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Hydrous Fo91 ringwoodite crystals were synthesized at 20 GPa and high-temperature conditions using a multi-anvil press. Recovered crystals were analyzed using electron microprobe analysis, Raman spectroscopy, infrared spectroscopy, synchrotron Mössbauer spectroscopy, single-crystal X-ray diffraction, and single-crystal Laue neutron diffraction, to carefully characterize the chemistry and crystallography of the samples. Analysis of the combined data sets provides evidence for the presence of tetrahedrally coordinated ferric iron and multiple hydrogen incorporation mechanisms within these blue-colored iron-bearing ringwoodite crystals. Tetrahedral ferric iron is coupled with cation disorder of silicon onto the octahedrally coordinated site. Cation disorder in mantle ringwoodite minerals may be promoted in the presence of water, which could have implications for current models of seismic velocities within the transition zone. Additionally, the presence of tetrahedrally coordinated ferric iron may cause the blue color of many ringwoodite and other high-pressure crystals. [ABSTRACT FROM AUTHOR]

Raman spectroscopic data were obtained for (Mg,Fe)2SiO4 samples during compression to 57 GPa. Single crystals of San Carlos olivine compressed hydrostatically above 41 GPa showed appearance of a new "defect" peak in the 820–840 cm–1 region associated with SiOSi linkages appearing between adjacent SiO44– tetrahedra to result in five- or sixfold-coordinated silicate species. Appearance of this feature is accompanied by a broad amorphous background. The changes occur at lower pressure than metastable crystalline transitions of end-member Mg2SiO4 forsterite (Fo-I) into Fo-II and Fo-III phases described recently. We complemented our experimental study using density functional theory (DFT) calculations and anisotropic ion molecular dynamics (AIMD) simulations to study the Raman spectra and vibrational density of states (VDOS) of metastably compressed Mg2SiO4 olivine, Fo-II and Fo-III, and quenched melts at high and low pressures. By 54 GPa all sharp crystalline peaks disappeared from observed Raman spectra indicating complete pressure-induced amorphization (PIA). The amorphous (Mg,Fe)2SiO4 spectrum contains Si-O stretching bands at lower wavenumber than expected for SiO44– indicating high coordination of the silicate units. The amorphous spectrum persisted on decompression to ambient conditions but with evidence for reappearance of tetrahedrally coordinated units. Nonhydrostatic compression of polycrystalline olivine samples showed similar appearance of the defect feature and broad amorphous features between 43–44 GPa. Both increased in intensity as the sample was left at pressure overnight but they disappeared during decompression below 17 GPa with recovery of the starting olivine Raman signature. A hydrated San Carlos olivine sample containing 75–150 ppm OH was also studied. Significant broadening of the SiO44– stretching peaks was observed above 43 GPa but without immediate appearance of the defect or broad amorphous features. However, both of these characteristics emerged after leaving the sample at 47 GPa overnight followed by complete amorphization that occurred upon subsequent pressurization to 54 GPa. During decompression the high-density amorphous spectrum was retained to 3 GPa but on final pressure release a spectrum similar to thermally quenched low-pressure olivine glass containing isolated SiO44– groups was obtained. Leaving this sample overnight resulted in recrystallization of olivine. Our experimental data provide new insights into the metastable structural transformations and relaxation behavior of olivine samples including material recovered from meteorites and laboratory shock experiments. [ABSTRACT FROM AUTHOR]

Pressure-temperature melting curves in two carbonate-bearing systems of relevance to Earth's mantle have been determined using the laser-heated diamond-anvil cell (LH-DAC). The solidus along the MgSiO3-MgCO3 join in the MgO-SiO2-CO2. system (MS-CO2) was defined from 15 to 80 GPa using in situ melting criteria, reaching a maximum temperature of ~2340 K at 80 GPa. The occurrence of melting has been confirmed with ex-situ textural and chemical analysis of recovered samples. The melting curve has a negative dT/dP slope at pressures between ~15 and 23 GPa resulting from the subsolidus phase transition of ilmenite- to perovskite-structured MgSiO3. The shallow slope of the melting curve at pressures higher than this transition indicate that for plausible mantle geotherms carbonate-bearing silicate lithologies will melt throughout the lower mantle. The solidus of a mixture along the MgCO3-CaCO3 join was determined as a proxy for alkali-free carbonate lithologies. Melting temperatures increase from 1860 K at 16 GPa to ~2100 K above 35 GPa, where the melting curve flattens. The melting reaction magnesite + post-aragonite (high-pressure CaCO3) = melt was confirmed using an in situ experiment. We conclude that crystalline Mg and Ca carbonate mixtures are unstable with respect to molten carbonate at conditions of the convective lower mantle. The flat melting curves at high pressures in both systems suggests that subducted carbonates will undergo melting at lower mantle conditions, a process that may be important for superdeep diamond formation and carbon storage in the deep mantle. [ABSTRACT FROM AUTHOR]

The article discusses the use of diamond samples for studying ancient samples of the Earth's interior including magnesiowüstite found in lower mantle, and discusses composition of diamond-hosted inclusions displaying magnesioferrite exsolution with formation under lower mantle conditions.

Earth's physical properties and mantle dynamics are strongly dependent on mantle grain size, shape, and orientation, but these characteristics are poorly constrained. Experimental studies provide an opportunity to simulate the grain growth kinetics of mantle aggregates. The experimentally determined grain sizes can be fit to the normal grain growth law (Gn – Gn0) = k0t∙exp(–ΔH/RT) and then be used to determine grain size throughout the mantle and geological time. The grain growth dynamics of spinel-orthopyroxene mixtures in the upper mantle are modeled here by experimentally producing small grain sizes in the range of 0.5 to 2 μm radius at pressures and temperatures equivalent to the spinel lherzolite stability field. To accurately measure the sizes of these small grains, we have developed a computer vision workflow; using a watershed transformation, which rapidly measures 68% more grains and produces a 20% improvement in the average grain size accuracy and repeatability when compared with manual methods. Using this automated approach, we have been able to identify a significant proportion of small grains, which have been overlooked when using manual methods. This additional population of grains, when fit to the normal grain growth law, highlights the influence of improved accuracy and sample size on the estimation of grain growth kinetic parameters. Our results demonstrate that automatic computer vision enables a systematic, fast, repeatable method of grain size analysis, across large data sets, improving the accuracy of experimentally determined grain growth kinetics. [ABSTRACT FROM AUTHOR]

Reactions involving carbon in the deep Earth have limited manifestations on Earth's surface, yet they have played a critical role in the evolution of our planet. The metal-silicate partitioning reaction promoted carbon capture during Earth's accretion and may have sequestered substantial carbon in Earth's core. The freezing reaction involving iron-carbon liquid could have contributed to the growth of Earth's inner core and the geodynamo. The redox melting/freezing reaction largely controls the movement of carbon in the modern mantle, and reactions between carbonates and silicates in the deep mantle also promote carbon mobility. The 10-year activity of the Deep Carbon Observatory has made important contributions to our knowledge of how these reactions are involved in the cycling of carbon throughout our planet, both past and present, and has helped to identify gaps in our understanding that motivate and give direction to future studies. [ABSTRACT FROM AUTHOR]