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We model apatite-saturation in carbonatite melts based on a compilation of experimental data ranging from 650 to 1430 °C and 1 to 60 kbar. The data show a very strong correlation of inverse temperature with the apatite solubility product, a relation expressed by the equation. ln ( C a O 5 · P O 2.5 3) = - 27450 / T + 5.79 (T in Kelvin). Within the available dataset, F and Cl do not play a discernable role. Application of the solubility product to natural Ca-carbonatites indicates that a few rocks with >8 wt% P 2 O 5 have cumulative apatite while most Ca-carbonatites (with typically <5 wt% P 2 O 5) are apatite undersaturated at their liquidus temperatures, defined by calcite crystallization. To address true carbonatite liquids, we model calcite fractionation and melt evolution for natural rock compositions with 5, 10 and 20 mol% H 2 O and/or (Na,K) 2 CO 3 added, 5% representing the lower bound for any carbonatite formation model. Both H 2 O or (Na,K) 2 CO 3 cause very similar liquidus depressions of ∼10 °C/mol%. The model result is that saturation of apatite occurs in most natural carbonatite melts only after >45, 30–55, and 10–30 mol% calcite-fractionation for 5, 10, and 20 mol% fluxing components added, respectively. We further estimate the melt fractions necessary to dissolve all apatite in carbonatite melts generated from carbonated MORB and pelites, opening the discussion on an unlikely restitic nature of subducted apatites. In both the crystallization and forward melting cases, apatite crystallization or dissolution is mostly governed by temperature, surprisingly, carbonatite melt evolution through calcite-fractionation has a minor influence on the solubility product. [ABSTRACT FROM AUTHOR]

Abstract Nitrogen is a key constituent of our atmosphere and forms the basis of life, but its early distribution between Earth reservoirs is not well constrained. We investigate nitrogen partitioning between metal and silicate melts over a wide range of conditions relevant for core segregation during Earth accretion, i.e. 1250–2000 °C, 1.5–5.5 GPa and oxygen fugacities of ΔIW-5.9 to ΔIW-1.4 (in log units relative to the iron–wüstite buffer). At 1250 °C, 1.5 GPa, D N metal melt / silicate melt ranges from 14 ± 0.1 at ΔIW-1.4 to 2.0 ± 0.2 at ΔIW-5, N partitioning into the core forming metal. Increasing pressure has no effect on D N metal melt / silicate melt , while increasing temperature dramatically lowers D N metal melt / silicate melt to 0.5 ± 0.15 at ΔIW-4. During early core formation N was hence mildly incompatible in the metal. The partitioning data are then parameterised as a function of temperature and oxygen fugacity and used to model the evolution of N within the two early prevailing reservoirs: the silicate magma ocean and the core. Depending on the oxidation state during accretion, N either behaves lithophile or siderophile. For the most widely favoured initially reduced Earth accretion scenario, N behaves lithophile with a bulk partition coefficient of 0.17 to 1.4, leading to 500–700 ppm N in closed-system core formation models. However, core formation from a magma ocean is very likely accompanied by magma ocean degassing, the core would thus contain ≤100 ppm of N, and hence, does not constitute the missing N reservoir. Bulk Earth N would thus be 34–180 ppm in the absence of other suitable reservoirs, >98% N of the chondritic N have hence been lost during accretion. Highlights • N partitioning between metal and silicate melts at core forming conditions. • N is lithophile during Earth's core formation. • The reduced mantle is a major N reservoir on present-day Earth. [ABSTRACT FROM AUTHOR]

Abstract Carbon isotope fractionation in the CO 2 –CO–CH 4 –C system was investigated at 300–1200 °C at near-atmospheric pressures by thermally decomposing a variety of organic materials in sealed quartz tubes. Measured gas speciations correspond well to the expected range from thermodynamic calculations. We show that chemical and isotopic equilibrium among gas species is obtained when applying a nickel catalyst for CO 2 /CH 4 , CH 4 /CO, and CO 2 /CO at ≤600 °C or without a catalyzing agent for CO 2 /CO at ≥800 °C. The experiments define carbon isotope fractionation factors for the CO 2 /CH 4 , CO 2 /CO and CH 4 /CO pairs as (i) 10 3 ln ⁡ α CO 2 / CH 4 = 8.9 (± 0.6) ⋅ 10 5 ⋅ (1 T 2 ) 0.825 (± 0.005) (200–1200 °C) (ii) 10 3 ln ⁡ α CO 2 / CO = 1.07 (± 0.05) ⋅ 10 6 ⋅ (1 T 2 ) 0.830 (± 0.003) (300–1200 °C) (iii) 10 3 ln ⁡ α CH 4 / CO = 1.1 (± 0.2) ⋅ 10 3 ⋅ (1 T 2 ) 0.462 (± 0.001) (300–1200 °C), which reproduce the experimental values within 0.2‰ for CO 2 /CH 4 and CO 2 /CO and within 0.12‰ for CH 4 /CO (T in K, 1 σ fit uncertainties in brackets, CO 2 /CH 4 includes the ≤600 °C experimental data of Horita, 2001). Carbon isotope fractionation factors at 1000 °C are still large for CO 2 /CH 4 and CO 2 /CO (6.6 and 7.5‰, respectively) but only 1.5‰ for CH 4 /CO. Elemental carbon precipitated through thermal decomposition of the organic starting materials yields δ 13 C values that depend on the X(O) = O/(O + H) of the organic starting material, i.e. the initial oxidation state of carbon in the organics. We further observe a catalytic effect of the quartz walls on chemical and isotopic exchange in the CO 2 /CO system, probably due to the activation of the silicate surface by H+ and OH− ions at >650 °C. Our experimental results yield improved calibrations of the CO 2 /CH 4 equilibria and the first experimental calibration of CO 2 /CO and CH 4 /CO carbon isotope fractionation. Applications are in the tracing of magmatic hydrothermal gas emissions, in carbon-precipitating COH-fluids, and in monitoring of coal-seam fires, but our results may also be applied for quality control during steel-making processes. Highlights • Experiment-based equilibrium carbon isotope fractionation factors for CH 4 –CO 2 –CO. • Nickel catalyzer facilitates chemical and isotopic equilibration of gas. • Hydrated silica surfaces catalyze CO 2 –CO carbon isotope exchange. [ABSTRACT FROM AUTHOR]

Banded iron formations (BIF) were common shelf and ocean basin sediments 3.5–1.8 Ga ago. To understand the fate of these dense rocks upon subduction, the melting relations of carbonated BIF were determined in Fe–Ca–(Mg)–Si–C–O 2 at 950–1400 °C, 6 and 10 GPa, oxidizing ( f O 2 = hematite–magnetite, HM) and moderately reducing ( f O 2 ∼ CO 2 -graphite/diamond, CCO) conditions. Solidus temperatures under oxidizing conditions are 950–1025 °C with H 2 O, and 1050–1150 °C anhydrous, but 250–175 °C higher at graphite saturation (values at 6–10 GPa). The combination of Fe 3+ and carbonate leads to a strong melting depression. Solidus curves are steep with 17–20 °C/GPa. Near-solidus melts are ferro-carbonatites with ∼22 wt.% FeO tot , ∼48 wt% CO 2 and 1–5 wt.% SiO 2 at f O 2 ∼ HM and ∼49 wt.% FeO tot , ∼20 wt% CO 2 and 19–25 wt.% SiO 2 at f O 2 ∼ CCO . At elevated subduction geotherms, as likely for the Archean, C-bearing BIF could melt out all carbonate around 6 GPa. Fe-rich carbonatites would rise but stagnate gravitationally near the slab/mantle interface until they react with the mantle through Fe–Mg exchange and partial reduction. The latter would precipitate diamond and yield Fe- and C-rich mantle domains, yet, Fe–Mg is expected to diffusively re-equilibrate over Ga time scales. We propose that the oldest subduction derived diamonds stem from BIF derived melts. [ABSTRACT FROM AUTHOR]

During ascent, kimberlites react with the lithospheric mantle, entrain and assimilate xenolithic material, loose volatiles and suffer from syn- and post-magmatic alteration. Consequently, kimberlite rocks deviate heavily from their primary melt. Experiments at 7 GPa, 1300–1480 °C, 10–30 wt% CO 2 and 0.46 wt% H 2 O on a proposed primitive composition from the Jericho kimberlite show that saturation with a lherzolitic mineral assemblage occurs only at 1300–1350 °C for a carbonatitic melt with <8 wt% SiO 2 and >35 wt% CO 2 . At asthenospheric temperatures of >1400 °C, where the Jericho melt stays kimberlitic, this composition saturates only in low-Ca pyroxene, garnet and partly olivine. We hence forced the primitive Jericho kimberlite into multiple saturation with a lherzolitic assemblage by adding a compound peridotite. Saturation in olivine, low- and high-Ca pyroxene and garnet was obtained at 1400–1650 °C (7 GPa), melts are kimberlitic with 18–29 wt% SiO 2 + Al 2 O 3 , 22.1–24.6 wt% MgO, 15–27 wt% CO 2 and 0.4–7.1 wt% H 2 O; with a trade-off of H 2 O vs. CO 2 and temperature. Melts in equilibrium with high-Ca pyroxene with typical mantle compositions have ≥2.5 wt% Na 2 O, much higher than the commonly proposed 0.1–0.2 wt%. The experiments allow for a model of kimberlite origin in the convective upper mantle, which only requires mantle upwelling that causes melting at the depth where elemental carbon (in metal, diamond or carbide) converts to CO 2 (at ∼250 km). If primary melts leading to kimberlites contain a few wt% H 2 O, then adiabatic temperatures of 1400–1500 °C would yield asthenospheric mantle melts that are kimberlitic (>18 wt% SiO 2 + Al 2 O 3 ) but not carbonatitic (<10 wt% SiO 2 + Al 2 O 3 ) in composition, carbonatites only forming 100–200 °C below the adiabat. These kimberlites represent small melt fractions concentrating CO 2 and H 2 O and then acquire part of their chemical signature by assimilation/fractionation during ascent in the subcratonic lithosphere. [ABSTRACT FROM AUTHOR]

High-pressure, high-temperature experiments have been performed at ∼1.2 GPa and 1360–2100 °C to investigate the partitioning of Pt between a silicate melt and a metallic melt. Our experiments indicate that nanonuggets encountered in previous experiments are experimental artifacts, formed at high temperature by oversaturation caused by high oxygen fugacity during the initial stages of an experiment. Experiments at high-acceleration using a centrifuging piston-cylinder show that nanonuggets can be removed by gravity during the experiment. Formation of nanonuggets can also be avoided by using initially reduced starting materials. The presence of iron is also a key element in reducing the formation of nanonuggets. Our nanonugget-free data are broadly consistent with previous nanonuggets-filtered data, and suggest that Pt partitioning becomes independent of oxygen fugacity below an oxygen fugacity of at least IW+2. Pt is thus possibly dissolved as a neutral species (or even an anionic species) at low fO 2 , instead of the more common Pt 2+ species present at higher fO 2 . Due to low concentration, the nature of this species cannot be determined, but atomic Pt or Pt − are possible options. Under core-formation conditions, Pt partitioning between metal and silicate is mostly independent of oxygen fugacity, silicate melt composition, and pressure. Partition coefficient during core formation can be expressed by the following equation: log D Pt M metal / silicate = 1.0348 + 14698 / T (in weight units). Calculations indicate that the Pt content (and by extension the Highly Siderophile Elements content) of the Earth’s mantle cannot be explained by equilibrium partitioning during core formation, requiring further addition of HSE to the mantle. The mass of this late veneer is approximately 0.4% of the total mass of the Earth (or 0.6% of the mass of the mantle). [ABSTRACT FROM AUTHOR]

We determined the melting phase relations and conditions of the dense hydrous magnesium silicate phase D (nominally MgSi 2 O 4 (OH) 2 ) on composition in MgO–SiO 2 –H 2 O (MSH), MgO–Al 2 O 3 –SiO 2 –H 2 O (MASH), and FeO–MgO–Al 2 O 3 –SiO 2 –H 2 O (FMASH), and on a mixture of phase D + olivine + enstatite (MSH) at 22–32 GPa and 1000–1800 °C. Contrasting to previous studies, we performed H 2 O-undersaturated experiments. Bulk compositions were synthetic mixtures of brucite + silica or brucite + olivine + enstatite on the silica-rich side of the tie-line perovskite–H 2 O. At 22–24 GPa, the maximum thermal stability of phase D is between 1350 and 1400 °C in MSH and FMASH, but 1600 °C at 24 GPa in the Fe-free, Al-bearing bulk composition (MASH). Apparently, addition of Al 2 O 3 increases the stability field of phase D by 200 °C, an effect that is counter balanced by addition of FeO. At 32 GPa, the stability of phase D (MSH and FMASH) is between 1350 and 1400 °C. At 22 GPa, phase D melts to a Mg-rich melt coexisting with MgSi-ilmenite + stishovite, whereas at 24–32 GPa melt coexists with perovskite and stishovite. Even melts from bulk compositions in the silica-rich part of the MSH system (molar bulk Mg/Si < 0.5) are magnesian-rich (Mg/Si molar ratio of 2–5) and are distinct from aqueous fluids and hydrous melts at lower pressures. The temperature stabilities determined in this study indicate that slabs that thermally relax when stagnating on top of the 660-km discontinuity or penetrating into the lower mantle will have their last dense hydrous magnesium silicate phase, i.e., phase D, melting and producing a magnesian and hydrous melt that will rise through the transition zone. Such a melt could be responsible for observed low velocity zones, and may be neutrally buoyant at the 410-km discontinuity and will affect the structure and dynamics of the mantle. [ABSTRACT FROM AUTHOR]

Abstract: Iron isotope fractionation during metal–silicate differentiation has been proposed as a cause for differences in iron isotope compositions of chondrites, iron meteorites and the bulk silicate Earth. Stable isotope fractionation, however, rapidly decreases with increasing temperature. We have thus performed liquid metal–liquid silicate equilibration experiments at 1250–1300°C and 1GPa to address whether Fe isotope fractionation is resolvable at the lowest possible temperatures for magmatic metal–silicate differentiation. A centrifuging piston cylinder apparatus enabled quantitative metal–silicate segregation. Elemental tin or sulphur was used in the synthetic metal-oxide mixtures to lower the melting temperature of the metal. The analyses demonstrate that eight of the 10 experimental systems equilibrated in a closed isotopic system, as was assessed by varying run durations and starting Fe isotope compositions. Statistically significant iron isotope fractionation between quenched metals and silicates was absent in nine of the 10 experiments and all 10 experiments yield an average metal–silicate fractionation factor of 0.01±0.04‰, independent of whether graphite or silica glass capsules were used. This implies that Fe isotopes do not fractionate during low pressure metal–silicate segregation under magmatic conditions. In large bodies such as the Earth, fractionation between metal and high pressure (>20GPa) silicate phases may still be a possible process for equilibrium fractionation during metal–silicate differentiation. However, the 0.07±0.02‰ heavier composition of bulk magmatic iron meteorites relative to the average of bulk ordinary/carbonaceous chondrites cannot result from equilibrium Fe isotope fractionation during core segregation. The up to 0.5‰ lighter sulphide than metal fraction in iron meteorites and in one ordinary chondrite can only be explained by fractionation during subsolidus processes. [Copyright &y& Elsevier]

Abstract: In this study we experimentally determine phlogopite/melt partition coefficients of Ra and other trace elements in a lamproitic system. This work was achieved using an analytical technique (LA-ICP-MS) with low detection limits (~0.01 fg) permitting the measurement of the very low Ra concentrations feasible in experiments (~1 ppb). D Ra phlogopite/melt was determined to 2.28±0.44 and 2.84±0.47 in two experiments, the ratio D Ra/D Ba is around 1.6. The compatibility of Ra in phlogopite results from an ionic radius being close to the apex of the lattice strain parabola for earth alkalis in the large XII-coordinated interlayer site of phlogopite. A re-evaluation of D Ra and D Ra/D Ba for magmatic minerals containing appreciable Ra, yields D Ra mineral/melt ranging from ~2.6 for phlogopite down to 2–3•10−5 for pyroxenes, and D Ra/D Ba mineral/melt from ~4 for leucite to 2•10−2 for orthopyroxene. The influence of melt composition on D Ra/D Ba is less than 10%. All investigated minerals have different D Ra/D Ba, strongly fractionating Ra from Ba. Thus, for magmatic systems, (226Ra)/Ba in the various minerals is not constant, these minerals do not form a straight line in the (226Ra)/Ba–(230Th)/Ba system at the time of crystallization and thus, there is no (226Ra)/Ba–(230Th)/Ba isochron at t 0. 226Ra–230Th–Ba mineral dating is thus applicable only to model ages calculated from mineral–glass pairs with known D Ra. [Copyright &y& Elsevier]

Abstract: Radium and other trace element partition coefficients have been experimentally determined for plagioclase and alkali-feldspar in equilibrium with silicate melts. In anorthitic plagioclase, alkalis (e.g. Na, K, Rb, Cs) and the heavy earth alkalis (e.g. Ba, Ra) are incompatibles, in albitic plagioclase Na is compatible, whereas Ca and Sr are always compatible in plagioclase. D Ra plagioclase/melt was determined to be 0.017±0.006 (An91), 0.025±0.009 (An81) and 0.47±0.08 (An34) for three different An contents, the ratio D Ra/D Ba increasing with decreasing An component. In alkali-feldspar, K, Rb, Ba, Ra, and Sr are compatible, whereas Na, Cs, and Ca are incompatible. D Ra alkali-feldspar/melt was determined to 2.54±0.04 for a Or75Ab21An4 composition, the ratio D Ra/D Ba is 0.55±0.01. For plagioclase, D Ra and X An are related as RT lnD Ra [kJ/mol]=−52.54(±5.75)X An −4.25(±1.51) and D Ra/D Ba =exp((−9.47(±6.21)X An −13.66(±1.52))/RT) [kJ/mol], allowing calculation of D Ra from the An content of magmatic plagioclase. For alkali-feldspar we propose the equation D Ra/D Ba =exp((−16.06(±0.58)+15.52(±0.67) X Or)/RT) [kJ/mol]. The experimentally determined partition coefficients are then used to recalculate Th–Ra–Ba model ages, leading to generally shorter feldspar residence times. The variations encompass 20 to 90% shorter crystallization ages with respect to the originally calculated ones, in two cases feldspar crystallization ages increase slightly. For several cases the measured plagioclase and melt have either never been in equilibrium or the Th–Ra–Ba system has been disturbed after crystallization. [Copyright &y& Elsevier]

Abstract: Radium and other trace element (i.e. Cs, Rb, Ba, Sr, Cd, Cu, Pb, Zn, La, Lu, Nd, Sm, Y, Sn) partition coefficients have been experimentally determined at atmospheric pressure for leucite in equilibrium with a phonolitic melt. Ra bulk concentrations of ∼0.25 ppm are sufficient for precise LA-ICP-MS measurements of Ra in leucite and melt. Partition coefficients increase for the alkalis from Rb to Cs, and for the alkaline earths from Sr to Ba to Ra, with the maximum of both lattice strain parabolas at a radius of 1.89±0.01 Å, i.e. near Cs and Ra. At 1190 °C, D Ra leucite/melt was determined to 2.3±0.7 and 1.8±0.5 for two different experiments, the average D Ra/D Ba is 4.2±0.5. Hitherto, crystallization ages calculated from the isotopic 226Ra disequilibrium assumed that D Ra equals D Ba. The experimentally determined partition coefficients lead to an order of magnitude younger 226Ra crystallization ages for leucite. Employing our partitioning data in combination with the isotopic results from Black, S., Macdonald, R., DeVivo, B., Kilburn, R.J., Rolandi, G. U-series disequilibria in young (A.D. 1944) Vesuvius rocks: Preliminary implications for magma residence times and volatile addition. J. Volcanol. Geotherm. Res. 82, 97–111, leucite crystal ages from several phonothephrite lavas of the 1944 eruption of Vesuvius become undefined, testifying for a more complicated process than equilibrium leucite crystallization and subsequent undisturbed evolution of these lavas. [Copyright &y& Elsevier]

Abstract: High-pressure liquids in the MgO–SiO2–H2O (MSH) system have been investigated at 11 and 13.5GPa and between 1000 and 1350°C. A bulk composition more magnesian than the tie-line forsterite–H2O was employed for the study. Rocking multi-anvil experiments were combined with a diamond trap set-up. After termination of the experiments, the liquid trapped in the diamond layer was analysed by laser ablation ICP-MS using the ‘freezing’ technique. At 11GPa, liquids coexist with one or two of phase A, clinohumite, chondrodite, and forsterite. A marked discontinuity in the evolution of liquid compositions near 1100°C is observed at 11GPa. A step of ∼13wt% H2O and 13wt% MgO is interpreted to result from overstepping the fluid-saturated solidus reaction mass balanced to 1.00(18) phase A+1.07(4) fluid=0.63(15) chondrodite+1.44(2) melt. At 13.5GPa liquids coexist with one or two of hydrous wadsleyite, clinohumite, superhydrous B, phase B, and forsterite. The discontinuity in liquid composition is no longer present, indicating that the second critical endpoint of the solidus has been overstepped. Thus, hydrous melts in the Mg-rich part of the MSH system (molar bulk Mg/Si>2) are chemically distinct from aqueous fluids at pressure up to 11GPa. Convergence of fluid and melt compositions along the solidus resulting in a supercritical liquid occurs between 11 and 13.5GPa, at which pressure the entire MSH system becomes supercritical. [Copyright &y& Elsevier]

Fluid saturated high-pressure experiments often result in strongly zoned experimental charges, this hinders experimentation in chemically homogeneous systems which in turn has serious consequences on equilibration, reaction progress, and (apparent) phase stabilities. In order to overcome these problems, a 600-ton press accommodating either a multianvil or end-loaded piston cylinder module has been mounted in such a way that it can be turned by 180°, thus inverting its position in the gravity field. During turning, hydraulic pressure, heating power, and cooling water remain connected allowing fully controlled pressures and temperatures during experiments.A series of experiments at 13 GPa, 950°C, on a serpentine bulk composition in the MgO-SiO2-H2O system demonstrates that continuous turning at a rate of 2 turns/min results in a nearly homogeneous charge composed of phase E + enstatite. The same experiment at static conditions resulted in four mineral zones: quench phase E, enstatite, enstatite + phase E, and phase E + phase A. Phase A disappears in experiments at a turning rate ≥1 turn/min. A static 15-min experiment shows that zonation already forms within this short time span. Placing two short capsules within a single static experiment reveals that the fluid migrates to the hot spot in each capsule and is not gravitationally driven toward the top. The zonation pattern follows isotherms within the capsule, and the degree of zonation increases with temperature gradient (measured as 10 °C within a capsule) and run time.Our preferred interpretation is that Soret diffusion causes a density-stratified fluid within the capsule that does not convect in a static experiment and results in temperature dependant chemical zonation. The aggravation of zonation and appearance of additional phases with run time can be explained with a dissolution-reprecipitation process where the cold spot of the capsule is relatively MgO enriched and the hot spot relatively SiO2 and H2O enriched (at 13 GPa and 950°C). Rocking and tilting of a stratified fluid induces Rayleigh-Taylor instabilities, causing chemical rehomogenization. If turning is faster than the time required to build significant chemical potential gradients in the fluid, chemical zonation in the distribution of the solids is suppressed. [Copyright &y& Elsevier]

The binary CaCO 3 -MgCO 3 constitutes the reference model system to reconstruct the petrogenesis of carbonated rocks, and of carbonatite magmas possibly generated in the Earth's mantle. We experimentally determined the melting of aragonite and magnesite to pressures of 12 GPa, and of calcite-magnesite mixtures at 3 and 4.5 GPa, at variable Ca/(Mg + Ca) (X Ca). The melting curves of aragonite, and magnesite have similar slopes, the latter melting ≈ 30 °C higher than aragonite. In the Ca-Mg binary, the minimum of the liquidus surface situates at an X Ca of 0.65–0.60, at 1200 °C − 3 GPa, and 1275 °C − 4.5 GPa. Together with available data at 1 and 6 GPa, the minimum liquid composition remains approximately constant with pressure. All available experimental data are then fit by the first thermodynamic model for CaCO 3 -MgCO 3 liquids. Surprisingly, although carbonate liquids should behave as relatively simple molten salts, the liquids display large non-ideality and a three-component, pressure dependent, asymmetric liquid solution model is required to model the liquidus surface. Attempts to use only the two end-member components fail, invariably generating a very wide magnesite-liquid loop in disagreement with the experimental evidence. The liquid model is then used to evaluate results of experimentally determined phase relationships for carbonated peridotites in CaO-MgO-SiO 2 -CO 2 (CMS-CO 2), and CaO-MgO-Al 2 O 3 -SiO 2 -CO 2 (CMAS-CO 2). Computations highlight that the liquid composition formed in a model carbonated mantle do not represent "minimum melts" but are more magnesian at high pressure. The pressure–temperature position of the solidus, as well as its dP/dT slope, including the appearance or absence of the "carbonatite ledge", depend on bulk composition, unless truly invariant assemblages occur. [ABSTRACT FROM AUTHOR]

• D C metal/silicate mainly increases with melt polymerization (NBO/T) from 14 to 640. • C contents in the silicate vary from 0.01 to 0.57 wt%, metal contains 6.6–7.6 wt%. • C appears to be one order of magnitude less siderophile than previously determined. In the accreting Earth and planetesimals, carbon was distributed between a core forming metallic melt, a silicate melt, and a hot, potentially dense atmosphere. Metal melt droplets segregating gravitationally from the magma ocean equilibrated near its base. To understand the distribution of carbon, its partitioning between the two melts is experimentally investigated at 1.5–6.0 GPa, 1300–2000 °C at oxygen fugacities of −0.9 to −1.9 log units below the iron-wuestite reference buffer (IW). One set of experiments was performed in San Carlos olivine capsules to investigate the effect of melt depolymerization (NBO/T), a second set in graphite capsules to expand the data set to higher pressures and temperatures. Carbon concentrations were analyzed by secondary ionization mass spectrometry (SIMS) and Raman spectra were collected to identify C-species in the silicate melt. Partition coefficients are governed by the solubility of C in the silicate melt, which varies from 0.01 to 0.6 wt%, while metal melts contain ∼7 wt% C in most samples. C solubility in the silicate melt correlates strongly with NBO/T, which, in olivine capsules, is mostly a function of temperature. Carbon partition coefficients D C metal/silicate at 1.5 GPa, 1300–1750 °C decrease from 640(49) to 14(3) with NBO/T increasing from 1.04 to 3.11. For the NBO/T of the silicate Earth of 2.6, D C metal/silicate is 34(9). Pressure and oxygen fugacity show no clear effect on carbon partitioning. The present results differ from those of most previous studies in that carbon concentrations in the silicate melt are comparatively higher, rendering C to be about an order of magnitude less siderophile, and the discrepancies may be attributed to differences in the experimental protocols. Applying the new data to a magma ocean scenario, and assuming present day mantle carbon mantle concentrations from 120 to 795 ppm, implies that the core may contain 0.4–2.6 wt% carbon, resulting in 0.14–0.9 wt% of this element for the bulk Earth. These values are upper limits, considering that some of the carbon in the modern silicate Earth has very likely been delivered by the late veneer. [ABSTRACT FROM AUTHOR]

At high temperatures, isotope partitioning is often assumed to proceed under equilibrium and trends in the carbon isotope composition within graphite and diamond are used to deduce the redox state of their fluid source. However, kinetic isotope fractionation modifies fluid- or melt-precipitated mineral compositions when growth rates exceed rates of diffusive mixing. As carbon self-diffusion in graphite and diamond is exceptionally slow, this fractionation should be preserved. We have hence performed time series experiments that precipitate graphitic carbon through progressive oxidization of an initially CH 4 -dominated fluid. Stearic acid was thermally decomposed at 800 °C and 2 kbar, yielding a reduced COH-fluid together with elemental carbon. Progressive hydrogen loss from the capsule caused CH 4 to dissociate with time and elemental carbon to continuously precipitate. The newly formed C0, aggregating in globules, is constantly depleted by − 6.2 ± 0.3 ‰ in 13C relative to the methane, which defines a temperature dependent kinetic graphite-methane 13C/12C fractionation factor. Equilibrium fractionation would instead yield graphite heavier than the methane. In dynamic environments, kinetic isotope fractionation may control the carbon isotope composition of graphite or diamond, and, extended to nitrogen, could explain the positive correlation of δ 13 C and δ 15 N sometimes observed in coherent diamond growth zones. 13C enrichment trends in diamonds are then consistent with reduced deep fluids oxidizing upon their rise into the subcontinental lithosphere, methane constituting the main source of carbon. • Tracking the 13C/12C distribution during oxidation of a methane-dominated COH-fluid. • CH 4 dissociation causes kinetic isotope effects recorded in co-precipitated C0. • At 800 °C, co-precipitated C0 is depleted in 13C by 6.2 ± 0.3 ‰ relative to CH 4. • Kinetic control of C isotope composition of fluid-precipitated graphite and diamond. [ABSTRACT FROM AUTHOR]

Carbon isotope exchange between carbon-bearing high temperature phases records the carbon (re-) processing in the Earth's interior, where the vast majority of global carbon is stored. Redox reactions between carbonate phases and elemental carbon govern the mobility of carbon, which then can be traced by its isotopes. We determined the carbon isotope fractionation factor between graphite and a Na 2 CO 3 -CaCO 3 melt at 900–1500 °C and 1 GPa; The failure to isotopically equilibrate preexisting graphite led us to synthesize graphite anew from organic material during the melting of the carbonate mixture. Graphite growth proceeds by (1) decomposition of organic material into globular amorphous carbon, (2) restructuring into nano-crystalline graphite, and (3) recrystallization into hexagonal graphite flakes. Each transition is accompanied by carbon isotope exchange with the carbonate melt. High-temperature (1200–1500 °C) equilibrium isotope fractionation with type (3) graphite can be described by Δ 13 C c a r b o n a t e - g r a p h i t e = 3.17 (± 0.07) · 10 6 T 2 (temperature T in K). As the experiments do not yield equilibrated bulk graphite at lower temperatures, we combined the ≥1200 °C experimental data with those derived from upper amphibolite and lower granulite facies carbonate-graphite pairs (Kitchen and Valley, 1995; Valley and O'Neil, 1981). This yields the general fractionation function Δ 13 C c a r b o n a t e - g r a p h i t e = 3.37 ± 0.04 · 10 6 T 2 usable as a geothermometer for solid or liquid carbonate at ≥600 °C. Similar to previous observations, lower-temperature experiments (≤1100 °C) deviate from equilibrium. By comparing our results to diffusion and growth rates in graphite, we show that at ≤1100 °C carbon diffusion is slower than graphite growth, hence equilibrium surface isotope effects govern isotope fractionation between graphite and carbonate melt and determine the isotopic composition of newly formed graphite. The competition between diffusive isotope exchange and growth rates requires a more careful interpretation of isotope zoning in graphite and diamond. Based on graphite crystallization rates and bulk isotope equilibration, a minimum diffusivity of Dgraphite = 2 × 10−17 m2s−1 for T > 1150 °C is required. This value is significantly higher than calculated from experimental carbon self-diffusion constants (∼1.6 × 10−29 m2 s−1) but in good agreement with the value calculated for mono-vacancy migration (∼2.8 × 10−16 m2 s−1). [ABSTRACT FROM AUTHOR]

The group 13 elements Ga and In are overabundant in bulk silicate Earth (BSE) when compared to lithophile elements of similar 50% nebular condensation temperature T c 50 . To understand whether evaporation from silicate melts provides a more accurate description of volatility during the later stages of planetary accretion, namely, at higher temperatures and oxygen fugacities than in the solar nebula, knowledge of the activities of GaO 1.5 and InO 1.5 in silicate melts and their stable gaseous species are required. To this end, we doped anorthite-diopside (An-Di) eutectic glasses with ∼1000 and ∼10,000 ppm of Ga and In and determined their equilibrium partial pressures above the silicate liquid by Knudsen Effusion Mass Spectrometry (KEMS) using Ir cells at 1550–1740 K over the log(f O 2) range ΔIW+1.5 to ΔIW+2.5 (IW = iron-wüstite buffer). We detect Ga0 and In0 as the dominant vapour species and determine activity coefficients of γ (GaO 1.5) = 0.036(6) at 1700 K and of γ (InO 1.5) = 0.017(12) at 1674 K. Using these activity coefficients, we calculate partial pressures of Ga and In, together with those of similarly volatile elements, K and Zn and show that their relative volatilities from An-Di eutectic melts are in the in order Ga > K ∼ In > Zn, different from those predicted from their T c 50 under nebular conditions but in line with their relative abundances in the BSE. This substantiates the view that the abundances of volatiles in BSE, such as Ga and In, may have been set by evaporation from silicate melts under oxidising conditions at later stages of planetary accretion. Moreover, chondrules likely never underwent significant evaporation during melting and their volatile-depleted nature is likely inherited from the earliest solid condensates. [ABSTRACT FROM AUTHOR]

The application of carbonate clumped isotope (Δ 47) thermometry in deep-time is often limited by modification of the original temperature signal by thermal resetting. New modeling approaches to estimate the initial isotopic composition of partially reset calcites and maximal burial temperatures, however, open promising avenues in temperature reconstruction. Such approaches strongly depend on laboratory-derived kinetic parameters of calcite materials, which may differ in their microstructure, water content and distribution, and minor and trace element composition, and thus may have different resetting kinetics. The rostra of belemnites, an extinct group of mollusks with a wide temporal and spatial occurrence in the Mesozoic, have been extensively used for deep-time paleoclimate reconstructions using oxygen isotope geochemistry. Belemnites are also important targets for clumped isotope-based temperature reconstructions, but often are found to have reset Δ 47 compositions. Here, we present results from heating experiments on belemnite rostral calcite and optical calcite and provide belemnite-specific kinetic parameters for clumped isotope resetting. We show that belemnite calcite is altered faster and at lower temperatures than optical calcite and all other calcites reported in previous studies. We suggest that fast initial resetting results from oxygen isotope exchange of belemnite calcite with internal skeletal water present as fluid inclusions or organic-derived water, a process completed within 2–4 min at the experimental temperatures used here. Extrapolation to geological timescales using different solid-state bond reordering models shows that belemnite calcite resetting starts at lower burial temperatures than brachiopod, spar, and optical calcites. This susceptibility to thermal resetting results in a measurable (+3 °C) increase of the apparent Δ 47 temperature even under shallow to moderate burial conditions (i.e., 40–50 °C for 106–107 years timescales). Following the overprint to higher apparent Δ 47 temperatures during burial, the belemnite Δ 47 may further re-equilibrate during exhumation resulting in a decrease of apparent Δ 47 temperatures. Such "retrograde resetting" is similar to what is observed for carbonatites and marbles during cooling, and may cause underestimation of the thermal resetting a sample experienced during its geological history. Overall, our results demonstrate the importance of material-specific kinetic parameters and we urge caution when interpreting Δ 47 -derived temperatures of biogenic carbonates from deep-time archives. [ABSTRACT FROM AUTHOR]

Abstract: The Newtonian viscosity of synthetic rhyolitic liquids with 0.15–5.24wt% dissolved water was determined in the interval between 580 and 1640°C and pressures of 1atm and 5–25kbar. Measurements were performed by combining static and accelerated (up to 1000g) falling sphere experiments on water-bearing samples, with high temperature concentric cylinder experiments on 0.15wt% H2O melts. These methods allowed viscosity determinations between 102 and 107 Pas, and cover the complete range of naturally occurring magmatic temperatures, pressures, and H2O-contents for rhyolites. Our viscosity data, combined with those from previous studies, were modeled by an expression based on the empirical Vogel–Fulcher–Tammann equation, which describes viscosities and derivative properties (glass transition temperature Tg , fragility m, and activation volume of viscous flow Va) of silicic liquids as a function of P-T-X (H2O). The fitted expressions do not account for composition-dependent parameters other than X (H2O) and reproduce the entire viscosity database for silicic liquids to within 3.0% average relative error on log η (i.e. std. error of estimate of 0.26 log units). The results yield the expected strong decrease of viscosity with temperature and water content, but show variable pressure dependencies. Viscosity results to be strongly affected by pressure at low pressures; an effect amplified at low temperatures and water contents. Fragility, as a measure for the deviation from Arrhenian behavior, decreases with H2O-content but is insensitive to pressure. Activation volumes are always largely negative (e.g., less than −10cm3/mol) and increase strongly with H2O-content. Variations in melt structure that may account for the observed property variations are discussed. [Copyright &y& Elsevier]

Even though Ga 2 O 3 and In 2 O 3 are broadly used as semi-conductors, thermodynamic data for their vaporisation reactions exhibit a large spread. Therefore, the vaporisation behaviour of solid Ga 2 O 3 and In 2 O 3 was determined by means of Knudsen Effusion Mass Spectrometry (KEMS). Ga 2 O 3 and In 2 O 3 were studied in an iridium Knudsen cell and heated over a temperature range of 1200–1750 K in order to identify the species present in the vapour phase, and determine their partial pressures. We find that M 2 O (where M = Ga or In) is the most abundant gas species above the solid oxide, followed by M and MO, in accord with tabulated data. Following the calculation of partial pressures and equilibrium constants, we propose Δ f H 298, 3 rd o G a 2 O g = −68966 ± 7442 Jmol-1 and Δ f H 298, 3 rd o I n 2 O g = −22245 ± 964 Jmol-1 from the 3rd law method. Deviations in Δ f H 298, 3 rd o i relative to literature KEMS measurements are generally within ∼2% relative, and can be ascribed to the use of different ionisation cross sections, Knudsen cell material, temperature calibrations, as well as tabulated Gibbs energy functions. However, comparison with ab initio studies suggests the data reported in this work is more accurate than in previous studies, given that the Δ f H 298, 3 rd o I n O g = 157744 ± 3681 Jmol-1 deviates by only ∼0.2% from the theoretical value. [ABSTRACT FROM AUTHOR]

Despite their relatively simple mineralogical composition (olivine + Fe–Ni metal + FeS ± pyroxene), the origin of pallasite meteorites remains debated. It has been suggested that catastrophic mixing of olivine fragments with Fe–(Ni)–S followed by various degrees of annealing could explain pallasites bearing solely or prevalently fragmented or rounded olivines. In order to verify this hypothesis, and to quantify the grain growth rate of olivine in a liquid metal matrix, we performed a series of annealing experiments on natural olivine plus synthetic Fe–S mixtures. The best explanation for the observed olivine grain size distributions (GSD) of the experiments are dominant Ostwald ripening for small grains followed by random grain boundary migration for larger grains. Our results indicate that olivine grain growth in molten Fe–S is significantly faster than in solid, sulphur-free metal. We used the experimentally determined grain growth law to model the coarsening of olivine surrounded by Fe–S melt in a 100–600 km radius planetesimal. In this model, an impact is responsible for the mixing of olivine and Fe–(Ni)–S. Numerical models suggest that annealing at depths of up to 50 km allow for (i) average grain sizes consistent with the observed rounded olivine in pallasites, (ii) a remnant magnetisation of Fe–Ni olivine inclusions as measured in natural pallasites and (iii) for the metallographic cooling rates derived from Fe–Ni in pallasites. This conclusion is valid even if the impact occurs several millions of years after the differentiation of the target body was completed. [ABSTRACT FROM AUTHOR]

Abstract: We present 53 in situ measurements of the unit-cell volume (V) of polycrystalline (Mg0.64Fe0.36)O ferropericlase (FP) at simultaneous high pressures (P) and temperatures (T) up to P =26.7GPa and T =2173K, including pressure–temperature conditions at the top of Earth''s lower mantle. FP volumes were determined through in situ energy-dispersive synchrotron X-ray diffraction in a multi-anvil press, using gold and MgO as pressure markers. Our data, combined with 112 previously published P–V–T measurements for the same FP sample, were fitted to high-temperature Birch-Murnaghan and Mie-Grüneisen equations of state (EOS). Experimental data are reproduced accurately, with a standard deviation lower than 0.31GPa for both EOS. EOS calculations show that the thermal expansion and P–T derivatives of the bulk modulus of this iron-rich FP are virtually identical to those of pure MgO to pressures >55GPa and temperatures of 3000K. This result is confirmed by measurements of the normalised unit-cell volumes for FP and MgO at identical simultaneous high P–T conditions, which are identical to within 0.6 per cent relative to 26.7GPa and 2173K. The pressure and temperature derivatives of the bulk modulus, and thermal expansion are concluded to be independent of iron content across the range of plausible FP compositions in Earth''s lower mantle. [Copyright &y& Elsevier]