Fe2+, is one of the most widely used parameters in petrology. We explore the effect of redox conditions on $${\text{Kd}}_{{{\text{Fe}}^{T} {-} {\text{Mg}}}}$$ Kd Fe T - Mg using experimental, olivine-saturated basaltic glasses with variable H2O (≤ 7 wt%) over a wide range of fO2 (iron-wüstite buffer to air), pressure (≤ 1.7 GPa), temperature (1025–1425 °C) and melt composition. The ratio of Fe3+ to total Fe (Fe3+/∑Fe), as determined by Fe K-edge µXANES and/or Synchrotron Mössbauer Source (SMS) spectroscopy, lies in the range 0–0.84. Measured Fe3+/∑Fe is consistent (± 0.05) with published algorithms and appears insensitive to dissolved H2O. Combining our new data with published experimental data having measured glass Fe3+/∑Fe, we show that for Fo65–98 olivine in equilibrium with basaltic and basaltic andesite melts, $${\text{Kd}}_{{{\text{Fe}}^{T} {-} {\text{Mg}}}}$$ Kd Fe T - Mg decreases linearly with Fe3+/∑Fe with a slope and intercept of 0.3135 ± 0.0011. After accounting for non-ideal mixing of forsterite and fayalite in olivine, using a symmetrical regular solution model, the slope and intercept become 0.3642 ± 0.0011. This is the value at Fo50 olivine; at higher and lower Fo the value will be reduced by an amount related to olivine non-ideality. Our approach provides a straightforward means to determine Fe3+/∑Fe in olivine-bearing experimental melts, from which fO2 can be calculated. In contrast to $${\text{Kd}}_{{{\text{Fe}}^{T} {-} {\text{Mg}}}}$$ Kd Fe T - Mg , the Mn–Mg exchange coefficient, $${\text{Kd}}_{{{\text{Mn}} {-} {\text{Mg}}}}$$ Kd Mn - Mg , is relatively constant over a wide range of P–T–fO2 conditions. We present an expression for $${\text{Kd}}_{{{\text{Mn}} {-} {\text{Mg}}}}$$ Kd Mn - Mg that incorporates the effects of temperature and olivine composition using the lattice strain model. By applying our experimentally-calibrated expressions for $${\text{Kd}}_{{{\text{Fe}}^{T} {-} {\text{Mg}}}}$$ Kd Fe T - Mg and $${\text{Kd}}_{{{\text{Mn}} {-} {\text{Mg}}}}$$ Kd Mn - Mg to olivine-hosted melt inclusions analysed by electron microprobe it is possible to correct simultaneously for post-entrapment crystallisation (or dissolution) and calculate melt Fe3+/∑Fe to a precision of ≤ 0.04.

Pressure Induced Spin Crossover and Magnetic Properties of Multiferroic Ba3NbFe3Si2O14

Recently, the iron containing langasite-type crystal Ba3NbFe3Si2O14 has attracted great attention as a new magnetically induced multiferroic. In this work, magnetic, structural and electronic properties of the multiferroic Ba3NbFe3Si2O14 were investigated by several methods, including synchrotron X-ray diffraction, Raman spectroscopy and synchrotron Mössbauer source technique at high quasi-hydrostatic pressures (up to 70 GPa), created in diamond anvil cells. At room temperature, two structural transitions at pressures of about 3.0 and 17.5 GPa were detected. Mössbauer studies at high pressures revealed a radical change in the magnetic properties during structural transitions. At pressures above 18 GPa, the crystal transforms into two magnetic fractions, and in one of them the Néel temperature (TN) increases by about four times compared with the TN value in the initial phase (from 27 to 115 K). At pressures above 50 GPa, a spin crossover occurs when the fraction of iron Fe3+ ions in oxygen octahedra transits from the high-spin (HS, S = 5/2) to the low-spin (LS, S = 1/2) state. This leads to a new change in the magnetic properties. The magnetic ordering temperature of the LS sublattice was found to be of about 22(1) K, and magnetic correlations between HS and LS sublattices were studied.

The validity of an experiment testing the influence of acceleration on time dilation using a rotating Mössbauer absorber and a Synchrotron Mössbauer Source

Three experiments are reviewed, performed (in 2014–2016) at ID18 of ESRF to measure the influence of acceleration on time dilation by measuring the relative shift between the absorption lines of two states of the same rotating absorber with accelerations anti-parallel and parallel to the incident beam. Statistically significant data for rotation frequencies up to 510 Hz in both directions of rotation were collected. For each run with high rotation, a stable statistically significant `vibration-free' relative shift between the absorption lines of the two states was measured. This may indicate the influence of acceleration on time dilation. However, the measured relative shift was also affected by the use of a slit necessary to focus the beam to the axis of rotation to a focal spot of sub-micrometre size. The introduction of the slit broke the symmetry in the absorption lines due to the nuclear lighthouse effect and affected the measured relative shift, preventing to claim conclusively the influence of acceleration on time dilation. Assuming that this loss of symmetry is of first order, the zero value of the relative shift, corrected for this loss, falls always within the experimental error limits, as predicted by Einstein's clock hypothesis. The requirements and an indispensable plan for a conclusive experiment, once the improved technology becomes available, is presented. This will be useful to future experimentalists wishing to pursue this experiment or a related rotor experiment involving a Mössbauer absorber and a synchrotron Mössbauer source.

Polarization Analysis in Mössbauer Reflectometry with Synchrotron Mössbauer Source

Polarization selection of the reflected radiation has been employed in Mössbauer reflectivity measurements with a synchrotron Mössbauer source (SMS). The polarization of resonantly scattered radiation differs from the polarization of an incident wave so the Mössbauer reflectivity contains a scattering component with 90° rotated polarization relative to the π-polarization of the SMS for some hyperfine transitions. We have shown that the selection of this rotated π→σ component from total reflectivity gives an unusual angular dependence of reflectivity characterized by a peak near the critical angle of the total external reflection. In the case of collinear antiferromagnetic interlayer ordering, the “magnetic” maxima on the reflectivity angular curve are formed practically only by radiation with this rotated polarization. The first experiment on Mössbauer reflectivity with a selection of the rotated polarization discovers the predicted peak near the critical angle. The measurement of the rotated π→σ polarization component in Mössbauer reflectivity spectra excludes the interference with non-resonant electronic scattering and simplifies the spectrum shape near the critical angle allowing for an improved data interpretation in the case of poorly resolved spectra. It is shown that the selected component of Mössbauer reflectivity with rotated polarization is characterized by enhanced surface sensitivity, determined by the “squared standing waves” depth dependence. Therefore, the new approach has interesting perspectives for investigations of surfaces, ultrathin layers and multilayers having complicated magnetic structures.

Nuclear resonance reflectivity from a [57Fe/Cr]30multilayer with the Synchrotron Mössbauer Source

Mössbauer reflectivity spectra and nuclear resonance reflectivity (NRR) curves have been measured using the Synchrotron Mössbauer Source (SMS) for a [57Fe/Cr]30periodic multilayer, characterized by the antiferromagnetic interlayer coupling between adjacent57Fe layers. Specific features of the Mössbauer reflectivity spectra measured with π-polarized radiation of the SMS near the critical angle and at the `magnetic' maximum on the NRR curve are analyzed. The variation of the ratio of lines in the Mössbauer reflectivity spectra and the change of the intensity of the `magnetic' maximum under an applied external field has been used to reveal the transformation of the magnetic alignment in the investigated multilayer.

Advances in testing the effect of acceleration on time dilation using a synchrotron Mössbauer source

New results, additional techniques and know-how acquired, developed and employed in a recent HC-1898 experiment at the Nuclear Resonance Beamline ID18 of ESRF are presented, in the quest to explore the acceleration effect on time dilation. Using the specially modified Synchrotron Mössbauer Source and KB-optics together with a rotating single-line semicircular Mössbauer absorber on the rim of a specially designed rotating disk, the aim was to measure the relative spectral shift between the spectra of two states when the acceleration of the absorber is anti-parallel and parallel to the source. A control system was used for the first time and a method to quantify the effects of non-random vibrations on the spectral shift was developed. For several runs where the effect of these vibrations was negligible, a stable statistically significant non-zero relative shift was observed. This suggests the influence of acceleration on time.