ALBERT

Description: Bartelkeite from Tsumeb, Namibia, was originally described by Keller et al. (1981) with the chemical formula PbFeGe 3 O 8 . By means of electron microprobe analysis, single-crystal X-ray diffraction, and Raman spectroscopy, we examined this mineral from the type locality. Our results show that bartelkeite is monoclinic with space group P 2 1 / m , unit-cell parameters a = 5.8279(2), b = 13.6150(4), c = 6.3097(2) Å, β = 127.314(2)°, and a revised ideal chemical formula PbFeGe VI Ge 2 IV O 7 (OH) 2 ·H 2 O ( Z = 2). Most remarkably, bartelkeite is isostructural with the high-pressure P 2 1 / m phase of lawsonite, CaAl 2 Si 2 O 7 (OH)·H 2 O, which is only stable above 8.6 GPa and a potential host for H 2 O in subducting slabs. Its structure consists of single chains of edge-sharing FeO 6 and Ge1O 6 octahedra parallel to the c -axis, cross-linked by Ge2 2 O 7 tetrahedral dimers. The average 〈Ge-O〉 bond lengths for the GeO 6 and GeO 4 polyhedra are 1.889 and 1.744 Å, respectively. The Pb atoms and H 2 O groups occupy large cavities within the framework. The hydrogen bonding scheme in bartelkeite is similar to that in lawsonite. Bartelkeite represents the first known mineral containing both 4- and 6-coordinated Ge atoms and may serve as an excellent analog for further exploration of the temperature-pressure-composition space of lawsonite.

Description: Aluminum hydroxysulfate, AlSO 4 (OH), is postulated to play a vital role in controlling the solubility of aluminum in sulfate-rich acidic soils and ground waters, but it has not yet been confirmed in nature. This study reports the synthesis of an AlSO 4 (OH) crystal at 700 °C and ~1.0 GPa in a hydrothermal diamond-anvil cell from a mixture of 95% H 2 SO 4 and Al 2 O 3 powder and its structure determination from single-crystal X-ray diffraction data. AlSO 4 (OH) is monoclinic with space group C 2/ c and unit-cell parameters a = 7.1110(4), b = 7.0311(5), c = 7.0088(4) Å, β = 119.281(2)°, and V = 305.65(3) Å 3 . Its crystal structure is characterized by kinked chains of corner-sharing AlO 6 octahedra that run parallel to the c -axis. These chains are linked together by SO 4 tetrahedra and hydrogen bonds, forming an octahedral-tetrahedral framework. Except for the numbers and positions of H atoms, AlSO 4 (OH) is isostructural with the kieserite-type minerals, a subgroup of the titanite group of minerals. A comparison of powder X-ray diffraction patterns indicates that our AlSO 4 (OH) is the same as that obtained by Shanks et al. (1981) through hydrolysis of Al 2 (SO 4 ) 3 solutions at temperatures above 310 °C. To date, AlSO 4 (OH) has been synthesized only at temperatures above 290 °C, implying that it may not stable in low-temperature environments, such as acidic soils and mine waters. The possible environments to find Al(OH)SO 4 may include places where sulfur-rich magma-derived fluids react with aluminous rocks under elevated temperature and pressure, and on Venus where a sulfur-rich atmosphere interacts with surface rocks at temperatures above 400 °C.

Description: The replacement of rutile by Fe-Ti oxides is a common phenomenon during the retrogression of eclogites. Here, we report an unusual case regarding the replacement of Fe-Ti oxides by rutile during greenschist-facies metamorphic overprinting of veins in amphibolites (retrograded eclogites) from the Dabie ultrahigh-pressure (UHP) terrane, eastern China. The veins mainly consist of plagioclase, Fe-Ti oxides, and quartz, which crystallized from a Ti-rich amphibolite-facies fluid that formed during exhumation of the eclogites. Two types of textures involving the replacement of Fe-Ti oxides by rutile are recognized in the veins: (1) the first type is characterized by the development of rutile coronas (Rt-C) and other silicates (high-Fe epidote, muscovite, and chlorite) around the external boundaries of the Fe-Ti oxide grains, and (2) the second type is characterized by the formation of symplectitic intergrowths of rutile (Rt-S) and magnetite after exsolved hemo-ilmenite (H-Ilm) lamellae in the Fe-Ti oxides. The micro-textures, mineral assemblages, and Zr-in-rutile thermometry indicate that both replacement reactions involved mineral re-equilibration processes in the presence of an infiltrating fluid phase at ~476–515 °C, taking place by an interface-coupled dissolution-precipitation mechanism. Thermodynamic modeling reveals that both replacement reactions occurred during oxidation processes under relatively high-oxygen fugacity ( f O 2 ) conditions, approximately 2.5–4.5 log f O 2 units higher than the fayalite-magnetite-quartz (FMQ) reference buffer. In situ Sr isotopic analyses of epidote (Ep-C) coexisting with the Rt-C suggest that the infiltrating fluid involved in the greenschist-facies replacement reactions was externally derived from the surrounding granitic gneisses (the wall rocks of the amphibolites). Compared with the rutile in the UHP eclogites (Rt-E) and amphibolites (Rt-A), the Rt-C is characterized by distinctly lower contents of Nb (〈10 ppm) and Ta (〈2 ppm) and Nb/Ta ratios (〈10) and higher contents of Cr (〉340 ppm) and V (〉1580 ppm). These results provide a geochemical fingerprint for distinguishing the low-pressure (LP) rutile from relic high-grade phases in retrograded HP-UHP rocks. Our results reveal that rutile can form during LP retrograde stage in UHP rocks by high- f O 2 fluid-induced replacement reactions. The unusual replacements of Fe-Ti oxides by rutile-bearing assemblages during retrogression provide important constraints on fluid-mineral reactions and f O 2 variations in exhumed UHP slabs.

Description: Single-crystal elasticity of candidate minerals in the Earth’s mantle, such as that of ferropericlase and bridgmanite, etc., is very important for understanding the seismic observations, geodynamic flow patterns, and testing geochemical and mineralogical models of the planet’s deep interior. Determination of the full elastic tensor typically requires measuring both compressional and shear wave velocities ( v P and v S ) of the candidate single crystal as a function of crystallographic orientations at high pressures, but it has been a huge technical challenge obtaining v P at pressures above 25 GPa using Brillouin light scattering coupled with in a diamond-anvil cell due to the spectral overlap of the sample v P with the v S of the diamond window. In this study, we present a new method to derive the full elastic tensor ( C ij ) of single crystals using only measured v S of a given crystal platelet as a function of the azimuthal angle. Experimentally determined v P and v S results from Brillouin measurements for cubic periclase (MgO) and spinel (MgAl 2 O 4 ), tetragonal stishovite (SiO 2 ), and orthorhombic zoisite [Ca 2 Al 3 Si 3 O 12 (OH)] at ambient conditions are used as examples to demonstrate the application of our approach from theoretical analyses and experimental prospective. For high-symmetry cubic minerals, such as cubic MgO and spinel, a suitable crystallographic plane with small tradeoffs between any two C ij in v S is required for the method to work well such that the obtained C ij using measured v S velocities alone can be within 3% of the values derived from using both v S and v P . Our analyses show that the (–1,0.5,0.2) platelet for periclase and the (1,1,0) platelet for spinel are respective optimal orientations for applying our method. For lower symmetry minerals, such as tetragonal stishovite and orthorhombic zoisite, three crystallographic planes, that are orthogonal to each other and are tilted at least 20° from the principal crystallographic planes, can be used to provide reliable constraints on C ij using measured v S alone. We have extended this method to derive C ij of the (–1,0.5,0.2) platelet for periclase at pressures of 5.8 and 11.3 GPa, in a high-pressure diamond-anvil cell to demonstrate the usefulness of the approach in studying the elasticity of Earth’s mantle minerals at relevant pressure-temperature conditions. Our proposed approach can be extended to all other crystal systems at high pressures to overcome the constant lack of experimental v P velocities at above 25 GPa, potentially providing new experimental and theoretical approaches in constraining the elastic tensor of the materials in the Earth’s deep interior, which will be an effective strategy to solve one of the most relevant difficulties involved in the experimental study of the elastic properties (especially elastic anisotropy) of minerals of the lower mantle.

Description: 〈span〉〈div〉Abstract〈/div〉Epidote spherulites are identified in a greenschist facies metavolcanic breccia enclosing a body of coesite-bearing eclogite at Ganghe in the Dabie ultrahigh-pressure metamorphic belt, east-central China. The epidote spherulites are formed by fibrous, radially arranged, and rare earth element (REE)-rich epidote crystals (ΣREE = 0.13–0.36 (or slightly higher) cations per formula unit, cpfu) and interfibrillar REE-poor epidote (ΣREE ≤ 0.10 cpfu). Some of the epidote spherulites are overgrown by radially arranged euhedral epidote crystals, which also form aggregates around preexisting quartz, plagioclase, and/or epidote. The epidote grains in such aggregates display oscillatory zoning, with REE content varying from a negligible amount to about 0.44 cpfu. Epidote also occurs as REE-poor individual euhedral crystals about the radial epidote aggregates or form loose clusters of randomly oriented crystals. Thermodynamic modeling of the mineral assemblages in the plagioclase pseudomorphs and in the matrix shows that they formed at greenschist facies metamorphic conditions (435–515 °C and 5–7 kbar). The epidote spherulites and radial euhedral epidote aggregates, however, do not belong to these assemblages and are non-equilibrium textures. They imply crystal growth under large degrees of supersaturation, with relatively low ratios of the diffusion rate (〈span〉D〈/span〉) to the crystal growth rate (〈span〉G〈/span〉). At low 〈span〉D/G〈/span〉 ratios, spiky interfaces are favorable for diffusion-controlled growth and the resultant texture is a collection of spikes around a growth center, forming a spherulite. The change of epidote texture from spherulite to radial euhedral crystal aggregate implies a decrease of supersaturation and an increase of 〈span〉D/G〈/span〉, such that the crystal morphology was controlled by its crystallographic structure. The crystallization of the individual epidote grains corresponds to a further drop of supersaturation and a further increase of the 〈span〉D/G〈/span〉 ratio, approaching to the equilibrium conditions. Transiently higher 〈span〉P-T〈/span〉 conditions are inferred from the spherulite-forming reactions, relative to the 〈span〉P-T〈/span〉 estimates for the equilibrium assemblages. The fibrous crystals in the spherulites having relatively large interfacial energies would inevitably adjust their shapes to equilibrium ones with low interfacial energies if the 〈span〉P-T〈/span〉-H〈sub〉2〈/sub〉O conditions were maintained for a sufficiently long period of time. The non-equilibrium epidote aggregates likely formed in response to 〈span〉P-T〈/span〉 and fluid pulses, possibly related to seismicity.〈/span〉

Description: Previous research on super-reducing ultrahigh-pressure (SuR UHP) phases from the Tibetan ophiolitic chromitites were mainly conducted on isolated grains extracted from extremely large samples. This approach has been questioned because of possible contamination. To elucidate the occurrence and origin of these SuR UHP minerals, we studied 33 thin sections and rock chips of three ophiolitic chromitites from the Yarlung Zangbo suture zone. Here we report and analyze unambiguously in situ SuR UHP assemblages from the ophiolitic chromitites by electron probe micro-analyzer, scanning microscope and Laser Raman spectroscope. The SuR UHP and associated phases include: (1) blue moissanite as inclusions in olivine (Fo 96–98 ), and in olivine domains between disseminated chromite grains; (2) multiple inclusions of moissanite + wüstite + native Fe in olivine; (3) FeNi and FeCr alloys in olivine and chromite; and (4) native Fe and Si in chromite. Crustal asphaltum and h-BN also occur as inclusions in chromite. Our documented in situ SuR UHP phases, combined with the previously inferred existence of ringwoodite + stishovite, all indicate that these assemblages formed under a highly reducing environment (oxygen fugacities several orders of magnitude lower than that of the iron-wüstite buffer) in the mantle transition zone (MTZ) and in the deep upper mantle. Diamond + moissanite with distinct 13 C-depleted compositions from chromitites have a metasedimentary carbon source. Associations with existing crustal minerals in chromitites demonstrate that carbon-bearing metasedimentary rocks were recycled into the mantle through subduction, and locally modified its composition. Finally we propose a three-stage model to explain the formation of SuR UHP phase-bearing chromitite. Discoveries of SuR UHP phases in Luobusa and other ophiolitic podiform chromitites from the polar Ural Mountains and from Myanmar imply existence of a new type of ophiolitic chromitite. Such occurrences provide an additional window to explore the physical-chemical conditions of the MTZ, mantle dynamics, and the profound recycling of crustal materials.

Description: Ophiolites are a newly documented host of diamonds on Earth. Abundant diamonds have indeed been separated from peridotites and chromitites of ophiolites in China, Myanmar, and Russia. In addition, diamond grains have recently been discovered in chromite from the Cretaceous Luobusa ophiolite (Tibet) and the early Paleozoic Ray-Iz ophiolite (polar Urals, Russia). These diamonds are accompanied by a wide range of highly reduced minerals, such as Ni–Mn–Co alloys, Fe–Si and Fe–C phases, and moissanite (SiC); these have been found as either mineral separates or inclusions in diamonds and indicate growth under superreducing conditions. The diamond-bearing chromite grains likely formed near the mantle transition zone and were then brought to shallow levels in the upper mantle to form podiform chromitites in oceanic lithosphere. Because these diamond grains occur widely in peridotites and chromitites of many ophiolites, we refer to them as ophiolite-hosted diamonds. It is possible that such diamonds may be common in the upper oceanic mantle.

Description: High-pressure synchrotron Mössbauer (SMS) and X-ray emission (XES) spectroscopic measurements were conducted to investigate the spin and valence states of iron in (Al,Fe)-bearing magnesium silicate glass (Mg 0.79 Fe 0.10 Al 0.10 Si 0.96 O 3 ) up to 126 GPa and 300 K. By analyzing the Fe K β emission spectra using the integrated relative difference (IRD) method, which accounts for the spectral broadening effects, the derived total spin momentum ( S ) of the iron in the glass shows no observable changes with pressure within the experimental uncertainties. A two-doublet fitting model representing two diverse local iron atomic environments was used to satisfactorily simulate the high-pressure SMS spectra of iron in the glass. The doublet with an averaged quadrupole splitting (QS) value of 1.94(±0.25) mm/s and chemical shift (CS) of 1.02(±0.25) mm/s at ambient conditions was assigned to be high-spin Fe 2+ , whereas the second doublet with QS = 0.83(±0.25) mm/s and CS = 0.49(±0.25) mm/s was assigned to be high-spin Fe 3+ . Increasing pressure continuously elevates the QS of Fe 2+ from ~2 mm/s at ambient pressure to 3.5 mm/s at 126 GPa, while Fe 3+ only exhibits a slight increase in the QS to 1.34(±0.25) mm/s. Comparing with previous experimental and theoretical studies on the local geometries and hyperfine parameters of silicate glasses and minerals, we conclude that the occurrence of the extremely high QS of Fe 2+ in our glass above ~40–50 GPa can be associated with the enhanced density and diverse distortions and geometries of the local Fe 2+ environments. Our combined XES and SMS results show that both Fe 2+ and Fe 3+ ions in Al-bearing silicate remain in the high-spin state, rather than undergoing a spin-pairing transition as proposed previously. Assuming that the silicate glass results can be used as an analog for understanding silicate melts, our results here indicate that iron ions likely experience significant changes in the local environments yet remain overall in the high-spin state in silicate melts at the extreme pressure and temperature conditions of the deep mantle.

Description: Qingsongite (IMA 2013-30) is the natural analog of cubic boron nitride (c-BN), which is widely used as an abrasive under the name "Borazon." The mineral is named for Qingsong Fang (1939–2010), who found the first diamond in the Luobusa chromitite. Qingsongite occurs in a rock fragment less than 1 mm across extracted from chromitite in deposit 31, Luobusa ophiolite, Yarlung Zangbu suture, southern Tibet at 29°13.86N and 92°11.41E. Five electron microprobe analyses gave B 48.54 ± 0.65 wt% (range 47.90–49.2 wt%); N 51.46 ± 0.65 wt% (range 52.10–50.8 wt%), corresponding to B 1.113 N 0.887 and B 1.087 N 0.913 , for maximum and minimum B contents, respectively (based on 2 atoms per formula unit); no other elements that could substitute for B or N were detected. Crystallographic data on qingsongite obtained using fast Fourier transforms gave cubic symmetry, a = 3.61 ± 0.045 Å. The density calculated for the mean composition B 1.100 N 0.900 is 3.46 g/cm 3 , i.e., qingsongite is nearly identical to synthetic c-BN. The synthetic analog has the sphalerite structure, space group F 3 m. Mohs hardness of the synthetic analog is between 9 and 10; its cleavage is {011}. Qingsongite forms isolated anhedral single crystals up to 1 μm in size in the marginal zone of the fragment; this zone consists of ~45 modal% coesite, ~15% kyanite, and ~40% amorphous material. Qingsongite is enclosed in kyanite, coesite, or in osbornite; other associated phases include native Fe; TiO 2 II, a high-pressure polymorph of rutile with the αPbO 2 structure; boron carbide of unknown stoichiometry; and amorphous carbon. Coesite forms prisms several tens of micrometers long, but is polycrystalline, and thus interpreted to be pseudomorphic after stishovite. Associated minerals constrain the estimated pressure to 10–15 GPa assuming temperature was about 1300 °C. Our proposed scenario for formation of qingsongite begins with a pelitic rock fragment that was subducted to mid-mantle depths where crustal B originally present in mica or clay combined with mantle N ( 15 N = –10.4 ± 3 in osbornite) and subsequently exhumed by entrainment in chromitite. The presence of qingsongite has implications for understanding the recycling of crustal material back to the Earth’s mantle since boron, an essential constituent of qingsongite, is potentially an ideal tracer of material from Earth’s surface.

Description: 〈span〉〈div〉Abstract〈/div〉In this study, we measured the sound velocities of single-crystal periclase by Brillouin light scattering (BLS) combined with in situ synchrotron X-ray diffraction (XRD) up to ~30 GPa and 900 K in an externally heated diamond-anvil cell (EHDAC). Our experimental results were used to evaluate the combined effects of pressure and temperature on the elastic moduli of single-crystal periclase using third-order Eulerian finite-strain equations. All of the elastic moduli increased with increasing pressure but decreased with increasing temperature, except the off-diagonal modulus 〈span〉C〈/span〉〈sub〉12〈/sub〉, which remained almost constant up to ~30 GPa and 900 K. The derived aggregate adiabatic bulk and shear moduli (〈span〉K〈/span〉〈sub〉S0〈/sub〉, 〈span〉G〈/span〉〈sub〉0〈/sub〉) at ambient conditions were 162.8(±0.2) and 130.3(±0.2) GPa, respectively, consistent with literature results. The pressure derivatives of the bulk [(〈span〉∂KS/∂P〈/span〉)〈sub〉300 K〈/sub〉] and shear moduli [(〈span〉∂G/∂P〈/span〉)〈sub〉300 K〈/sub〉] at ambient conditions were 3.94(±0.05) and 2.17(±0.02), respectively, whereas the temperature derivatives of these moduli [(〈span〉∂KS/∂T〈/span〉)〈sub〉P〈/sub〉 and (〈span〉∂G/∂T〈/span〉)〈sub〉P〈/sub〉] at ambient conditions were –0.025(±0.001) and –0.020(±0.001) GPa/K, respectively. A comparison of our experimental results with the high-pressure (〈span〉P〈/span〉) and high-temperature (〈span〉T〈/span〉) elastic moduli of ferropericlase (Fp) in the literature showed that all the elastic moduli of Fp were linearly correlated with the FeO content up to approximately 20 mol%. These results allowed us to build a comprehensive thermoelastic model for Fp to evaluate the effect of Fe-Mg substitution on the elasticity and seismic parameters of Fp at the relevant 〈span〉P-T〈/span〉 conditions of the lower mantle. Our modeling results showed that both the increase of the Fe content in Fp and the increasing depth could change the compressional wave anisotropy (〈span〉AV〈/span〉〈sub〉P〈/sub〉) and shear wave splitting anisotropy (〈span〉AV〈/span〉〈sub〉S〈/sub〉) of Fp in the upper parts of the lower mantle. Furthermore, using our modeling results here, we also evaluated the contribution of Fp to seismic lateral heterogeneities of thermal or chemical origin in the lower mantle. Both the thermally induced and Fe-induced heterogeneities ratios (〈span〉R〈/span〉〈sub〉S/P〈/sub〉 = 〈span〉∂〈/span〉ln〈span〉V〈/span〉〈sub〉S〈/sub〉/〈span〉∂〈/span〉ln〈span〉V〈/span〉〈sub〉P〈/sub〉) of Fp from 670 to 1250 km along a representative lower mantle geotherm increased by ~2–5% and ~15%, respectively. The thermally induced 〈span〉R〈/span〉〈sub〉S/P〈/sub〉 value of Fp20 is ~30% higher than Fp10, indicating that the Fe content has a significant effect on the thermally induced 〈span〉R〈/span〉〈sub〉S/P〈/sub〉 of Fp. Compared to the seismic observation results (〈span〉R〈/span〉〈sub〉S/P〈/sub〉 = 1.7–2.0) in the upper regions of the lower mantle, the Fe-induced 〈span〉R〈/span〉〈sub〉S/P〈/sub〉 value of Fp is more compatible than the thermally induced 〈span〉R〈/span〉〈sub〉S/P〈/sub〉 value of Fp20 (the expected composition of Fp in the lower mantle) within their uncertainties. Thus, we propose that Fe-induced lateral heterogeneities can significantly contribute to the observed seismic lateral heterogeneities in the Earth's lower mantle (670–1250 km).〈/span〉