No description available.

Understanding Earth’s accretion and primary differentiation is a long-standing goal of geology. The segregation of FeNi metal from molten silicate to form Earth’s core is expected to deplete and fractionate the highly siderophile elements (HSEs). Estimates of the primitive upper mantle (PUM) composition however, reveal only modest HSE depletions and chondritic element ratios. Past experiments to determine if the mantle composition is set by high-temperature metal-silicate equilibrium have involved measuring the solubility of HSEs in silicate melt at conditions more reducing than the iron-wustite (IW) buffer. Accurate determination of solubilities at such conditions has been hindered by the formation of dispersed metal inclusions; this work describes methods to circumvent the problem. Results of three separate studies are presented which document the solubility of Re, Pt and Au in molten silicate which is demonstrably nugget-free. Data obtained from experiments done at 0.1 MPa–2 GPa, 1573–2573 K and ~ IW -1.5 to +3 reveal: 1) Re, Pt and Au solubility increases with increasing temperature, 2) Re solubility increases with increasing oxygen fugacity (fO2), consistent with dissolution as oxide species, 3) Below ~ IW +3, Pt and Au solubility increases with decreasing fO2, consistent with dissolution as neutral or silicide species, and 4) that Au is amongst the most soluble HSE in molten silicate, with values increasing with temperature, but insensitive to changes in P, fO2 and melt composition, making it well suited as a geothermometer for core formation. Partition coefficients calculated from these and previous solubility measurements indicate that metal-silicate equilibrium is unable to reproduce the Re/Os and Pt/Os ratios required by PUM Os isotope systematics if simultaneously accounting for the observed absolute element abundances. Instead, results support late accretion of material following core formation, elevating element abundances and endowing chondritic inter-element ratios. Experimental results are incorporated into a terrestrial accretion model, which differs from the standard approach by explicitly accounting for the distribution of oxygen. Model results show siderophile element abundances in PUM are best reproduced if the mantle undergoes oxidation during accretion and metal-silicate equilibrium occurs near the peridotite solidus.

Understanding Earth’s accretion and primary differentiation is a long-standing goal of geology. The segregation of FeNi metal from molten silicate to form Earth’s core is expected to deplete and fractionate the highly siderophile elements (HSEs). Estimates of the primitive upper mantle (PUM) composition however, reveal only modest HSE depletions and chondritic element ratios. Past experiments to determine if the mantle composition is set by high-temperature metal-silicate equilibrium have involved measuring the solubility of HSEs in silicate melt at conditions more reducing than the iron-wustite (IW) buffer. Accurate determination of solubilities at such conditions has been hindered by the formation of dispersed metal inclusions; this work describes methods to circumvent the problem. Results of three separate studies are presented which document the solubility of Re, Pt and Au in molten silicate which is demonstrably nugget-free. Data obtained from experiments done at 0.1 MPa–2 GPa, 1573–2573 K and ~ IW -1.5 to +3 reveal: 1) Re, Pt and Au solubility increases with increasing temperature, 2) Re solubility increases with increasing oxygen fugacity (fO2), consistent with dissolution as oxide species, 3) Below ~ IW +3, Pt and Au solubility increases with decreasing fO2, consistent with dissolution as neutral or silicide species, and 4) that Au is amongst the most soluble HSE in molten silicate, with values increasing with temperature, but insensitive to changes in P, fO2 and melt composition, making it well suited as a geothermometer for core formation. Partition coefficients calculated from these and previous solubility measurements indicate that metal-silicate equilibrium is unable to reproduce the Re/Os and Pt/Os ratios required by PUM Os isotope systematics if simultaneously accounting for the observed absolute element abundances. Instead, results support late accretion of material following core formation, elevating element abundances and endowing chondritic inter-element ratios. Experimental results are incorporated into a terrestrial accretion model, which differs from the standard approach by explicitly accounting for the distribution of oxygen. Model results show siderophile element abundances in PUM are best reproduced if the mantle undergoes oxidation during accretion and metal-silicate equilibrium occurs near the peridotite solidus.

Volatile heavy metals are liberated from magmas during eruptive and passively degassing volcanic activity. Volcanic emanations have been estimated to contribute 20-40% of volatile elements such as Bi, Pb, As or Sb, and up to 40-50 % of Cd and Hg annually (Nriagu, 1989). Some workers, however, believe these ranges are too high (Hinkley, 1999) or too low (Zreda-Gostynska and Kyle, 1997) leading to considerable differences in global inventory budgets of these metals and the degree to which they load the atmosphere. The objective of this work is to investigate the behavior of volatile heavy metals such as Au, Tl, As, Pb etc. in subaerially erupted magmas and experimentally in silicate melts. Analysis of natural pumice samples confirm the futile, sporadic nature of Hg and associated heavy metals, suggesting these metals are fully degassed prior to deposition. Diffusion experiments were conducted in natural basalt, dacite and synthetic rhyolite (Ab-Or-Qz minimum eutectic) over a range of temperatures (1200 – 1430 °C) at 0.1 MPa. Starting compositions were doped with a heavy metal cocktail (Bi, Pb, Tl, Au, Re, Sb, Sn, Cd, Mo, As, Cu) and loaded into open top Pt capsules. One set of experiments examined the effect of melt composition (polymerization) on element diffusion, and the second investigated the effects of ligands on diffusion by adding known concentrations of Cl and S. During experiments of varying duration, concentration gradients arose in the volatile trace metals due to their varying volatility, as measured (normal to the melt/gas interface) by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) in quenched glasses. Diffusion profiles followed an Arrhenius relationship from which diffusion coefficients (D) and activation energies (Ea) were obtained for Au, Tl, As, Cd, Re, Pb and Bi (in decreasing order of volatility). Results show Au and Tl are the most volatile in dacite and rhyolite yielding LogDDac Au = -10.7 ± 0.1 m2/s and LogDDac Tl = -10.9 ± 0.1 m2/s in dacite, and LogDRhy Au = -10.9 ± 0.1 m2/s and LogDRhy Tl = -11.3 ± 0.3 m2/s in rhyolite respectively. The D for Au could not be measured in basalt but Tl was the fastest diffusing species LogDBas Tl = -10.8 ± 0.2 m2/s. Ligands Cl and S were shown to increase the volatilities of all metals, with S having a more profound effect. Diffusivities were applied to a simple 1D bubble growth model (Smith 1955). Model results indicate diffusion coefficients play a major role in metal fractionation processes occurring at depths that ultimately dictate what metal ratios are measured at the surface of volcanoes.

No description available.

Le soufre est un composant volatil important des magmas qui présente différents états d'oxydation en fonction des conditions d’oxydoréduction et de la phase dans laquelle il se trouve : dans le liquide silicaté, il est typiquement dissous comme S⁶⁺ et/ou S²⁻ , dans la phase gazeuse il se trouve principalement comme SO₂ (S⁴⁺ ) et H₂S (S²⁻). L’Etna, pour lequel les conditions d’oxydoréduction sont faiblement contraintes, est utilisée comme cas d’étude pour examiner le comportement du soufre dans les magmas basaltiques hydratés pendant la différenciation et le dégazage. Cette recherche combine l'étude des inclusions vitreuses avec une étude expérimentale en conditions magmatiques sur la solubilité du S dans les basaltes alcalins hydratés.Les résultats expérimentaux suggèrent l’important contrôle de la ƒO₂ sur la teneur en S dans les magmas hydratés de l’Etna, et le partage du S entre les phases fluid and liquid. Les inclusions vitreuses ont été piégées à différentes profondeurs à l'intérieur du système magmatique. Elles décrivent une tendance continue de différenciation, marquée par une cristallisation fractionnée, à partir de la composition picritique (FS) vers le basalte plus récent dégazé (2013). Le contenu en S dans le liquide de l'Etna est extrêmement variable et atteint 4150 ppm dans les inclusions vitreuses les plus primitives. Les spectres XANES Fe³⁺/ΣFe des certaines inclusions vitreuses donnent des rapports Fe³⁺/ΣFe généralement décroissants à partir du liquide le plus primitif (FS) jusqu’au plus évolué (2013). Les simulations effectué par le logiciel MELTS confirme que la diminution du rapport Fe³⁺/ΣFe est principalement due au processus de différenciation magmatique, renforcé par le dégazage du S à ƒO₂ < NNO + 1. Cette réduction du magma provoque à son tour la diminution de la solubilité du S dans les basaltes hydratés de l’Etna, et peut constituer un éventuel activateur de l’exsolution du S, à l’origine de l’important dégazage du S observé au cours des dernières décennies à l’Etna. / Sulfur is an important volatile component of magmas that presents different oxidation states, depending on the redox conditions and on the phase of occurrence: in silicate melts it is typically dissolved as S⁶⁺ and/or S²⁻ , in the gas phase it occurs principally as SO₂ (S⁴⁺ ) and H₂S (S²⁻). Mount Etna, in which magmatic redox conditions are poorly constrained, is used as a case study to investigate sulfur behavior in hydrous basaltic magmas during magma differentiation and degassing. This research integrates the study of natural olivine-hosted melt inclusions with an experimental study on S solubility in hydrous alkali basalts at magmatic conditions.Experimental results suggest the important control of ƒO₂ on the S abundance in Etnean hydrous magma and its partitioning between fluid and melt phases. Melt inclusions were entrapped at different depths inside the magmatic system (up to ~ 18 km, below crater level). They delineate a continuous differentiation trend, marked by fractional crystallization, from the picritic basalt (FS) toward the most evolved and degassed (2013) basalt. S content in Etnean melt is extremely variable and reaches 4150 ppm in the primitive melt inclusions. XANES Fe³⁺/ΣFe spectra in some glass inclusions, resulted in the generally decreasing of Fe³⁺/ΣFe ratios from the most primitive (FS) to the most evolved (2013) melts. MELTS software confirms that the Fe³⁺/ΣFe decrease is due principally to the melt differentiation process, enhanced to the S degassing at ƒO₂ < NNO+1. Magma reduction, in turn, induces the decrease of the sulfur solubility in the hydrous Etnean basalt, as well as of the sulfide saturation, and may constitute a possible enhancer of S exsolution, triggering the important S degassing observed in the last decades in Mt. Etna.

Dans les premiers instants de l'histoire des planètes telluriques, la chaleur d'accrétion, le chauffage radioactif et la différenciation noyau-manteau apparaissent comme des sources d'énergie capables de fondre le manteau terrestre significativement. L'évolution d'un océan de magma suite à ces évènements catastrophiques dépend des propriétés physiques des matériaux silicatés en conditions mantelliques et de la dynamique convective complexe d'un manteau en cristallisation. Actuellement, certains auteurs proposent que la structure actuelle du manteau profond pourrait être associée à des reliques de la cristallisation d'un océan de magma primitif. Nous avons développé un modèle thermodynamique capable de modéliser de façon auto cohérente des séquences de cristallisation dans les conditions du manteau profond. A partir de ce modèle, nous avons montré que le magma s'enrichit progressivement en fer au cours de la cristallisation. Le liquide résiduel devient ainsi plus dense que la phase solide. Ce modèle thermodynamique suggère un scénario de cristallisation de l'océan de magma similaire à celui proposé par (Labrosse et al., 2007). Celui-ci prédit que la structure actuelle de la base du manteau hériterait de la cristallisation d'un océan de magma primitif. Afin d'étudier l'influence de ce contraste de densité et des profils de liquidus sur la dynamique syn- cristallisation d'un océan de magma, nous avons développé un code de convection multiphasique intégrant changement phase, percolation / compaction et cristallisation fractionnée. Dans ce mémoire, nous présentons des modèles dynamiques préliminaires de cristallisation dans le cas univariant / Early in the history of terrestrial planet, heat of accreation, radioactive deacay and core-mantle segratation may have melted the silicate mantle significantly. Magma ocean evolution depends on both physical properties of materials at relevant P-T conditions and the complex dynamics of a convecting cristallizing mantle. Present deep Earth mantle structures might be direclty linked to the crystallization of a potential magma ocean. We propose a complete thermodynamic model of the solid-liquid equilibrium in the MgO-FeO-SiO2 system which allows to compute self-consistenltly crystallization sequence at deep mantle conditions. The present study shows that, at thermodynamic equilibrium, the first solids that crystallize in the deep mantle are lighter than the liquid as they are more Mg-rich. This further enriches the melt in iron and this residual melt becomes much denser than the solid phase. Both the anti-freeze effect of iron and its high density suggest a mantle crystallization scenario similar to that described in Labrosse et al. (2007) where the ULVZ are iron rich and very fusible remnants of a primordial basal magma ocean. In addition, we have developped a multiphase convection code accounting for solid-liquid phase change, compaction and fractionnal cristallization. This mechanical model is dedicated to the investigation of the effects of various temperature profile and solid liquid density cross-overs on the dynamics of a cristallizing mantle. In this thesis, we show preliminary models illustrating the effect of chemical density contrasts between melt and solid in the case of univariant crystallization

Nous avons étudié des magmas ferrifères silicatés magnésiens à la pression du manteau terrestre en utilisant la dynamique moléculaire (First Principles Molecular Dynamics). Les résultats de l’équation d’état que nous avons obtenus à partir de nos simulations ont été utilisés pour créer un modèle chimique et minéralogique pour les zones de très basse vitesse sismique (ULVZ, anomalies régionales dans le manteau proche de la limite noyau-manteau). De plus, nous avons étudié le comportement du Ni, du Co et du Fe dans ces magmas et établi la dépendance du spin en fonction de la concentration, de la pression, de la température et du degré de polymérisation du magma silicaté. Nous avons montré qu’une baisse du spin moyen peut être corrélée au changement de pente (kink) observé précédemment pour les coefficients de partage du Ni et du Co. Nous avons analysé la structure du magma pour toutes les compositions étudiées en fonction de la pression. Nos résultats donnent un nouvel aperçu de la coordination des éléments majeurs et traces dans les magmas silicatés de différents degrés de polymérisation. Nous interprétons l’anomalie de coordination Ni-O en fonction de la pression comme un changement d’état de spin. L’effet de la polymérisation du magma silicaté sur les coefficients de partage du Co, du Ni et du W entre le métal et le magma silicaté a été étudié par expériences multi-enclumes en conditions isobares et isothermes. Nous avons réalisé des simulations FPMD de magmas à des degrés de polymérisation similaires aux expériences afin d’expliquer le caractère de plus en plus lithophile du W lorsque le degré de polymérisation du magma silicaté diminue. Nous proposons une explication structurale pour expliquer l’affinité décroissante apparente du W dans les magmas silicatés dépolymérisés. / We explore Fe-bearing Mg-silicate melts through the pressure regime of the Earth’s mantle using First Principles Molecular Dynamics (FPMD). The equation of state results we obtained from our simulations are used to create a chemical and mineralogical model for Ultra-Low Velocity Zones (anomalous region on the mantle side of the core-mantle boundary). Furthermore we study the behaviour of Ni, Co, and Fe in these melts, and asses their spin-crossover dependencies on their concentration, pressure, temperature, and the degree of polymerization of the silicate melts. We show that a decrease in the average spin can be correlated with the previously observed kink in the partitioning coefficient of Ni and Co. We investigate the melt structure of all the compositions studied as a function of pressure. Our results provide new insight into the coordination of major and trace elements in silicate melts with different degrees of polymerization. We interpret the anomalous Ni-O coordination trend with pressure as the result of the spin state change. The effect of silicate melt polymerization on the partitioning of Co, Ni, and W between a metal and silicate melt, is investigated at isobaric and isothermic conditions using multi-anvil experiments. We have performed FPMD simulations of melts with similar degrees of polymerization as the experiments in order to explain the increasing lithophile character of W with the decrease in polymerization of the silicate melt. We propose a structural explanation for tungsten’s apparent increased affinity for depolymerized silicate melts.

Impaktite des Chicxulubkraters wurden petrographisch (Polarisationsmikroskopie, REM, KL) und chemisch (RFA, TRFA, PGE, EMS) untersucht, um das Verhalten von Ejekta während des atmosphärischen Transports zu erforschen. Die proximalen Impaktite der UNAM-7 Bohrung bestehen aus einer suevitischen Brekzie (222.2 bis 384.4 m) und einer basalen, polymikten Brekzie mit geringem Silikatschmelzanteil. Letztere beinhaltet Evaporit-Megablöcke und Karbonatschmelzpartikel; Zersetzung von Kalzit und Anhydrit ist durch Entgasungsbläschen indiziert. An der distalen Kreide-Paläogen Grenze von El Guayal (520 km SW vom Kraterzentrum) beinhaltet eine 10 m mächtige suevitische Abfolge in einer oberen Untereinheit akkretionäre Lapilli und darüber eine Toneinheit. Das Auftreten von Karbonatschmelzen mit der PGE-angereicherten Impaktorkomponente in der Toneinheit belegt den Zusammenhang der K-P Grenze mit dem Chicxulub-Impakt. Die folgenden Stadien können für die Ablagerung und Alteration der Ejekta unterschieden werden: (1) Ein Hochgeschwindigkeitsauswurf beschleunigte Zersetzungsprodukte und initiierte einen Gasstrom. (2) Karbonatschmelzen wurden mit Anhydrit-Megablöcken ausgeworfen und initiierten einen lateral ausbreitenden Ejektavorhang. Kalzitrückreaktionen erhitzte das Material während des Transports. (3) Die Ejektionswolke kollabierte teilweise, wobei der zurückfallende Suevit vom Impaktormaterial, das in die Stratosphäre verteilt wurde, fraktioniert wurde. Die Kombination von Silikatschmelze mit Kalzit initiierte einen heißen, gas-angetriebenen Strom. In einer oberen, moderat temperierten, turbulenten Aschewolke kondensierte Wasserdampf, und durch Akkretion von Asche entstanden akkretionäre Lapilli. (4) Die Impaktorkomponente wurde mit den feinsten Ejektamassen für Wochen bis Jahre abgelagert. (5) Der Transport von Ejekta in der heißen Ejektionswolke induzierte Alterationsprozesse in den Ablagerungen. Es kann geschlussfolgert werden, dass ein gewisser Anteil des CO2 zu Kalzit zurückreagierte, währenddessen SOX Gase vollständig in die Atmosphäre freigesetzt wurden. Diese Beobachtungen inklusive des Auftretens von Karbonatschmelzen unterstützen die Aussage, dass der freigesetzte Anteil von CO2 in die Atmosphäre in der Vergangenheit überbewertet wurde. / Impactites of the Chicxulub crater were studied petrographically (polarisation microscopy, SEM, CL) and chemically (XRF, TXRF, PGE, EMPA) to investigate the evolution of ejecta during transit through the atmosphere. At the proximal UNAM-7 borehole, the sequence of impactites consists of a suevitic breccia (222.2 to 348.4 m) on top of a polymict silicate melt-poor breccia. The latter is intercalated with evaporite megablocks representing an analogue to the Bunte Breccia of the Nördlinger Ries crater. It contains carbonate melt particles; calcite and anhydrite decomposition is indicated by degassing vesicles. At the distal Cretaceous-Palaeogene site of El Guayal (~520 km SW of the crater centre), a ~10 m thick suevitic succession contains at its upper subunit accretionary lapilli and on top a clay unit. Intermixing of calcite with hot silicate melt resulted in recrystallisation and decomposition of calcite. In the clay unit, the presence of carbonate melt spheroids together with the PGE-enriched impactor component links the Chicxulub impact with the K-P boundary. The following stages can be distinguished for the deposition and alteration of the ejecta: (1) Jetting accelerated decomposition products and initiated a vapour flow. (2) Carbonate melts were excavated with anhydrite megablocks and initiated a lateral extending ejecta curtain. Calcite reformations heated the material during transport. (3) The expanding ejecta plume partially collapsed separating the falling suevite from impactor material that had been lifted into the stratosphere. The combination of silicate melt with calcite initiated a hot, gas-driven, basal flow. In an upper, moderately tempered, turbulent ash cloud, steam condensed and accretion of ash-sized material formed accretionary lapilli. (4) The impactor component was deposited with the finest ejecta for weeks to years. (5) The prolonged transport of ejecta in the hot ejecta plume induced alteration processes observed in the deposits. It can be concluded that a certain amount of CO2 has back-reacted to calcite, whereas SOX gases were completely liberated. These observations including the abundant presence of carbonate melts support that the amount of CO2 released to the atmosphere during the Chicxulub impact was overestimated previously.