Fabrizio Nestola

Hydrous mantle transition zone indicated by ringwoodite included within diamond

The ultimate origin of water in the Earth’s hydrosphere is in the deep Earth—the mantle. Theory and experiments have shown that although the water storage capacity of olivine-dominated shallow mantle is limited, the Earth’s transition zone, at depths between 410 and 660 kilometres, could be a major repository for water, owing to the ability of the higher-pressure polymorphs of olivine—wadsleyite and ringwoodite—to host enough water to comprise up to around 2.5 per cent of their weight. A hydrous transition zone may have a key role in terrestrial magmatism and plate tectonics, yet despite experimental demonstration of the water-bearing capacity of these phases, geophysical probes such as electrical conductivity have provided conflicting results, and the issue of whether the transition zone contains abundant water remains highly controversial. Here we report X-ray diffraction, Raman and infrared spectroscopic data that provide, to our knowledge, the first evidence for the terrestrial occurrence of any higher-pressure polymorph of olivine: we find ringwoodite included in a diamond from Juína, Brazil. The water-rich nature of this inclusion, indicated by infrared absorption, along with the preservation of the ringwoodite, is direct evidence that, at least locally, the transition zone is hydrous, to about 1 weight per cent. The finding also indicates that some kimberlites must have their primary sources in this deep mantle region.

The high-temperature P21/c-C2/c phase transition in Fe-free pyroxene (Ca0.15Mg1.85Si2O6): structural and thermodynamic behavior

Abstract A high-temperature in situ single-crystal X-ray diffraction study was performed from room T to 1150 °C on two crystals of Fe-free P21/c clinopyroxenes of composition Ca0.15Mg1.85Si2O6 [cell parameters at room T: a = 9.651(2) Å, b = 8.846(2) Å, c = 5.202(1) Å, b = 108.38(2)°, V = 421.4 (2) Å3] synthesized by isothermal annealing for 624 h at T = 1370 °C, P = 1 atm. A first order P21/c-C2/c phase transition was found slightly below 1000 °C [Tc = 926(39) °C]. The transition was revealed by discontinuous changes in intensities and cell parameters. Prolonged heating at high temperature induced a non-reversible increase in the transition temperature up to more than 1150 °C, without apparent changes in the order of the phase transition. Coupling with strain due to incipient exsolution in a formerly almost defect-free sample is suggested to be responsible for increase in Tc. TEM observations of a sample from the same starting material after further annealing for 72 h at T = 1050 °C, P = 1 atm are consistent with the proposed incipient exsolution model. Annealing was found to induce the formation of a mottled texture oriented parallel to (101). Results from structure refinement of data collected below the transition at T = 25, 500, 650, 800, and 1000 °C showed only minor changes in the chain configurations, which are highly differentiated up to 1000 °C, confirming the strong first-order character of the transition.

EosFit7-GUI: A new graphical user interface for equation of state calculations, analyses and teaching

EosFit7-GUI is a full graphical user interface designed to simplify the analysis of thermal expansion and equations of state (EoSs). The software allows users to easily perform least-squares fitting of EoS parameters to diffraction data collected as a function of varying pressure, temperature or both. It has been especially designed to allow rapid graphical evaluation of both parametric data and the EoS fitted to the data, making it useful both for data analysis and for teaching.

Geobarometry from host-inclusion systems: The role of elastic relaxation

Abstract Minerals trapped as inclusions within other host minerals can develop residual stresses on exhumation as a result of the differences between the thermo-elastic properties of the host and inclusion phases. The determination of possible entrapment pressures and temperatures from this residual stress requires the mutual elastic relaxation of the host and inclusion to be determined. Previous estimates of this relaxation have relied on the assumption of linear elasticity theory. We present a new formulation of the problem that avoids this assumption. We show that for soft inclusions such as quartz in relatively stiff host materials such as garnet, the previous analysis yields entrapment pressures in error by the order of 0.1 GPa. The error is larger for hosts that have smaller shear moduli than garnet.

High pressure behavior, transformation and crystal structure of synthetic iron-free pigeonite

Abstract A single-crystal high pressure X-ray investigation was performed up to P = 6.5 GPa on a synthetic clinopyroxene of composition Ca0.15(1)Mg1.85(1)Si2.00(1)O6 [Di15En85, unit-cell parameters at room pressure: a = 9.6525(6)Å, b = 8.8461(2)Å, c = 5.2036(5)Å, β = 108.370(5)°, V = 421.68(4)Å3]. A first order P21/c-C2/c displacive phase transition was found at P = 5.1 GPa; the transition was revealed by the disappearance of the b reflections (h + k = odd) and by sharp changes in the unit-cell parameters. Reversals through the transformation show that, if present, hysteresis is smaller than 0.1 GPa. The volume variation has been described by a third-order Birch-Murnaghan equation of state with V0 = 421.68(8) Å3, KT0 = 102(2) GPa, and K′ = 8(1) for the low-symmetry phase (P21/c) and with V0 = 411.06(3) Å3 and KT0 = 108(2) GPa for the high-symmetry phase (C2/c), with K′ fixed to the value obtained for the low-symmetry form. The axial compressibility shows the following scheme: βb > βa ≅ βc > βasinβ for both phases. In comparison with pure clinoenstatite, Di15En85 shows a similar step in unit-cell parameters at the transition, the disappearence of hysteresis and a decrease of transition pressure and of bulk modulus. Full intensity data sets were collected at room pressure, 2.6 and 4.5 GPa for the P21/c phase and at P = 6.2 GPa for the C2/c phase. A slight increase of the intensity of h + k odd reflections and of the difference in the A and B chain kinking angles were observed. A comparison of the structural behavior of the P21/c phase at high temperature and high pressure shows opposite behavior for M2-O bond lengths and O3-O3-O3 kinking angle.

Olivine with diamond-imposed morphology included in diamonds. Syngenesis or protogenesis?

The identification of syngenetic inclusions in diamond (i.e. inclusions of minerals that crystallized at the same time and by the same genesis as their host) has long been of paramount importance in diamond studies. However, the widespread assumption that many or most inclusions in diamonds are syngenetic is based on qualitative morphological criteria and few direct measurements. In order to provide statistically significant information on inclusion–host genetic relations for at least one kimberlite, we have determined the crystallographic orientations of 43 olivine inclusions with diamond-imposed morphology, a feature generally interpreted to indicate syngenesis, in 20 diamonds from the Udachnaya kimberlite (Siberia). Our unprecedented large data set indicates no overall preferred orientation of these olivines in diamond. However, multiple inclusions within a single diamond frequently exhibit similar orientations, implying that they were derived from original single monocrystals. Therefore, regardless of the possible chemical re-equilibration during diamond-forming processes, at least some of the olivines may have existed prior to the diamond (i.e. they are protogenetic). Our results imply that a diamond-imposed morphology alone cannot be considered as unequivocal proof of syngenicity of mineral inclusions in diamonds.

Electrical conductivity of hydrous wadsleyite

In situ complex impedance spectroscopy of H 2 O-bearing wadsleyite was performed in a multianvil apparatus at 14 GPa at temperatures up to 950 °C in order to determine electrical conductivity. With increasing H 2 O content in wadsleyite the electrical conductivity increases at a rate higher than observed in previous studies. The activation enthalpy in the temperature range studied where proton conduction dominates is low (0.66 eV) suggesting an inevitable crossover to small polaron conduction at moderately higher temperatures, depending on H 2 O concentration. Although the solubility of H 2 O in wadsleyite is significant for a silicate mineral (>3 wt%), the presence of more than trace dissolved H 2 O in wadsleyite is likely to result in a conductivity too high compared to recent estimates of transition-zone conductivity. The use of complex impedance spectroscopy shows that the frequency dependence of electrical properties is very different in the case of H 2 O-bearing silicate phases. At frequencies below 1000 Hz complex impedance spectra contain strong features which likely result from the sample–electrode interface such that including the low-frequency data would lead to artificially low conductivities.

Crystal chemistry of hydrous forsterite and its vibrational properties up to 41 GPa

Abstract The crystal structure of hydrous pure magnesium forsterite (Mg2SiO4) containing 8900 ppmw H2O, synthesized at 12 GPa and 1250 °C, has been refined. The major hydration mechanism appears to be M1 cation vacancy with protonation of the O1-O2 octahedral edge of M1. Raman spectra up to 41 GPa show strong coupling between the two Ag modes (824.4 and 856.2 cm-1). Mode Grüneisen parameters γi related to Mg(2)O6 translation mode decrease relative to anhydrous forsterite. This is attributed to the fact that the M2 site shows full occupancy compared to the M1 site, and the vacancy predominantly occurs at the M1 site. Pressure dependencies of four OH vibrations in the region 3548-3615 cm-1 suggest that positional ordering of hydrogen ion (proton) takes place with increasing pressure. The OH mode at 3615 cm-1 shows complex response as a function of pressure: the (∂ν/∂P) slope changes from -0.43 below 10 GPa to 1.97 between 10 and 20 GPa, and does not show clear pressure dependence above 20 GPa. Single-crystal X-ray data were used to assign the OH band to the structural sites, O1 and O2. The proton is closer to O2 than O1. The distance between the proton and O2 is 0.96 Å. In view of the empirical relation between OH-stretching frequencies and O···O distances (Libowitzky 1999), the pressure dependence of the OH mode (3615 cm-1) is well correlated with the O1-O2 distance and the degree of hydrogen bonding.

High-pressure phase transformation in LiFeGe2O6 pyroxene

Abstract A synthetic pyroxene with composition LiFeGe2O6 and space group P21/c at ambient conditions was investigated by single-crystal X-ray diffraction using a diamond anvil cell. The unit-cell parameters and crystal structure were determined at eight different pressures up to 8.7 GPa. Between 4.16 and 4.83 GPa, the sample shows a strongly first-order phase transition as indicated by a drastic drop in a, c, β, and unit-cell volume. The transition is marked by the disappearance of b-type reflections (h + k = odd) forbidden in a C-centered lattice. The volume bulk modulus of the P21/c phase is estimated to be 110 GPa as compared to 147 GPa of the C2/c one. The crystal structure evolution as a function of pressure is mainly influenced by the kinking of tetrahedral chains; the A and B non-symmetry equivalent chains of the P21/c phase undergo strong deformations up to 4.16 GPa (A chain ~2%, B chain ~5.3%). At the transition, the two chains become symmetry equivalent and the single tetrahedral chain of the C2/c phase shows only minor deformations with pressure (~1.9%) due to its already strong kinking (~130°). Such behavior is the main reason for the strong difference in compressibility between the low- and high-symmetry forms.

Inclusions under remnant pressure in diamond: a multi-technique approach

Measurement of the remnant pressure sustained by a mineral inclusion within a diamond can allow calculation of the source pressure and temperature conditions of the diamond formation. While Raman spectroscopy (point analyses and 2D mapping) has been the most commonly used non-destructive method for measuring this remnant pressure, two new techniques (quantitative birefringence analysis and in situ X-ray diffraction) have recently been developed. In this paper we apply all of these techniques to the study of two diamonds. The first is a diamond twin (macle) from Shandong (China), containing two olivine inclusions. Analysis of the largest inclusion by the birefringence and X-ray techniques returned compatible values for the remnant pressure of 0.1–0.2 GPa. However, while 2D Raman mapping of the diamond host qualitatively confirmed the presence of a radial stress field, the low remnant pressures values were too small to be detected by pressure shifts in the olivine’s Raman spectrum. The second sample studied was a rounded dodecahedral diamond (unknown origin) containing several coesite inclusions. Its morphology prevented quantitative birefringence analysis or Raman mapping being performed but Raman analyses on three inclusions (2.2–2.5 GPa) provided similar results to those obtained by X-ray diffraction from a single inclusion (2.7 GPa); all of these values lie within the range of previous published remnant pressures for coesite in diamond. Calculation of the source pressure and temperature conditions for both diamond samples returned anomalously low values, both below the diamond stability field for a range of mantle temperatures (950–1350 °C). The reasons for this are due to violations of the fundamental assumptions upon which such studies of these elastic effects in diamond are based. Although previous studies showed that olivine inclusions have the potential to record reasonable formation pressures under favourable conditions, the present study re-iterates the serious concerns of using the coesite-in-diamond geobarometer. It is inferred that, as the remnant pressures in the coesite-diamond pair are so high, there is a high probability that the diamond will undergo some plastic deformation, thus reducing the elastic behaviour and leading to severe underestimation of entrapment pressure.