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The Earth’s inner core exhibits significant anisotropy in both seismic velocity and attenuation as well as hemispherical and depth variations. These observations point to an inner core that is both complex and dynamic. However, interpretation of these observations without knowledge of the attenuation processes active in the inner-core is difficult. To address this we have used zinc, as a low-pressure analogue of the hexagonal close pack (hcp) structured iron that forms the inner core, to provide first-order constraints on the anelasticity of hcp metals at seismic frequencies and high temperatures. A D-DIA apparatus was combined with X-radiography were utilised to measure the anelastic response of zinc up to a homologous temperature (T/Tm) of ≈ 0.8. To analyse the data we developed an improved image processing method that reduces systematic errors and gives up to 3 orders of magnitude improvement in strain measurement precision. Using this algorithm, in the frequency range 0.1 to 0.003Hz, significant dissipation and softening of zinc’s Young’s modulus is observed. The softening occurs in the absence of significant impurities or a fluid phase and appears to be caused by, or related to, recrystallisation of the samples in response to the stress. The recrystallisation results in a steady-state grain-size and low dislocation density. The predicted reduction in shear wave speed is 2-3 times greater than that of for compressional waves, which is consistent with anelasticity playing a significant role in the seismic velocity of the inner core. Therefore anelastic effects in hcp iron must be considered in the interpretation of inner-core.

One of the most remarkable observations regarding volatile elements in the solar system is the depletion of N in the bulk silicate Earth (BSE) relative to chondrites, leading to a particularly high and non-chondritic C:N ratio. The N depletion may reflect large-scale differentiation events such as sequestration in Earth's core or massive blow off of Earth's early atmosphere, or alternatively the characteristics of a late-added volatile-rich veneer. As the behavior of N during early planetary differentiation processes is poorly constrained, we determined together the partitioning of N and C between Fe–N–C metal alloy and two different silicate melts (a terrestrial and a martian basalt). Conditions spanned a range of fO2 from ΔIW−0.4 to ΔIW−3.5 at 1.2 to 3 GPa, and 1400 °C or 1600 °C, where ΔIW is the logarithmic difference between experimental fO2 and that imposed by the coexistence of crystalline Fe and wustite. N partitioning ( D N metal / silicate ) depends chiefly on fO2, decreasing from 24 ± 3 to 0.3 ± 0.1 with decreasing fO2. D N metal / silicate also decreases with increasing temperature and pressure at similar fO2, though the effect is subordinate. In contrast, C partition coefficients ( D C metal / silicate ) show no evidence of a pressure dependence but diminish with temperature. At 1400 °C, D C metal / silicate partition coefficients increase linearly with decreasing fO2 from 300 ± 30 to 670 ± 50 . At 1600 °C, however, they increase from ΔIW−0.7 to ΔIW−2 ( 87 ± 3 to 240 ± 50 ) and decrease from ΔIW−2 to ΔIW−3.3 ( 99 ± 6 ) . Enhanced C in melts at high temperatures under reduced conditions may reflect stabilization of C–H species (most likely CH4). No significant compositional dependence for either N or C partitioning is evident, perhaps owing to the comparatively similar basalts investigated. At modestly reduced conditions (ΔIW−0.4 to −2.2), N is more compatible in core-forming metal than in molten silicate ( 1 ≤ D N metal / silicate ≤ 24 ), while at more reduced conditions (ΔIW−2.2 to ΔIW−3.5), N becomes more compatible in the magma ocean than in the metal phase. In contrast, C is highly siderophile at all conditions investigated ( 100 ≤ D C metal / silicate ≤ 700 ). Therefore, sequestration of volatiles in the core affects C more than N, and lowers the C:N ratio of the BSE. Consequently, the N depletion and the high C:N ratio of the BSE cannot be explained by core formation. Mass balance modeling suggests that core formation combined with atmosphere blow-off also cannot produce a non-metallic Earth with a C:N ratio similar to the BSE, but that the accretion of a C-rich late veneer can account for the observed high BSE C:N ratio.

Using vibrational spectroscopy and SIMS, we determined the solubility and speciation of C–O–H–N dissolved volatiles in mafic glasses quenched from high pressure under reduced conditions, with f O 2 from −3.65 to +1.46 relative to the iron–wustite buffer (IW). Experiments were performed on martian and terrestrial basalts at 1.2 GPa and 1400 °C in graphite containers with variable availability of H 2 O, and in the presence of FePt alloys or Fe–C liquids. The dominant C–O–H–N species varies systematically with f O 2 and H 2 O content: the carbonate ion prevails above IW + 1, but for dry conditions between IW−2 and IW + 1, C O species are most important. Below IW, reduced NH-bearing species are present. At the most reducing and hydrous (∼0.5 wt% H 2 O) conditions, small amounts of CH 4 are present. Concentrations of C diminish as conditions become more reduced, amounting to 10 s to ∼100 ppm in the interval ∼IW−2 to IW + 1 where C O species dominate, and as little as 1–3 ppm at more reduced conditions. Concentrations of non-carbonate carbon, dominated by C O species, correlate with CO fugacities along a trend implying that the species stoichiometry has just one C O group and suggesting that carbonyl complexes (transition metals with multiple carbon monoxide ligands) are not important species under these conditions. C partition coefficients between Fe–C liquid and silicate melt increase with decreasing f O 2 , becoming as great as 10 4 for the most reducing conditions investigated. The low solubility of C in silicate liquids under reducing conditions means that most C during the magma ocean stage of planetary differentiation is either segregated to the core or in the overlying atmosphere. Precipitation of C-rich phases in a carbon-saturated magma ocean is also possible, and is one mechanism by which some C can be retained in the mantle of a planet. The predominant magmatic carbonaceous species for both martian and lunar volcanism is likely C O.

International audience; Melting experiments were performed on a silica-rich peridotite composition at 10-17 GPa to determine majoritic garnet-melt partition coefficients (D) for major and trace elements. Our results show that D for many elements, including Na, Sc, Y and rare earth elements (REE), varies significantly with increasing pressure or proportion of majorite component. Lu and Sc become incompatible at 17 GPa, with D decreasing from 1.5 at 10 GPa to 0.9 at 17 GPa. As predicted from lattice strain, log D for isovalent cations entering the large site of majoritic garnet exhibits a near-parabolic dependence on ionic radius. Our data are used to refine a previously published predictive model for garnet-melt partitioning of trivalent cations, which suffered from a lack of calibration in the 10-20 GPa range. Our results suggest that Archean Al-depleted komatiites from Barberton (South Africa) may have been generated by partial melting of dry peridotite at depths between 200 and 400 km. We also speculate that transition zone diamonds from Kankan (Guinea), which contain inclusions of majoritic garnet, may have formed from the partial reduction of CO2-rich magmas that subsequently transported them to the surface. This hypothesis would provide an explanation for the REE patterns of majoritic garnet trapped within these diamonds, including Eu anomalies. Finally, we show that segregation of majoritic garnet-bearing cumulates during crystallisation of a deep Martian magma ocean could lead to a variety of Lu/Hf and Sm/Nd ratios depending on pressure, leading to a range of ε143Nd and ε176Hf isotope signatures for potential mantle sources of Martian rocks.

Laser heated diamond-anvil-cell experiments performed on a “tetragonal almandine pyrope phase” (TAPP) bulk composition constrain high-temperature phase relations between 6 and 48 GPa. Phase assemblages determined by synchrotron X-ray diffraction consist of TAPP + garnet ± pseudobrookite ± enstatite from 6 to 10 GPa, garnet from 13 to 22 GPa, Mg-perovskite + garnet from 23 to 30 GPa, and Mg-perovskite above 30 GPa. TAPP has the same structure at 1 atm and at high pressure based on in situ X-ray diffraction data, and is stable at a maximum pressure of 10–13 GPa at 1300–1700 K. These results rule out direct incorporation of TAPP in diamond at transition-zone or lower-mantle pressures. Possible origins of TAPP are (1) entrapment by diamonds in the upper mantle, and (2) retrograde formation from a high-pressure garnet or Mg-perovskite precursor. We suggest that single-phase TAPP inclusions and possibly composite TAPP-bearing inclusions originated as Mg-perovskite in the lower mantle in mafic protoliths, and that TAPP forms upon retrograde conversion at pressures below 13 GPa upon exhumation of the diamonds from the lower mantle to shallower depths of less than 400 km.

Experiments at 20^97 GPa and 2000 K in the system CaO^MgO^ TiO2^SiO2 constrain phase relations involving Mg-rich and Ca-rich perovskite solid solutions at conditions relevant to the Earth’s deepTransition Zone and lower mantle. Bulk compositions were investigated with molar Ti/(Tiþ Si) up to 0·5 within the quasi-ternary ‘perovskite plane’ , which is defined by a reciprocal solution among the components MgSiO3, MgTiO3, CaSiO3, and CaTiO3. Multi-anvil experiments at 20 GPa and 2000 K on bulk compositions within the plane produce akimotoite coexisting with Ca-perovskites that lie close to the CaSiO3^CaTiO3 join. Higher-pressure experiments using a laser-heated diamond anvil cell constrain the position of a two-perovskite field that extends into the perovskite plane from the solvus along the MgSiO3^CaSiO3 binary join, where limited mutual solubility exists between MgSiO3 and CaSiO3 perovskites. On the join MgSiO3^MgTiO3, MgTiO3 solubility in MgSiO3 perovskite increases with pressure, with MgSi0·8Ti0·2O3 perovskite stable at 50 GPa. Limited reciprocal solution at 25 GPa results in an expansive two-perovskite field that occupies much of the Si-rich portion of the perovskite plane. Solution ofTi into Mg-rich and Ca-rich perovskites enhances the solubility of reciprocal Ca and Mg components, respectively. Increase in pressure promotes reciprocal solution, and the two-phase field collapses rapidly with pressure toward the MgSiO3^CaSiO3 join.We find that a single-phase, orthorhombic perovskite with near equimolar Ca and Mg is stable in a composition with Ti/ (Tiþ Si) of only 0·05 at 97 GPa, requiring that by this pressure the two-phase field occupies a small area close to the MgSiO3^CaSiO3 join. On the basis of experiments at 1500 K, temperature has only a mild effect on the position of the Ca-rich limb of the solvus. Ca(Ti,Si)O3 mineral inclusions in deep sublithospheric diamonds could not have formed in equilibrium with Mg-perovskite owing to their virtual lack of MgSiO3 component at pressures of Mg-perovskite stability, but may have equilibrated withTransition Zone MgSiO3-rich phases at lower pressures; this observation can be extended generally to near-endmember CaSiO3 inclusions. On an iron-free basis, the average bulk compositions of clinopyroxene^ilmenite and orthopyroxene^ilmenite megacrysts from kimberlites plot in single-perovskite fields at pressures greater than about 45 and 65 GPa, respectively, when projected onto the perovskite plane.We predict that the effect of iron will not be large, and estimate that single-phase perovskites may form at somewhat lower pressures than in the iron-free system. Thus, the origin of pyroxene^ilmenite megacrysts from single-phase perovskite solutions in the lower mantle is plausible on the basis of phase relations, although a lower pressure magmatic origin appears more likely. Deeply subducted Ti-rich lithologies such as ocean-island basalt will crystallize a single perovskite rather than a two-perovskite assemblage beginning at pressures of 80 GPa. Normal mid-ocean ridge basalt and primitive mantle peridotite are expected to remain within a two-phase perovskite field until Mg-perovskite transforms to post-perovskite.

Quantitative thermobarometry of inclusions in zoned garnet from a Franciscan eclogite block record a counter-clockwise P–T path from blueschist to eclogite and back. Garnet retains prograde zoning from inclusion-rich Alm52Grs30Pyp6Sps12 cores to inclusion-poor Alm62Grs25Pyp12Sps1 mantles, with overgrowths of highly variable composition. Barometry using the Waters–Martin version of the garnet–phengite–omphacite thermobarometer yields conditions of 7–15 kbar, 400–500°C (garnet cores), 18–22 kbar, ∼550°C (mantles), and 10–14 kbar, 350–450°C (overgrowths), in agreement with clinozoisite–sphene–rutile–garnet–quartz barometry. These pressures are ∼10–15 kbar less than those obtained using more recent, fully thermodynamic calibrations of the phengite–omphacite–garnet thermobarometer. Low early temperatures suggest that the block was subducted in a thermally mature subduction zone and not at the inception of subduction when prograde temperature is expected to be higher. Franciscan high-grade blocks likely represent crust subducted throughout the history of this convergent margin, rather than only at the inception of the subduction zone.

The solubility behavior of K2O, Na2O, Al2O3, and SiO2 in silicate-saturated aqueous fluid and coexisting H2O-saturated silicate melts in the systems K2O-Al2O3-SiO2-H2O and Na2O-Al2O3-SiO2-H2O has been examined in the 1- to 2-GPa pressure range at 1100°C. Glasses of Na- and K-tetrasilicate compositions with 0, 3, and 6 mol% Al2O3 were used as starting materials. In both systems, the oxides dissolve incongruently in aqueous fluid and silicate melt. When recalculated to an anhydrous basis, the aqueous fluids are enriched in alkalis and depleted in silica and alumina relative to their proportions in the starting materials. The extent of incongruency is more pronounced in the Na2O-Al2O3-SiO2-H2O system than in the K2O-Al2O3-SiO2-H2O system. The partition coefficients of the oxides, Doxidefluid/melt, are linear and positive functions of the oxide concentration in the fluid for each composition. There is a slight dependence of the partition coefficients on bulk composition. No effect of pressure could be discerned. For alkali metals, the fluid/melt partition coefficients range from 0.06 to 0.8. For Al2O3 this range is 0.01 to 0.2, and for SiO2, it is 0.01 to 0.32. For all compositions, DK2Ofluid/melt∼DNa2Ofluid/melt>DSiO2fluid/melt>DAl2O3fluid/melt for the same oxide concentration in the fluid. DK2Ofluid/melt, DNa2Ofluid/melt, and DSiO2fluid/melt correlate negatively with the Al2O3 content of the systems. This correlation is consistent with a solubility model of alkalis that involve associated KOH°, NaOH°, silicate, and aluminate complexes.

The melting curve of FeSi has been determined to 150 GPa in the laser-heated diamond anvil cell (LH-DAC) on the basis of discontinuities in the power versus temperature function. A multianvil experimental cross-check at 12 GPa using textural criteria as a proxy for melting is in good agreement with our LH-DAC results. The melting point of FeSi reaches similar to 4000 K at the core mantle boundary and an extrapolated value of 4900 K at the inner-core boundary (ICB). We also present the melting curve as determined by the Lindemann melting law; this agrees well with our experimental curve to 70 GPa and then diverges to higher temperatures, reaching 6200 K at the ICB. These temperatures are substantially higher than previous LH-DAC determinations. The boundary of the epsilon-FeSi -> CsCl-FeSi subsolidus transition has also been determined by synchrotron-based X-ray diffraction at high pressures, and the results confirm a negative Clapeyron slope for the transition. We conclude that if present, FeSi is likely to be solid within the D '' layer and is unlikely to be present within the inner core for any plausible bulk core silicon content.

Partial melting in the Earth's mantle plays an important part in generating the geochemical and isotopic diversity observed in volcanic rocks at the surface. Identifying the composition of these primary melts in the mantle is crucial for establishing links between mantle geochemical 'reservoirs' and fundamental geodynamic processes. Mineral inclusions in natural diamonds have provided a unique window into such deep mantle processes. Here they provide exper8imental and geochemical evidence that silicate mineral inclusions in diamonds from Juina, Brazil, crystallized from primary and evolved carbonatite melts in the mantle transition zone and deep upper mantle. The incompatible trace element abundances calculated for a melt coexisting with a calcium-titanium-silicate perovskite inclusion indicate deep melting of carbonated oceanic crust, probably at transition-zone depths. Further to perovskite, calcic-majorite garnet inclusions record crystallization in the deep upper mantle from an evolved melt that closely resembles estimates of primitive carbonatite on the basis of volcanic rocks. Small-degree melts of subducted crust can be viewed as agents of chemical mass-transfer in the upper mantle and transition zone, leaving a chemical imprint of ocean crust that can possibly endure for billions of years.