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We have determined the S contents of 14 different silicate melts equilibrated with Air/SO 2 mixtures at 1 atm pressure and temperatures of 1473–1773 K in 50 K intervals. Under these oxidizing conditions sulfur dissolves in the melts predominantly as sulfate (S6+ or SO 4 2–). The sulfur contents of the quenched products were determined by EPMA (Electron Probe Micro-Analysis) and by SIMS (Secondary Ion-Mass Spectrometry) with the latter being employed for low sulfur contents (i.e., ≤60 ppm). Equilibrium S contents were found to increase strongly with decreasing temperature and with some compositional parameters, especially CaO content. The data were used to calculate the sulfate capacity (log C S6+), for each melt composition at each temperature. Sulfate capacity is related to the equilibrium constant for sulfur dissolution and is defined as follows in terms of SO 4 2– concentration and fugacities of oxygen and sulfur: l o g C S 6 + = l o g SO 4 m e l t 2 - (w t. %) - 1 2 log fS 2 - 3 2 l o g fO 2 By determining log C S6+ at different Air/SO 2 ratios we showed that log C S6+ is independent of sulfur content and hence that Henry's Law is obeyed in the experimental composition and temperature range. We used stepwise linear regression to fit sulfate capacity to an equation incorporating compositional and temperature terms. Parameters which did not pass the F-test at the 95% confidence level were excluded. This approach led to the following fit equation: l o g C S 6 + = - 12.659 + (3692 X CaO - 7592 X SiO 2 - 13736 X TiO 2 + 3762 X Al O 1.5 + 34483) / T The standard error of the fit is 0.154 and R2 is 0.985. The data were used to simulate scenarios for SO 2 -degassing from ascending silicate melts of basaltic and andesitic compositions. The important observations are that SO 2 partitions strongly into an H 2 O-rich fluid phase and that most S is lost at relatively high pressures in the crust. So, for example we estimate that ≥85% of the initial S should be lost before the ascending magma reaches 100 MPa pressure. [ABSTRACT FROM AUTHOR]

• A new experimental method of controlling chlorine fugacity in silicate melts. • Chlorine dissolves by a mechanism in which 2 dissociated Cl− ions replace 1 O2− ion. • Chlorine remains stably dissolved in basalt to pressures <5 MPa. • Typical Cl 2 fugacities in basalt are ∼10−6 to 10−8 at the FMQ oxygen buffer. We have performed experiments at 0.5–2 GPa and 1200–1500 °C to investigate the dissolution behaviour of chlorine in silicate melts. The experiments were performed with chlorine fugacities controlled by mixtures of Ag, AgCl and AgI and oxygen fugacity buffered at C-CO-CO 2 (CCO) and Re-ReO 2. The results demonstrate that the initial chlorine dissolution mechanism involves the replacement of O2− in the silicate melt by two dissociated Cl− ions according to the reaction: Cl 2 + [O2− melt = 2[Cl− melt + 0.5O 2. The same dissolution mechanism applies to hydrous, fluid-saturated basalt at 100–200 MPa/1050 °C. Experiments using both an Fe-free haplobasaltic composition (An 50 Di 28 Fo 22) and an Icelandic basalt followed the predicted dependence of Cl concentration on f (Cl 2)0.5 and f (O 2)0.25. This Henrian behaviour extends from 0 to at least 2.6 wt% Cl dissolved in the haplobasaltic composition, 1.6 wt% Cl in anhydrous basalt and ∼1.5 wt% Cl in fluid-saturated basalt. Deviations from Henry's Law behaviour at higher concentrations are consistent with progressive association of Cl− ions. In the Henry's Law region Cl concentration in the An 50 Di 28 Fo 22 composition is given by (wt%): log Cl melt = 1.206 32 - 940 40 P T - 0.25 l o g f O 2 + 0.5 l o g f Cl 2 P is in GPa, T in kelvin, values in brackets are 1 standard error, and f (Cl 2) and f (O 2) refer to standard states of pure gas at 0.1 MPa and the temperature of interest. For the natural anhydrous basalt we obtain: log Cl melt = 0.984 64 - 930 70 P T - 0.25 l o g f O 2 + 0.5 l o g f Cl 2 By considering the P-T dependences of the Cl contents of melts we find that the concentrations observed in nature are extremely stable in basalt to very low pressures. Basalts containing the typical concentration range of 0.05–0.5 wt% Cl should, for example, only begin to degas their chlorine significantly, as HCl, at pressures in the range 0–5 MPa. Data on hydrous, fluid-saturated basalt at 100–200 MPa are, when corrected for dissolution of Ca, Na and K in the fluid, broadly consistent with our results for anhydrous basalt. Finally, we use recently evaluated thermodynamic data for sodalite (Na 4 Al 3 Si 3 O 12 Cl) to calculate the conditions under which this phase would stabilise in trachytes and phonolites. We find that the appearance of sodalite as a liquidus phase reflects a combination of low liquidus temperature and high Na 2 O activity rather than unusually high chlorine fugacity. [ABSTRACT FROM AUTHOR]

We have performed experiments at 1.5 GPa over the temperature range 1400–2100 °C to determine the partitioning of lithophile elements (U, Th, Eu, Sm, Nd, Zr, La, Ce, Yb) between sulfide liquid, low-S metals and silicate melt. The data demonstrate pronounced increases in partitioning of all the lithophile elements into sulfide at very low FeO contents (<1 wt%) of the silicate melt such that D U = [ U ] sulfide [ U ] silicate exceeds 1 and may be >10 in some cases. Similarly D Sm may be >2 under the same conditions of low silicate FeO. This strong partitioning behaviour is found only be important in S-rich metals, however because the observed effect of low FeO on partitioning is uniquely confined to metallic melts close to stoichiometric FeS in composition. The results and the effects of FeS content of the metal and FeO content (or activity) of the silicate may be understood in terms of exchange reactions such as: UO 2 + 2 FeS = 2 FeO + US 2 Silicate sulfide silicate sulfide High concentrations of FeS (in metal) and low FeO contents of the silicate melts drive the reaction to the right, favouring high US 2 in the sulfide and hence high D U . The effect is, we find, enhanced by the high solubility of S in the silicate (up to 11 wt%) at low FeO contents. This S content greatly reduces the activity coefficient of FeO in the silicate melt, enhancing the displacement of the reaction to the right. For sulfide-silicate partitioning at 1.5 GPa and 1400 °C we obtain D Nd /D Sm of about 1.4 and D Th ∼ 0.1D U . With increasing temperature the differences between these geochemically similar element pairs decreases such that, at 2100 °C D Nd /D Sm is 1.0 and D Th /D U is about 0.3. We used these results, together with D U and D Sm to model addition of a putative Mercury-like component (with FeS core) to early Earth. We find that the 1400° results could lead to a significant (∼11 ppm) 142 Nd anomaly in silicate Earth and add >8 ppb U to the core, but lead to an unreasonably high Th/U of silicate Earth (4.54). Based on the 2100 °C results the 142 Nd anomaly would be 0 but addition of the sulfur-rich body could add up to 10 ppb of U to the core, generating, when the accompanying 21 ppb Th is also considered, ∼3 TW of the energy required for the geodynamo. In this case, the Th/U ratio of silicate Earth would approximate 4.3, within the range of some estimates. [ABSTRACT FROM AUTHOR]

The majority of attempts to reconstruct the evolutionary history of the hominin clade proceed as if the hominin fossil record is a precise, accurate, and comprehensive record of human evolutionary history. In this contribution we review the various ways in which the apparent scarcity of early hominins on the landscape means that the existing hominin fossil record almost certainly falls short of this assumption, especially with respect to taxic diversity, as well as the spatial and temporal distribution of known taxa. We also suggest that interpretations of the hominin fossil record are particularly affected by practices that likely violate the principles of reproducibility, as well as by confirmation bias. The hominin fossil record should be seen for what it is; an incomplete record of human evolutionary history that limits what should be said about it. Generated narratives should be treated as heuristic devices, not as accurate and comprehensive descriptions of past events. [ABSTRACT FROM AUTHOR]

Comparative genomic analysis of the nuclear receptor family suggests that the testicular receptor 2, Nr2c1 , undergoes positive selection in the human-chimpanzee clade based upon a significant increase in nonsynonymous compared to synonymous substitutions. Previous in situ analyses of Nr2c1 lacked the temporal range and spatial resolution necessary to characterize cellular expression of this gene from early to mid gestation, when many nuclear receptors are key regulators of tissue specific stem or progenitor cells. Thus, we asked whether Nr2c1 protein is associated with stem cell populations in the mid-gestation mouse embryo. Nr2c1 is robustly expressed in the developing olfactory epithelium. Its expression in the olfactory epithelium shifts from multiple progenitor classes at early stages to primarily transit amplifying cells later in olfactory epithelium development. In the early developing central nervous system, Nr2c1 is limited to the anterior telencephalon/olfactory bulb anlagen, coincident with Nestin -positive neuroepithelial stem cells. Nr2c1 is also seen in additional cranial sensory specializations including cells surrounding the mystacial vibrissae, the retinal pigment epithelium and Scarpa's ganglion. Nr2c1 was also detected in a subset of mesenchymal cells in developing teeth and cranial bones. The timing and distribution of embryonic expression suggests that Nr2c1 is primarily associated with the early genesis of mammalian cranial sensory neurons and craniofacial skeletal structures. Thus, Nr2c1 may be a candidate for mediating parallel adaptive changes in cranial neural sensory specializations such as the olfactory epithelium, retina and mystacial vibrissae and in non-neural craniofacial features including teeth. [ABSTRACT FROM AUTHOR]

The accretion of the Earth was marked by the high-pressure segregation of the core, accompanied by dissolution of about 10% of one or more “light” elements into the metal. Cosmochemical data suggest that, of these 10% “light” elements, the core contains ∼1.7% S (Dreibus and Palme, 1996) and there is evidence that volatile elements such as S accreted to the Earth late in planetary growth, plausibly as a sulfide “matte” (O’Neill, 1991). Given that metallurgical data indicate that dissolution of even small amounts of sulfur in liquid Fe can have profound effects on the activities of some trace components, we have undertaken a study of the affect of S on the metal–silicate partitioning of a number of the most important chalcophile and siderophile elements. We performed experiments at 1.5 GPa and 1460–1650 °C on metal–silicate partitioning of W, Mo, Ni, Co, Cu, Ag, Mn, Cd, Tl, Cr, Sb, In, Pb, Ga, Ge, V and Zn under conditions where the S content of the metal was varied from 0 to 37 wt%. Mn and Ag were found to exhibit the highest ratio of chalcophile to siderophile behaviour while W, Ga and Sb are the most “chalcophobic” of the elements studied. In terms of the 1-parameter epsilon model (Wagner, 1962) we derived values for each element at 1600 °C as follows (negative values indicate chalcophile behaviour): ε Cu S = - 2.57 ; ε Mn S = - 6.41 ; ε Ag S = - 4.15 ; ε Sb S = 4.36 ; ε Cd S = - 3.78 ; ε In S = - 0.24 ; ε Tl S = - 6.21 ; ε Ga S = 6.54 ; ε Pb S = - 3.73 ; ε Cr S = - 3.70 ; ε Ge S = 7.03 ; ε V S = - 3.14 ; ε Zn S = - 1.68 ; ε Mo S = 2.27 ; ε W S = 6.47 ; ε Ni S = 2.17 ; ε Co S = 2.40 . We use these new data in conjunction with published pressure–temperature dependences of metal–silicate partitioning to test the effects of accreted S on the calculated trace element concentrations in bulk silicate Earth. The approach employs a continuous accretion model in which the oxidation state of the Earth and pressure of core segregation both increase during accretion. We find that, without addition of S in the latter stages of accretion, the Mo/W ratio of silicate Earth would be several times larger than that observed. Addition of ∼2% S accompanied by small amounts of carbon in the last 15% of accretion, however, enables us to match the observed concentrations of these elements in silicate Earth. This confirms an earlier conclusion that the Mo/W ratio of silicate Earth requires late sulfide addition to the core (Wade et al., 2012). Further support for late sulfide addition to the core comes from the depletion factors of volatile chalcophile elements Cu, Ga, Sb, Ag, Zn, Pb, Cd, In and Tl in silicate Earth relative to lithophile elements of similar volatility. We find that depletions of these elements are well correlated with their partition coefficients into sulfide (FeS) liquids at 1.5 GPa and temperatures of 1460–1650 °C. In contrast there is essentially no correlation between their depletion factors and sulfur-free liquid Fe–silicate partition coefficients. [ABSTRACT FROM AUTHOR]

Abstract: In order to place better constraints on the conditions of core formation on Earth and other planetary bodies we have performed experiments to determine the partitioning of Mo and W between liquid Fe-rich metal and liquid silicate at pressures of 1.5–24GPa and temperatures of 1803–2723K. Experiments performed in MgO capsules at 1.5GPa/1923K indicate that Mo is in the +4 oxidation state in the silicate at oxygen fugacities >2log units below the IW (Fe–FeO) buffer. In contrast W6+ is the dominant tungsten oxidation state in the silicate at 1.5GPa/1923K and 1.8–3.3log units below the IW buffer. When our 15 data for pressures between 6 and 24GPa are combined with those of we find evidence neither for a change in oxidation state of W above 6GPa nor for a change in pressure dependence of partitioning in the experimental fO2 range. Metal–silicate partitioning of both Mo and W shows strong dependence on silicate melt composition with both elements becoming more siderophile as the melt becomes more SiO2-rich. Although the trends in the partitioning data can be related to silicate melt composition in terms of the ratio of nonbridging oxygens to tetrahedral cations we find that use of a regular solution model for the silicate melt results in a significantly better fit to the data. We combined our results with those in the literature to obtain partitioning equations applicable to the Earth. In terms of weight partitioning we define and as follows: The experimental data, when corrected for compositional effects, yield the following expressions for a pyrolite mantle: The value in brackets corresponds to 1 standard error of the fit. These expressions were combined with the continuous accretion model of to investigate the constraints which they place on the accretionary process. We find, however, that, for accretionary paths consistent with the silicate Earth contents of Ni, Co, V, Cr and Nb, W should partition twice as strongly into the core as Mo. This is in stark contrast to the estimated core–mantle partition coefficients of ∼40 for W and 90–140 for Mo. Neither changes to the accretionary path nor the assumption of partial disequilibrium can readily alter this result. The answer appears to reside with the identity of one of the light elements in the core. We investigated the effect of S on our accretionary model by adding 2% of this element (consistent with cosmochemical estimates) to the core. If S is added at constant S/Fe ratio throughout accretion the net effect is negligible. If, however, S is added exclusively during the last 10–20% of accretion D Mo and D W become consistent with the silicate Earth contents of these elements. Only small additional adjustments to the model are required to accommodate changes in partitioning of Ni, Co, V, Cr and Nb. We conclude that the Mo and W contents of the silicate Earth indicate that S (and other moderately volatile elements) was added to the Earth during core formation but only during the last ∼20% of accretion. This conclusion is the same as that reached by from the Ag isotopic composition of silicate Earth. [Copyright &y& Elsevier]

Abstract: We have performed simulations of the electron microbeam analysis of metal–silicate partitioning experiments in which a small (∼50μm) ball of metal is embedded in a silicate matrix. This geometry approximates that found in many high pressure diamond-cell experiments aimed at determining the partition coefficients (D i) of elements between metal and silicate. The simulations involved using the (publicly available) Monte Carlo simulation package PENEPMA which is based on the general-purpose electron–photon transport code PENELOPE. The code allows complex geometrical structures to be defined by quadric surfaces, which enclose homogeneous materials specified by the user. The package simulates both electron and photon transport, keeps track of the X-ray production mechanism and includes the continuum contribution. Our principal results came from simulating the effects of secondary fluorescence on the apparent metal–silicate partition coefficient of Ni for the case of an Ni-rich metal (26.8% Ni, 73.2% Fe) embedded in a silicate matrix. If, as is frequently the case, analysis of the silicate is performed 5–10μm from the silicate–metal interface then secondary fluorescence of Ni in the metal will increase the apparent NiO concentration in the silicate by ∼1000ppm. In the case considered, this is 25% of the Ni content of the silicate and the fluorescence effect decreases the apparent D Ni from 110 to 84. For this type of experiment, therefore, the fluorescence effect reduces the apparent partition coefficient. We performed similar simulations of the analysis of the centre of a 10μm diameter metal ball embedded in the silicate matrix to determine apparent Si and O concentrations in the metal. In this case, because the electrons do not penetrate so far into the metal, the fluorescence effect is very small and can be neglected. Finally, we show that focused ion beam (FIB) sections, while providing the potential physically to separate metal from silicate present additional, but not insurmountable, problems of standardisation. The best results would be obtained with a section >3μm in thickness. We recommend that, in general, experimentalists consider simulating analysis of a typical experimental product before publication of results which may be affected by secondary fluorescence. [Copyright &y& Elsevier]

Abstract: We have determined the liquid metal–liquid silicate partitioning of Ni, Co, Mo, W, V, Cr and Nb at 1.5GPa/1923K and 6GPa/2123K under conditions of constant silicate melt composition with variable amounts of Si in the Fe-rich metallic liquid. Partitioning of Ni, Co, Mo, W and V is sensitive to the Si content of the metal with, in all five cases, increasing Si tending to make the element more lithophile than for conditions where the metal is Si-free. In contrast, metal–silicate partitioning of Cr and Nb is, at constant silicate melt composition, insensitive to the Si content of the metal. The implications of our data are that if, as indicated by the Si isotopic composition of the silicate Earth (), the core contains significant amounts of Si, the important siderophile elements Ni, Co, W and Mo were more lithophile during accretion and core formation than previously believed. We use our new data in conjunction with published metal–silicate partitioning results to develop a model of continuous accretion and core segregation taking explicit account of the partitioning of Si (this study) and O (from ) between metal and silicate and their effects on metal–silicate partitioning of siderophile elements. We find that the effect of Si on the siderophile characteristics of Ni, Co and W means that the pressures of core segregation estimated from these elements are ∼5GPa lower than those derived from experiments in which the metal contained negligible Si (e.g., ). The core–mantle partitioning of Cr and Nb requires that most of Earth accretion took place under conditions which were much more reducing than those implied by the current FeO content of the mantle and that the oxidation took place late in the accretionary process. Paths of terrestrial accretion, oxidation state and partitioning which are consistent with the current mantle contents of Ni, Co, W, V, Cr and Nb lead to Si and O contents of the core of ∼4.3wt.% and 0.15%, respectively. [Copyright &y& Elsevier]

Abstract: We performed a series of synthesis experiments at 1atm pressure to investigate the substitution mechanisms of 1+ and 3+ ions into olivine. Forsterite crystals were grown from bulk compositions that contained the element of interest (e.g. Li) and different amounts of additional single trace elements. By working at constant (major element) liquid composition and temperature we eliminated all compositional effects other than those due to the trace elements. Mineral–melt pairs were then analysed to determine the compositional-dependence of the partition coefficient (D), which corresponds to , and where [element] refers to weight concentration of the element in the respective phase. We find that Li forms a stable coupled substitution with Sc and, at above ∼500ppm Sc in the crystal, Li+ and Sc3+ ions form an ordered neutral complex ([LiSc]). This complex dissociates at lower trace element concentrations and a second, concentration-independent, mechanism begins to dominate. This second solution mechanism is most likely 2Li+ ⇔Mg2+ where one of the Li atoms is in an interstitial position in the crystal lattice. Natural olivines show Li contents slightly greater than Sc (on an atomic basis), indicating that both substitution mechanisms are significant. Unlike Sc, Al does not appear to form a stable complex with Li in the olivine structure. Sodium is highly incompatible in olivine with of ∼0.00015–0.03. Olivine–liquid partitioning of Na+ is independent of Sc3+ or Al3+ concentration. This indicates that the coupled substitution of Na+ with any 3+ ions is unlikely. Instead, the relevant substitution mechanism appears to be 2Na+ ⇔Mg2+. Although independent of 3+ ion concentration, is inversely correlated with the Li concentration of both melts and crystals, implying that Na competes (unsuccessfully) with Li to replace Mg in the olivine structure. Aluminium is highly incompatible in forsterite . Values of are similar for all phase pairs synthesised from starting materials containing between 10 and 100,000ppm Al. This suggests that Al is principally incorporated in forsterite by replacing one Mg and one Si atom , where the Al atoms on octahedral (Mg) and tetrahedral (Si) sites are dissociated from one another. The incorporation of gallium into forsterite is influenced by the presence of Li. Where Li concentration in the crystal is much greater than that of Ga (on an atomic basis) we find an excellent correlation between and melt Li content. This relationship indicates that Ga3+ and Li+ replace 2Mg2+ on octahedral sites and that the Ga and Li atoms are, like Sc and Li, strongly associated in the crystal structure. The mechanism by which scandium is incorporated into forsterite is strongly governed by the presence Li. As discussed above, ordered complexes form readily in forsterite in Li-rich experiments. Under Li-absent but Sc-rich conditions (Sc in the crystal >∼500ppm), is proportional to the concentration of Sc in the melt. This indicates that Sc incorporation is charge-balanced by the formation of magnesium vacancies , and that both species are associated . At lower Sc concentrations (<500ppm in the crystal), the concentration-dependence of partitioning indicates that the complexes dissociate. Our results demonstrate that partitioning of 1+ and 3+ ions into olivine is complex and involves a range of point defects which yield strongly composition-dependent crystal–melt partition coefficients. Since physical and chemical properties of natural olivine, such as diffusion of 6Li and 7Li and H2O solubility, depend on the concentrations of the defects identified in this study, our results provide an important insight into how determining substitution mechanisms can improve our understanding of large-scale mantle processes and properties. [Copyright &y& Elsevier]

Abstract: We present the results of melting experiments to 7.7GPa and 2200°C on a synthetic ‘chondrite-like’ composition suitable to model early planetesimal differentiation. Our principal observation is that two immiscible liquid alloys coexist to about 5.5GPa in Fe–Ni–S–C–O compositional space, with one liquid alloy being enriched in S and the other enriched in C. The chemical distinctions between the two liquid alloys progressively weaken as pressure increases. This is related to the contraction of the miscibility gap with increasing pressure. With the moderately C- and S-depleted composition used in this study, we observed closure of the miscibility gap at about 5.5GPa. Our results have implications for core formation on planetary bodies that have undergone extensive melting. Because of the characteristics of the immiscibility region, core formation and differentiation histories would have been largely dependent on the size of these bodies and their C and S contents. Our results indicate that relatively small bodies with elevated S and C contents would have likely experienced significant core stratification. [Copyright &y& Elsevier]

Abstract: We have combined metal–silicate partitioning data from the literature with new experimental results at 1.5–8GPa and 1480–2000°C to parameterize the effects of pressure, temperature and composition on the partitioning of V, Cr and Nb between liquid Fe metal (with low S and C content) and silicate melt. Using information from the steelmaking literature to correct for interactions in the metal phase, we find that, for peridotitic silicate melts, metal–silicate partition coefficients are given by:where is the partition coefficient for element i. Partitioning of V and Cr is insensitive to silicate melt composition, but Nb shows a considerable compositional effect. The new data enable us to examine different models of terrestrial accretion and core formation. If we fix the Fe content of the mantle at the current value and use the Ni content of the mantle to estimate pressure of equilibration then temperatures about 1200°C above the silicate liquidus are required to match vanadium partitioning to the current concentration of V in the mantle. Under these conditions the core concentrations of Si (∼15%) and Nb (>60% of Earth’s budget) are implausibly high. A more realistic approach is to assume that the metal of accreting planetesimals equilibrated at the base of a deep magma ocean whose temperature was close to the silicate liquidus. As the magma ocean deepened in proportion to the size of the Earth the metal was continuously extracted to the core without further re-equilibration in the lowermost mantle. In this case the V, Cr and Nb contents of core and mantle can easily reproduce the expected values provided the Earth became more oxidized as it accreted (O’Neill H. S. (1991) The origin of the Moon and the early history of the Earth—a chemical model. 2. The Earth Geochim. Cosmochim. Acta 55(4), 1159–1172; Wade J. and Wood B. J. (2005) Core formation and the oxidation state of the Earth. Earth Planet. Sci. Lett. 236, 78–95; Wänke H. and Dreibus G. (1988) Chemical-composition and accretion history of terrestrial planets. Philos. Trans. Roy. Soc. Lond. A-Math. Phys. Eng. Sci. 325(1587), 545–557). Increasing oxidation requires the oxidized Fe content of the mantle to increase from 0.5% to 1.0% to the current value of 6.26% as the Earth grew. This model , with magma ocean thickness corresponding to 35% of mantle depth, reproduces the calculated core–mantle partitioning of Ni, and Co and yields a Si content of the core of approximately 6%, in good agreement with cosmochemical estimates. [Copyright &y& Elsevier]

We have determined the partitioning of 39 trace elements between majorite-rich garnet and silicate melt at 25 GPa and ∼2300 °C. Concentrations in crystalline and quenched melt phases were determined by both SIMS and laser ablation-ICPMS techniques with, in general, very good agreement in partition coefficients. As anticipated from size and charge considerations, DLu (where DLu=[Lu]Majorite/[Lu]Melt=0.77) is much greater than DLa (=0.02) and K, U and Th are strongly incompatible. Partition coefficients for isovalent cations entering either the X- or Y-site exhibit a near-parabolic dependence on the radius of the incorporated cations as predicted from the lattice strain model. This underlines the important contribution made by the crystal structure to the control of trace element partitioning. Our data indicate that anhydrous melting at high pressures in the presence of residual garnet could lead to the generation of Al-depleted komatiites. The partitioning data have also been used to assess the potential effects of majoritic garnet fractionation into the deep mantle during a putative magma ocean event in early earth history. Crystallisation of a maximum of 14% majoritic garnet would have slightly depleted the primitive upper mantle in Si and Al without disturbing the chondritic abundance patterns of other refractory elements. The resultant reservoir would have a high Lu/Hf ratio, but a chondritic Sm/Nd ratio and would not be enriched in heat producing elements. [Copyright &y& Elsevier]

Many experimental data demonstrate distinct differences between crystal-liquid partition coefficients Di measured under high temperature, anhydrous conditions and those determined at lower temperatures in the presence of H2O. In this study we develop a thermodynamic method of separating the effects of H2O from those of temperature. We then apply the method to predict partitioning of REE between clinopyroxene, garnet and silicate melt over wide ranges of temperature, pressure and H2O content.Our initial inputs are the model of for REE partitioning between clinopyroxene and anhydrous melt and the melting temperatures of diopside on the join CaMgSi2O6-H2O . We then make the hypothesis that the effect of H2O on the activity of REE clinopyroxene component (REEMgAlSiO6) in the melt is the same as the measured effect on CaMgSi2O6 component. This leads to predictions of REE partition coefficients for clinopyroxene coexisting with hydrous melt at any P,T and H2O content up to 45 weight %. The results agree with observed REE partition coefficients with a standard deviation which is the same as that for the anhydrous data. We conclude, therefore, that the ‘H2O-effect’ may, in this case, be accurately predicted. We extend the approach to garnet by using the join Mg3Al2Si3O12-H2O to estimate the effects of H2O on all ‘garnet-like’ components in the melt. This enables calculation of garnet-melt REE partition coefficients for melts containing up to 25% H2O.The observation that H2O influences major and trace component activities in a similar manner enables us to make some generalisations about the combined effects on partitioning of decreasing temperature and increasing water content of the melt. The relative enthalpies of fusion ΔHf of major and trace components dictate whether trace element partition coefficients increase or decrease with H2O addition:ΔHftrace > ΔHfmajor gives increasing Dtrace with addition of H2O and ΔHftrace < ΔHfmajor leads to decreasing Dtrace with addition of H2O.Note that Dtrace is strictly the ratio of mole fractions of charge-balanced components such as REEMgAlSiO6 in solid to melt phases. In the cases considered here, however, Dtrace closely approximates D =SHAPE="BUILT" ALIGN="C" STYLE="S">[trace]solid[trace]melt where [trace] refers to weight concentration. For clinopyroxene ΔHfREE < ΔHfmajor and H2O addition leads to substantial decreases in REE partition coefficients. For garnet, ΔHfREE &ap; ΔHfmajor and addition of H2O results in little change in DREE. The consequences are illustrated with respect to Yb contents of melts of spinel lherzolite. For partial melting under anhydrous conditions, Yb is relatively compatible in clinopyroxene and the Yb contents of the product melts remain close to 5 times source values at all F (fraction of equilibrium batch melting) between 0 and 0.25. Hydrous peridotite produces melts, because of low DYb, which are strongly enriched in Yb (up to at least 10 times source) and whose Yb concentrations depend strongly on F. This means that the low Yb concentrations observed in many island arc tholeiites cannot be due to the addition of water to fertile peridotite. [Copyright &y& Elsevier]

There is, therefore, no theoretical or observational reason for fixing the entropy of the S-Fe electron exchange reaction in melts at a value fixed by the entropies of solids in an analogous solid-solid reaction. Reply to: O'Neill, H.S., 2021: "Comment on 'Compositional and temperature effects on sulfur speciation and solubility in silicate melts' " by Nash et al. [Earth Planet. Sulfur speciation, sulfur-iron electron exchange, redox Keywords: sulfur speciation; sulfur-iron electron exchange; redox EN sulfur speciation sulfur-iron electron exchange redox N.PAG N.PAG 1 03/08/21 20210415 NES 210415 1 Introduction We begin by stating that we reject the assertion of [8] that, in [6] we made an inappropriate temperature-extrapolation of our 1300°C experimental data. [Extracted from the article]

Abstract We have determined the chemical speciation of dissolved sulfur and the sulfur concentration at fixed oxygen and sulfur fugacities for a wide range of silicate melt compositions (from Fe-rich basalt to dacite). Each melt was equilibrated at 1300 °C and 1-atmosphere pressure at oxygen fugacities (f O 2) between −1.67 and +1.6 log units relative to the Fayalite–Magnetite–Quartz (FMQ) buffer and absolute sulfur fugacities between −5.1 and −1.2 log units. The f O 2 and f S 2 of the experiments were controlled by using gas mixtures of CO–CO 2 –SO 2. The speciation of sulfur in the quenched glasses was determined using both X-ray Absorption Near-Edge Spectroscopy (XANES), and from the dependence of equilibrium sulfur concentration on the f S 2 / f O 2 ratio measured by secondary-ion mass spectrometry (SIMS) and electron microprobe. The speciation of dissolved sulfur in each melt undergoes an abrupt transformation from S2− to S6+ with increasing f O 2 , and this transition is shifted ∼0.5 log units higher in f O 2 as melt FeO concentration increases from ∼5 wt% to ∼18 wt%. Since sulfide concentrations at constant f O 2 and f S 2 are consistently greater for more FeO-rich melts, the compositional effect on speciation may be explained by the well-known sensitivity of the sulfide capacity (C S 2 − ) of the melt to FeO concentration. S6+/S2− ratios for the glasses exhibit a linear relationship with Fe3+/Fe2+, indicating that the redox couples for iron and sulfur can be directly related to one another. We used thermodynamic data to model the interrelationship between Fe and S oxidation states in terms of the equilibrium FeS + 8 FeO 1.5 = 8 FeO + FeSO 4 Fitting the data to our experiments at 1300 °C we obtained the following expression for the temperature-dependence of speciation: log ⁡ ( S 6 + S 2 − ) = 8 log ⁡ ( Fe 3 + Fe 2 + ) + 8.7436 × 10 6 T 2 − 27703 T + 20.273 This equation fits the data for all our compositions and is also consistent with earlier results at 1050 °C and 950 °C. We used the interdependence of S and Fe oxidation states to infer electron transfer between Fe2+ and S6+ during quenching of glasses from Mauna Kea, Hawaii. The effect is sufficient to cause significant overestimation of equilibrium Fe3+/ Σ Fe in natural glasses and corresponding overestimate of f O 2 by about 0.8 log units. Glasses equilibrated under the most oxidizing conditions (containing S6+ only) have equilibrium S concentrations that are negatively correlated with their mole fractions of tetrahedral (Si + Ti) cations. Highlights • Measurements of S6+/S2− in 6 silicate melts varying from Martian basalt to terrestrial dacite. • The change in S6+/S2− of a silicate melts with f O 2 follows the thermodynamically predicted trend. • S6+/S2− and Fe3+/Fe2+ ratios of silicate melts are quantitatively linked. • Electron exchange between Fe2+ and S6+ during quench may modify Fe3+/Fe2+ in natural glasses. [ABSTRACT FROM AUTHOR]