[en] Ordering of Fe³⁺ and Fe²⁺ between octahedral (Oh) and tetrahedral (Td) sites in synthetic members of the magnetite (Fe₃O₄) - ulvoespinel (Fe₂TiO₄) solid-solution series was determined using Fe L_2,3-edge X-ray magnetic circular dichroism (XMCD) coupled with electron microprobe and chemical analysis, Ti L-edge spectroscopy, Fe K-edge EXAFS and XANES, Fe₅₇ Moessbauer spectroscopy, and unit cell parameters. Microprobe analysis, cell edges and chemical FeO determinations showed that the bulk compositions of the samples were stoichiometric magnetite-ulvoespinel solid-solutions. Surface sensitive XMCD showed that the surfaces of these oxide minerals were more sensitive to redox conditions and some samples required re-equilibration with suitable solid-solid buffers. Detailed site-occupancy analysis of these samples gave XMCD-Fe²⁺/Fe³⁺ ratios very close to stoichiometric values. L_2,3-edge spectroscopy showed that Ti⁴⁺ was restricted to Oh sites. XMCD results showed that significant Fe²⁺ only entered Td when the Ti content was > 0.40 apfu while Fe²⁺ in Oh increased from 1 a.p.f.u in magnetite to a maximum of ∼1.4 apfu in USP45. As the Ti content increased from this point, the steady increase in Fe²⁺ in Td sites was clearly observable in the XMCD spectra, concurrent with a slow decrease in Fe²⁺ in Oh sites. Calculated magnetic moments showed a steady decrease from magnetite (4.06 μ_B) to USP45 (1.5 μ_B) and then a slower decrease towards the value for ulvoespinel (0 μ_B). Two of the synthesized samples were also partially maghemitized by re-equilibrating with an oxidizing Ni-NiO buffer and XMCD showed that Fe²⁺ oxidation only occurred at Oh sites, with concomitant vacancy formation restricted to this site. This study shows the advantage of using XMCD as a direct measurement of Fe oxidation state in these complex magnetic spinels. These results can be used to rationalize the magnetic properties of titanomagnetites, and their oxidized titanomaghemitized analogues, in Earth's crustal rocks.

[en] Despite the abundance of garnet in deforming regions of the Earth, such as subduction zones, its rheological properties are not well defined by laboratory measurements. Here we report measurements of steady-state plastic properties of pyrope in its stability field (temperature up to 1573 K, pressure up to 6.8 GPa, strain rate ∼10-5 s-1) using a Deformation-DIA apparatus (D-DIA) coupled with synchrotron radiation. Synthetic pyrope (Py100) and natural pyrope (Py70Alm16Gr14) are both studied in a dry environment. Transmission electron microscopy (TEM) investigation of the run products indicates that dislocation glide, assisted by climb within grains and dynamic recrystallization for grain-boundary strain accommodation, is the dominant deformation process in pyrope. Both synthetic-and natural-pyropes' stress and strain-rate data, as measured in situ by X-ray diffraction and imaging, are best fitted with the single flow law:

[en] Pressure-volume-temperature data have been obtained for CaGeO3 perovskite up to 9.6 GPa and 1100 K using a cubic anvil, DIA-type high-pressure apparatus in conjunction with synchrotron X-ray diffraction. The data were analyzed using Birch-Murnaghan equation of state and thermal pressure approach with the bulk modulus at ambient pressure, KT0, and its pressure derivative, K'T0, constrained by previous measurements. A fit of the unit-cell volume data to the high-temperature Birch-Murnaghan (HTBM) equation of state gives (_partialKT/_partialT)P = -0.025 ± 0.015 GPa/K, a = 1.047 ± 0.356 x 10-5/K, and b = 3.282 ± 0.735 x 10-8/K2 for the thermal expansion α expressed by a + bT. The thermal pressure approach yields αKT = 4.04 ± 0.37 x 10-3 GPa/K and (_partial2P/_partialT2)V = 6.17 ± 1.28 x 10-6 GPa/K2. The energy dispersive X-ray diffraction data reveal no indication of a structural phase transition over the P-T range of the current experiment. A systematic relationship, KS0 = 6720/V(molar) - 13.07 GPa, has been established based on these isostructural analogues, which predicts KS0 = 261(15) for MgSiO3 perovskite and 225(8) for CaSiO3 perovskite, respectively.

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[en] Residual elastic strain in naturally deformed, quartz-containing rocks can be measured quantitatively in a petrographic thin section with high spatial resolution using Laue microdiffraction with white synchrotron x-rays. The measurements with a resolution of one micrometer allow the quantitative determination of the deviatoric strain tensor as a function of position within the crystal investigated. The observed equivalent strain values of 800-1200 microstrains represent a lower bound of the actual preserved residual strain in the rock, since the stress component perpendicular to the cut sample surface plane is released. The measured equivalent strain translates into an equivalent stress in the order of ∼ 50 MPa

[en] Deformation experiments were carried out in a Deformation-DIA high-pressure apparatus (D-DIA) on oriented Mg2SiO4 olivine (Fo100) single crystals, at pressure (P) ranging from 2.1 to 7.5 GPa, in the temperature (T) range 1373-1677 K, and in dry conditions. These experiments were designed to investigate the effect of pressure on olivine dislocation slip-system activities, responsible for the lattice-preferred orientations observed in the upper mantle. Two compression directions were tested, promoting either [100] slip alone or [001] slip alone in (010) crystallographic plane. Constant applied stress (σ) and specimen strain rates (Formula) were monitored in situ using time-resolved X-ray synchrotron diffraction and radiography, respectively. Transmission electron microscopy (TEM) investigation of the run products reveals that dislocation creep assisted by dislocation climb and cross slip was responsible for sample deformation. A slip transition with increasing pressure, from a dominant [100]-slip to a dominant [001]-slip, is documented. Extrapolation of the obtained rheological laws to upper-mantle P, T, and σ conditions, suggests that [001]-slip activity becomes comparable to [100]-slip activity in the deep upper mantle, while [001] slip is mostly dominant in subduction zones. These results provide alternative explanations for the seismic anisotropy attenuation observed in the upper mantle, and for the 'puzzling' seismic-anisotropy anomalies commonly observed in subduction zones.

[en] The structural behavior of a cancrinite, Na_5.96Ca_1.52[Al₆Si₆O₂₄](CO₃)_1.57·1.75H₂O, was determined by using in situ synchrotron X-ray powder diffraction data [λ = 0.91806(5) (angstrom)] at room pressure and from 25 to 982 ^oC. The sample was heated at a rate of about 9.5 ^oC/min, and X-ray traces were collected at about 15 ^oC intervals. The satellite reflections in cancrinite were lost at about 504 ^oC, where a phase transition occurs. All the unit-cell parameters for cancrinite also show a discontinuity at 504 ^oC. Initially, the [Ca·CO₃] clusters and their vacancies are ordered in the channels, and this ordering is destroyed on heating to give rise to the phase transition. Cancrinite loses water continuously until about 625 ^oC; thereafter an anhydrous cancrinite phase exists. From 25 to 952 ^oC, a minimal amount of CO₂ is lost from the structure. Over this temperature range, the average < Al-O-Si> bridging angle, which is an indication of the degree of rotation of the tetrahedra, increases from 143.7(4) to 147.7(5)^o. Rotations of the tetrahedra are caused by expansion of the Na1-O2 bond lengths

[en] We appreciate the opportunity to respond to the comment by Nasdala (2009) concerning our interpretation of infrared spectra used to investigate the change in the structure of Pb-irradiated zircon as a function of increasing fluence (Zhang et al. 2008a, 2008b). Nasdala is correct in cautioning experimentalists to carefully match the analytical technique to the expected irradiation damage profile in order to optimally probe the irradiation effects, and in fact, this point was emphasize by Ewing et al. (2003) in a review of radiation effects in zircon. However, Nasdala's discussion fails to fully appreciate three important points: (1) There is a difference between in situ irradiations of TEM samples that must be electron transparent, ∼200 nm thick, as were completed by Weber et al. (1994), and more bulk-like irradiations that were completed in the Zhang et al. (2008a and 2008b) studies; (2) The particle-solid interactions change along the path of an implanted ion, that is the distribution and nature of the damage changes with depth as the ion loses energy, resulting in the greatest number of ballistic interactions near the end of the particle trajectory (see Figure 1 of Ewing et al. 2003); (3) In comparing natural zircon damaged by alpha-decay events with ion irradiated zircon, one must be aware that the recoil nucleus and the alpha particle cause different types of damage, and the use of the Pb-implantation experiment is meant to simulate only the alpha-recoil damage.