ALBERT

Description: Aluminous bridgmanite (Al-Bm) is the dominant phase in the Earth’s lower mantle. In this study, the Mössbauer spectra of an Al-Bm sample Mg 0.868 Fe 0.087 Si 0.944 Al 0.101 O 2.994 were recorded from 65 to 300 K at 1 bar. The temperature dependence of the center shift was fitted by the Debye model and yielded the Debye temperatures of 305 ± 3 K for Fe 2+ and 361 ± 22 K for Fe 3+ . These values are lower than those of Al-free bridgmanite by 17 and 24%, respectively, indicating that the presence of Fe and Al increases the average Fe-O bond length and weakens the bond strength. At 300 K, the calculated recoil-free fractions of Fe 2+ (0.637 ± 0.006) and Fe 3+ (0.72 ± 0.02) are similar and therefore the molar fractions of Fe 2+ and Fe 3+ are nearly the same as the area fractions of the corresponding Mössbauer doublets. At 900 K, the calculated recoil-free fractions of Fe 3+ is 46% higher than that of Fe 2+ , implying that the molar fraction of Fe 3+ is only 41% for a measured spectral area fraction of 50%, and that the area fractions of iron sites may change with temperature without any changes in the valence state or spin state of iron. We infer that Fe 3+ accounts for 46 ± 2% of the iron in the Al-Bm and it enters the A site along with Al 3+ in the B site through the coupled-substitution mechanism. An Fe 2+ component with large quadrupole splitting (~4.0 mm/s) was observed at cryogenic conditions and interpreted as a high-spin distorted iron site.

Description: Understanding carbon speciation in Earth materials is important to unravel the geochemical evolution of the Earth’s atmosphere, the composition of mantle partial melts, and the overall distribution of carbon in the deep mantle. In an effort to provide the systematic protocols to characterize carbon-bearing fluid inclusions and other carbon-bearing species using high-resolution 13 C solid-state NMR, one of the element-specific probes of local structure around carbon, we explore the atomic configurations around the carbon species formed during the reaction between 13 C-enriched amorphous carbon and MgSiO 3 enstatite synthesized at 1.5 GPa and 1400 °C using 13 C MAS NMR spectroscopy and Raman spectroscopy. The Raman spectra for the fluid inclusion show the presence of multiple molecular species (e.g., CO 2 , CO, CH 4 , H 2 O, and H 2 ) and reveal heterogeneous distribution of these species within the inclusion. 13 C MAS NMR results show that the sharp peak at 125.2 ppm is dominant. While the peak could be assigned to either molecular CO 2 in the fluid phase or fourfold-coordinated carbon ( [4] C), the peak is likely due to fluid CO 2 , as revealed by Raman analyses of micrometer-sized fluid inclusions in the sample. The peaks at 161.2, 170.9, and 173.3 ppm in the 13 C NMR spectrum correspond to the carbonate ions (CO 3 2– ) and additional small peak at 184.5 ppm can be attributed to carbon monoxide. Based on the established relationship between 13 C abundance and peak intensity in the 13 C MAS NMR, the estimated 13 C amounts of CO 2 , CO 3 2– , and CO species are much larger than those estimated from carbon solubility in the crystals, thus, indicating that those carbon species are from external phases. The 13 C NMR spectrum for amorphous carbon showed a peak shift from ~130 to ~95 ppm after compression, thereby suggesting that the amorphous carbon underwent permanent pressure-induced densification, characterized by the transition from sp 2 to sp 3 hybridization and/or pressure-induced changes in sp 2 carbon topology. While direct probing of carbon species in the crystalline lattice using NMR is challenging, the current results and method can be utilized to provide quantitative analysis of carbon-species in the fluid-inclusions in silicates, which is essential for understanding the deep carbon cycle and volcanic processes.

Description: Fe-S-P compounds have been observed in many meteorites and could be the important components in planetary cores. Here we investigated the phase stability of Fe 3 (S,P) solid solutions and synthesized high-quality Fe 3 (S 1–x P x ) high-pressure phases in the multi-anvil press. The physical properties of Fe 3 (S 0.5 P 0.5 ) were further studied in the diamond-anvil cell by synchrotron X-ray diffraction and emission spectroscopy. The solubility of S in the Fe 3 (S,P) solid solution increases with increasing pressure. The minimum pressure to synthesize the pure Fe 3 S and Fe 3 (S 0.13 P 0.87 ) is about 21 and 8 GPa, respectively. The observed discontinuity in unit-cell parameters at about 18 GPa is caused by the high-spin to low-spin transition of iron, supported by X-ray emission spectroscopy data. The sulfur solubility in Fe 3 (S,P) solid solutions could be an excellent pressure indicator if such solid solutions are found in nature.

Description: We have investigated the high-pressure behavior of Fe 3 O 4 by in situ X-ray diffraction measurements from 11 to 103 GPa. Up to 70 GPa, the previous observed high-pressure Fe 3 O 4 phase (h-Fe 3 O 4 ) is stable, with a CaTi 2 O 4 -type structure. The compression curve shows an abnormal volume contraction at about 50 GPa, likely associated with the magnetic moment collapse observed at that pressure. Fitting the compression data up to 45 GPa to the Birch-Murnaghan equation of state yields a bulk modulus, K T0 = 172 GPa, and V 0 = 277 Å 3 , with fixed K' = 4. At a pressure between 64 and 73 GPa, a new structural transition was observed in Fe 3 O 4 , which can be attributed to a martensitic transformation as described by Yamanaka et al. (2008) for post-spinel structural transition. The diffraction data can be best fitted with a Pnma space group. No breakdown of Fe 3 O 4 was observed up to at least 103 GPa. The new high-pressure polymorph is about 6% denser than the h-Fe 3 O 4 phase at 75 GPa.

Description: Icosahedrite, the first natural quasicrystal with composition Al 63 Cu 24 Fe 13 , was discovered in several grains of the Khatyrka meteorite, a CV3 carbonaceous chondrite. The presence of icosahedrite associated with high-pressure phases like ahrensite and stishovite indicates formation at high pressures and temperatures due to an impact-induced shock. Previous experimental studies on the stability of synthetic icosahedral AlCuFe have either been limited to ambient pressure, for which they indicate incongruent melting at ~1123 K, or limited to room-temperature, for which they indicate structural stability up to about 35 GPa. These data are insufficient to experimentally constrain the formation and stability of icosahedrite under the conditions of high pressure and temperature that formed the Khatyrka meteorite. Here we present the results of room-temperature, high-pressure diamond-anvil cells measurements of the compressional behavior of synthetic icosahedrite up to ~50 GPa. High P - T experiments were also carried out using both laser-heated diamond-anvil cells combined with in situ synchrotron X-ray diffraction (at ~42 GPa) and multi-anvil apparatus (at 21 GPa) to investigate the structural evolution and crystallization of possible coexisting phases. The results demonstrate that the quasiperiodic order of icosahedrite is retained over the P - T range explored. We find that pressure acts to stabilize the icosahedral symmetry at temperatures much higher than previously reported. Direct solidification of AlCuFe quasicrystals from an unusual Al-Cu-rich melt is possible but it is limited to a narrow temperature range. Alternatively, quasicrystals may form after crystallization through solid-solid reactions of Al-rich phases. In either case, our results show that quasicrystals can preserve their structure even after hypervelocity impacts spanning a broad range of pressures and temperatures.

Description: We have synthesized magnesium-iron silicate perovskites with the general formula Mg 1–x Fe 3+ x+y Si 1–y O 3 , in which the iron cation is exclusively trivalent. To investigate the crystal chemistry of Fe 3+ -bearing perovskite, six samples (both with and without Al) were analyzed using scanning electron microscopy, electron microprobe, X-ray diffraction, and Mössbauer spectroscopy. Results indicate that Fe 3+ substitutes significantly into both the octahedral and dodecahedral sites in the orthorhombic perovskite structure, but prefers the octahedral site at Fe 3+ concentrations between 0.04 and 0.05 Fe per formula unit, and the dodecahedral site at higher Fe 3+ concentrations. We propose a model in which Fe 3+ in the A/B site (in excess of that produced by charge coupled substitution) is accommodated by Mg/O vacancies. Hyperfine parameters refined from the Mössbauer spectra also indicate that a portion of dodecahedral sites undergo significant structural distortion. The presence of Fe 3+ in the perovskite structure increases the unit-cell volume substantially compared to either the Mg end-member, or Fe 2+ -bearing perovskite, and the addition of Al did not significantly alter the volume. Implications for increased compressibility and a partially suppressed spin transition of Fe 3+ in lower mantle perovskite are also discussed.

Description: The potassium (K) and water (H 2 O) cycles in subduction zones are predominately controlled by the stability of K- and H 2 O-bearing minerals, such as K-mica, lawsonite, and dense hydrous magnesium silicates (DHMS). K-micas (muscovite or phlogopite) are the principal H 2 O and K hosts in subduction zones and Earth’s upper mantle and play a significant role in the deep H 2 O and K cycles. The Mg-10 Å phase, normally appearing in hydrated peridotite in high-pressure experiments, has been considered as an important water-carrier in subducted hydrated peridotite. In this study, we found a K-bearing Al-10 Å phase in the MORB+H 2 O system (hydrated basalt) at high pressures according to X-ray diffraction and stoichiometry. We experimentally constrained its stability field at high pressure. By considering newly and previously documented compositions of the 10 Å phase and micas, we confirmed a continuous solid solution or mixed layering between the 10 Å phase and K-mica at the interlayer site, suggesting that the K cycle and the H 2 O cycle in subduction zones are coupled. From the discussion of the effect of f H 2 O on stability of the Al-10 Å phase, we conclude that a cold subduction zone can host and carry more bulk H 2 O and K into Earth’s deep mantle than a hot one. This work expands the stability regions of the 10 Å phase from the ultramafic system (Mg-10 Å phase) to the mafic system (Al-10 Å phase), and emphasizes the significance of the 10 Å phase for the deep H 2 O and K cycle in subduction zone.

Description: The stability field of siderite has been determined up to 10 GPa. Decarbonation of siderite occurs at pressures below 6 GPa with a Clapeyron slope of about 0.0082 GPa/K. At higher pressure, we observed direct melting of siderite without decarbonation. The melting temperature is about 1550 °C at 10 GPa. Our experimental results, compared with previous studies on the decomposition curve of magnesite, indicate that Fe has a significant effect on the stability of magnesite-siderite solid solutions under upper mantle conditions. The reaction products are strongly dependent on the oxygen fugacity of the system. The disproportionation reaction during decomposition of siderite might be an important mechanism to explain the stability of carbon as graphite (diamond) in the Earth’s mantle.