Ekaterina A. Sirotkina

Structural and chemical characterization of Mg[(Cr,Mg)(Si,Mg)]O4, a new post-spinel phase with sixfold-coordinated silicon

Abstract The crystal structure and chemical composition of a crystal of Mg(Mg,Cr,Si)2O4 post-spinel phase synthesized in the model system MgCr2O4-Mg2SiO4 at 16 GPa and 1600 °C have been investigated. The compound was found to crystallize with a distorted orthorhombic calcium-titanate (CaTi2O4) structure type, space group Cmc21, with lattice parameters a = 2.8482(1), b = 9.4592(5), c = 9.6353(5) Å, V = 259.59(1) Å3, and Z = 4. The structure was refined to R1 = 0.018 using 345 independent reflections. The loss of the inversion center is due to the ordering of cations at the octahedral sites: Cr is mainly hosted at the M1 site, whereas Si at the M2 site. Such an ordered distribution induces a distortion thus provoking a change in coordination of Mg, which becomes sevenfold-coordinated instead of the usual eightfold coordination observed in post-spinel phases. Electron microprobe analysis gave the Mg[(Cr0.792Mg0.208) (Si0.603Mg0.397)]O4 stoichiometry for the studied phase. The successful synthesis of this phase can provide new constraints on thermobarometry of wadsleyite/ringwoodite-bearing assemblages.

Chromium solubility in perovskite at high pressure: The structure of (Mg1–xCrx)(Si1–xCrx)O3 (with x = 0.07) synthesized at 23 GPa and 1600 ºC

Abstract The crystal structure and chemical composition of a crystal of (Mg1-xCrx)(Si1-xCrx)O3 perovskite (with x = 0.07) synthesized in the model system Mg3Cr2Si3O12-Mg4Si4O12 at 23 GPa and 1600 °C have been investigated. The compound was found to be orthorhombic, space group Pbnm, with lattice parameters a = 4.8213(5), b = 4.9368(6), c = 6.9132(8) Å, V = 164.55(3) Å3. The structure was refined to R = 0.046 using 473 independent reflections. Chromium was found to substitute for both Mg at the dodecahedral X site (with a mean bond distance of 2.187 Å) and Si at the octahedral Y site (mean: 1.814 Å), according to the reaction Mg2+ + Si4+ = 2Cr3+. Such substitutions cause a shortening of the and a lengthening of the distances with respect to the values typically observed for pure MgSiO3 perovskite. Although high Cr-contents are not considered in the pyrolite model, Cr-bearing perovskite may be an important host for chromium in the lower mantle. The successful synthesis of perovskite with high-Cr content and its structural characterization are of key importance because the study of its thermodynamic constants combined with the data on phase relations in the lower-mantle systems can provide new constraints on thermobarometry of perovskite-bearing assemblages.

Chromium-bearing phases in the Earth’s mantle: Evidence from experiments in the Mg2SiO4–MgCr2O4 system at 10–24 GPa and 1600 °C

Abstract Phase relations in the system Mg2SiO4–MgCr2O4 were studied at 10–24 GPa and 1600 °C using a high-pressure Kawai-type multi-anvil apparatus. We investigated the full range of starting compositions for the forsterite-magnesiochromite system to derive a P–X phase diagram and synthesize chromium-bearing phases, such as garnet, wadsleyite, ringwoodite, and bridgmanite of a wide compositional range. Samples synthesized at 10 GPa contain olivine with small chromium content and magnesiochromite. Mg2SiO4 wadsleyite is characterized by the pressure-dependent higher chromium solubility (up to 7.4 wt% Cr2O3). The maximal solubility of chromium in ringwoodite in the studied system (~ 18.5 wt% Cr2O3) was detected at P = 23 GPa, which is close to the upper boundary of the ringwoodite stability. Addition of chromium to the system moves the boundaries of olivine/wadsleyite and wadsleyite/ringwoodite phase transformations to lower pressures. Our experiments simulate Cr-rich phase assemblages found as inclusions in diamonds, mantle xenoliths, and UHP podiform chromitites.

Bridgmanite-like crystal structure in the novel Ti-rich phase synthesized at transition zone condition

Abstract A new Ti-bearing bridgmanite-like phase with a threefold commensurate superstructure of the ideal MgSiO3-perovskite structure was observed in a [Mg5/6Al1/6][Si1/2Ti1/3Al1/6]O3 crystal synthesized in the model system Mg3Al2Si3O12–MgTiO3 at 20 GPa and 1600 °C. The compound was found to be orthorhombic, space group Pnma, with lattice parameters a = 14.767(3), b = 6.958(1), c = 4.812(1) Å, V = 494.4(2) Å3, which represents a 3a × b × c superstructure of the typical Pnma perovskite structure. The structure was refined to R = 0.024 using 846 independent reflections. The superstructure mainly arises from the ordering of titanium in one of the octahedral positions. Crystal-chemical details of the different polyhedra in the superstructure are discussed in comparison to pure MgSiO3. This is the first documented superstructure of a bridgmanite phase, and Ti-rich bridgmanite in the lower mantle arising from local Tienrichments may exhibit different physical properties and elemental partitioning behavior from Ti-poor, peridotitic bridgmanite. The study also shows that large amounts of Ti can stabilize bridgmanite-like compounds at considerably lower pressure than lower mantle conditions.

Incorporation of high amounts of Na in ringwoodite: Possible implications for transport of alkali into lower mantle

Abstract Here we report on the coexistence between Na-rich ringwoodite and bridgmanite in the system MgSiO3−Na2CO3−Al2O3 at 24 GPa and 1700 °C. In our experiments ringwoodite incorporates up to 4.4 wt% Na2O, with Na entering the octahedral site together with Si, according to the mechanism: Mg2+ → ⅔Na+ + ⅓Si4+. The volume of the unit cell increases along with the Na content. A similar behavior is observed for the unit-cell volume of Na-bearing bridgmanite, although the mechanism of Na incorporation into this structure remains unknown because of the lack of sufficient crystallographic data. Na2O is compatible in ringwoodite relative to bridgmanite with a partition coefficient (D) of 5 (+5/–4), but is incompatible in ringwoodite relative to carbonate-rich melt/fluid, with the D value ranging between 0.5 and 0.1. Al is highly enriched in bridgmanite relative to the other coexisting phases. Carbonatitic melt metasomatism in the deep transition zone may lead to local Na-enrichment, and ringwoodite may be an important host for Na in the deep transition zone. Subsequent convection or subduction of metasomatized mantle may lead to enrichment of alkaline elements in the upper and lower mantle.

Synthesis of inverse ringwoodite sheds light on the subduction history of Tibetan ophiolites

Tibetan ophiolites are shallow mantle material and crustal slabs that were subducted as deep as the mantle transition zone, a conclusion supported by the discovery of high-pressure phases like inverse ringwoodite in these sequences. Ringwoodite, Mg2SiO4, exhibits the normal spinel structure, with Mg in the octahedral A site and Si in the tetrahedral B site. Through A and B site-disorder, the inverse spinel has four-coordinated A cations and the six-coordinated site hosts a mixture of A and B cations. This process affects the density and impedance contrasts across the boundaries in the transition zone and seismic-wave velocities in this portion of the Earth. We report the first synthesis at high pressure (20 GPa) and high temperature (1600 °C) of a Cr-bearing ringwoodite with a completely inverse-spinel structure. Chemical, structural, and computational analysis confirm the stability of inverse ringwoodite and add further constraints to the subduction history of the Luobusa peridotite of the Tibetan ophiolites.

The influence of low aluminum concentrations on the composition and conditions of crystallization of majorite–knorringite garnets: Experiment at 7.0 GPa and 1500–1700°C

Crystallization of garnet in high-chromium restite formed under the conditions of partial melting in the spinel facies and subsequently subducted into the garnet depth facies was studied experimentally in the MgO–Al2O3–Cr2O3–SiO2 system. The crystallization of garnet and the dependence of its composition on the temperature and bulk composition of the system with low Al concentration were studied as well. Experiments in the knorringite–majorite–pyrope system with 5, 10, and 20 mol % Prp were carried out at 7 GPa. The phase associations for the starting composition of pure knorringite Mg3Cr2Si3O12 included chromiumbearing enstatite MgSiO3 (up to 3.2 wt % Cr2O3) and eskolaite Cr2O3. Addition of Al resulted in crystallization of high-chromium majoritic garnet. The portion of garnet in the samples always exceeded the concentration of pyrope in the starting composition owing to the formation of the complex majorite–knorringite–pyrope series of solid solutions. With increasing content of pyrope (from 5 to 20 mol %) and increasing temperature, the modal concentration of garnet increased significantly (from 6–12 to 22–37%). The garnet was characterized by high concentrations of the pyrope (23–80 mol %) and knorringite (22–70 mol %) components. The excess of Si (>3 f.u.) with decreasing Cr concentration provided evidence for the contribution of the majorite–knorringite trend to the variation in garnet composition. On the basis of the natural data, most of the garnets composing xenoliths of ultrabasic rocks in kimberlites and occurring as inclusions in diamonds are low-chromium; i.e., their protolith was not subjected to partial melting, at least in the spinel depth facies.