Der Transport von volatilen Elementen in den Erdmantel – Experimentelle Studien des tiefen Stickstoffkreislaufs und der (F,OH)-Position in Topas

Volatile Elemente sind für die Geowissenschaften von besonderem Interesse, weil sie maßgeblichen Einfluss auf die Bildung von Atmosphäre und Ozeanen haben, eine Schlüsselrolle in sedimentären, magmatischen und metamorphen Prozessen spielen und somit die Evolution der Erdkruste und des Erdmantels wesentlich mitbestimmt haben. Stickstoff und Wasserstoff gehören zu den wichtigsten volatilen Elementen. Die Bestimmung ihrer Häufigkeit, sowie ihrer Verteilung und ihres Austausches zwischen Atmosphäre, Erdkruste und Erdmantel sind seit Jahren Thema vieler Forschungsarbeiten. Trotzdem ist das Wissen über den Stickstoffkreislauf sehr lückenhaft, insbesondere über den Stickstoffaustausch zwischen der Erdkruste und dem Erdmantel ist bisher nur sehr wenig bekannt. Nominell wasserhaltige Hochdruckphasen leisten einen wesentlichen Beitrag zum Wasserreservoir des Erdmantels, denn sie können beträchtliche Mengen an Wasser und häufig auch weiteren flüchtigen Elementen wie z.B. Halogenen in ihren Kristallstrukturen speichern. Daher sind Stabilitäten und kristallchemischen Eigenschaften von wasserhaltigen Hochdruckphasen von besonderer Relevanz und großem Interesse. Das Ziel diese Doktorarbeit ist also die Erweiterung des Wissen über die geochemischen Kreisläufe von Stickstoff und Wasserstoff im Erdinneren. Stickstoff wird als Ammonium, NH4, in den Erdmantel subduziert. Es entsteht bei der Zersetzung von sedimentierter organischer Materie und wird dann in kaliumreiche Minerale der Erdkruste eingebaut. Werden diese NH4-haltigen Sedimente entlang einer kalten Geotherme versenkt, so kann das enthaltene NH4 in die neuentstehenden Hochdruckphasen eingebaut werden. Diese zeigt die Synthese von NH4-Phengit, NH4-Cymrit, NH4-Wadeit, und NH4-Hollandit. Die Entgasung von molekularem Stickstoff, N2, am mittelozeanischen Rücken setzt voraus, dass es ein Stickstoffreservoir im darunterliegenden peridotitischen Mantel gibt. Die Synthese von NH4-haltigem Klinopyroxen deutet darauf hin, dass dies ein wichtiger Stickstoffträger im Mantel sein könnte. Abschätzungen zeigen, dass Klinopyroxen im Mantel über ein Stickstoffspeicherkapazität of etwa 1012 mol N2 verfügt. Die Zersetzung einer solchen NH4-Komponente in Klinopyroxen oberen Bereich des Mantels würde zur Freisetzung von Stickstoff und zur Rückführung in die Atmosphäre führen und somit den Kreislauf schließen. Topas ist eine wichtige Modellphase für den Transport von volatilen Elementen in den Erdmantel, weil er über eine einfache chemische Zusammensetzung und ein relativ großes Stabilitätsfeld verfügt. Detaillierte Untersuchungen an der (F,OH)-Mischkristallreihe offenbarten eine unerwartete Komplexität des Wasserstoffverhaltens, welches auf die lokale Ordnung von F und OH in der Kristallstruktur zurückzuführen ist. Fluor und OH sind statistisch auf derselben kristallographischen Position verteilt. Zwei eng benachbarte Positionen beeinflussen sich gegenseitig, so dass man im IR Spektrum zwei OH-Streckschwingungen findet. Der Anteil an lokal geordneten OH...OH-Bereichen in der Kristallstruktur hängt von der OH-Konzentration des Topases ab und lässt sich mit Hilfe der Wahrscheinlichkeitsrechnung vorhersagen. Das Auftreten solch geordneter Bereiche reduziert lokal die Symmetrie und beeinflusst so auch weitere kristallchemische Eigenschaften wie z.B. die Piezo- und Pyroelektrizität, die Strukturausdehnung und die Stabilität. Temperaturabhängige Untersuchungen an (F,OH)-Topas und Topas-OH zeigen zwei Phasentransformationen. Bei tiefen Temperaturen ändert sich die Symmetrie von P1 nach Pbn21, bei hohen Temperaturen von Pbn21 nach Pbnm. Das Verhalten von (F,OH)-Topas bei zunehmendem Druck und bei abnehmender Temperatur ist vergleichbar. Dies trifft auf Topas-OH nicht zu. Die wahrscheinlichste Erklärung ist die unterschiedliche Geometrie der Wasserstoffbindungen. Die Studien dieser Dissertation demonstrieren, dass detailliertes Wissen über Kristallstrukturen und kristallchemische Eigenschaften von großer Bedeutung ist, um globale Prozesse wie das Recycling von volatilen Elementen im Erdmantel verstehen und modellieren zu können.Volatile elements are of particular interest in geosciences, because they are essential for the formation of the atmosphere and oceans, play a key role in sedimentary, magmatic, and metamorphic processes, and thus in the evolution of crust and mantle. Nitrogen and hydrogen are major volatile elements. Their abundances, partitioning and cycling between the major reservoirs, atmosphere, crust, and mantle, have been investigated in many studies during the last years. Still, the present knowledge of the nitrogen cycle is only fragmentary, in part because of the missing data on the nitrogen cycling between crust and deep mantle. The contribution of the nominally hydrous high-pressure silicates to the water reservoir in the mantle cannot be neglected. This is because they have a high transport capacity for water and often also other volatiles, e.g., halogens. Hence, the stabilities and crystal chemical properties of hydrous high-pressure phases are of great interest. This thesis tries to extend the knowledge about the two element cycles, nitrogen and hydrogen, in the Earth's interior. Nitrogen is primarily transported to the deep Earth as ammonium, NH4, which originates from the decomposition of organic matter followed by its incorporation into K-rich sediments. With ongoing subduction in cold slabs, NH4 from the sediments can be redistributed into newly formed high-pressure phases such as NH4-phengite, NH4-cymrite, NH4-Si-wadeite, NH4-hollandite, which were successfully synthesized in this thesis. This provides the means for nitrogen and hydrogen transport to the Earth’s mantle. Degassing of molecular nitrogen, N2, at mid-ocean ridges implies a nitrogen reservoir in normal mantle rocks, i.e., peridotites. The synthesis of NH4-bearing clinopyroxene suggests that this mineral might be an important carrier of nitrogen in the mantle. Rough calculations resulted in a storage capacity of about 1012 mol N2. The breakdown of such an NH4 component in clinopyroxene at mid-upper mantle conditions would lead to a nitrogen release back to the atmosphere and close the nitrogen cycle. An important model phase for volatile transport into the mantle is topaz because of it’s simple chemical composition and the large extent of it's stability. Detailed investigation of the (F,OH)-solid solution series unraveled an unexpected complexity of the behavior of hydrogen in the crystal structure. A crucial result from these investigations is the importance of short-range ordering in the crystal structure on crystal chemical properties. Fluorine and OH are statistically distributed in the same crystallographic site. Two closely adjacent sites interact with each other, which result in the occurrence of two OH stretching bands in the IR spectra. The amount of local OH…OH ordering depends on the OH concentration in the topaz crystal and can be predicted from probability calculations. This short-range ordering reduces locally the symmetry and thus, affects further crystal chemical properties such as piezo- and pyroelectricity, structural increase, and the stability. Temperature-dependent investigations show that (F,OH)-bearing topaz and topaz-OH undergo phase transitions with symmetry changes from P1 to Pbn21 at low temperature and from Pbn21 to Pbnm at high temperature. A comparison with pressure-dependent studies show a similar behavior of (F,OH)-bearing topaz at decreasing temperature and increasing pressure, whereas the influences on topaz-OH are different. The most likely explanation may be the differences in the hydrogen bond geometry. The studies of this thesis demonstrate that detailed crystal knowledge is essential to understand and model the replenishment of the Earth’s mantle with volatiles via subduction

Cr(III) solubility in aqueous fluids at high pressures and temperatures

Trivalent chromium is generally considered relatively insoluble in aqueous fluids and melts. However, numerous counterexamples in nature indicate Cr(III) mobilization by aqueous fluids during metamorphism or hydrothermal alteration of chromite-bearing rocks, or by pegmatite melts. So far, very little is known about the chromium concentrations and speciation in such fluids.In this study, the solubility of eskolaite (Cr2O3) in 1.6–4.2 m aqueous HCl solutions was determined in situ at elevated pressures up to 1 GPa and temperatures ranging between 400 and 700 °C using synchrotron micro–X-ray fluorescence spectroscopy (μ-XRF). Determined concentrations of dissolved Cr ranged between about 900–18,000 ppm, with the highest concentrations found at 500 °C and 861 MPa. The Cr(III) solubility in aqueous HCl fluids is retrograde in the studied temperature range and increases with pressure.In addition, Cr(III) complexation in these fluids was explored by Raman spectroscopy on a 12.3 mass% HCl fluid in equilibrium with eskolaite at 400 and 600 °C, 0.3–1.6 GPa. All spectra show two prominent Cr–Cl stretching bands at about 275 and 325 cm−1, which display some fine structure, and in some spectra weak bands in the region between 380 and 500 cm−1. The sum of the integrated intensities of the two dominant bands reveals qualitatively the same changes with temperature along an isochore, with pressure at constant temperature, and with the time required for equilibration as the Cr(III) concentrations in the fluid determined by μ -XRF. Complementary ab initio molecular dynamics simulations of a 4 m HCl solution at two different densities (0.8 and 0.97 g/cm3) and temperatures (427 and 727 °C) were performed to investigate the vibrational properties of variousView the MathML sourceCrClx(H2O)y3-x and View the MathML sourceCrClx(H2O)y(OH)z3-x-z complexes with 3⩽x+z⩽43⩽x+z⩽4 and 0⩽y⩽20⩽y⩽2. Quasi-normal mode analysis reveals that both the tetrahedral symmetric and antisymmetric Cr–Cl stretching vibrations of View the MathML sourceCrCl4(H2O)0-2- have characteristic frequencies in the range of the two strongest experimentally observed Raman bands, whereas Cr–O stretching vibrations of hydroxy-chloride complexes occur at wavenumbers above 400 cm−1.Solubility and complexation of Cr(III) depend strongly on the activities of Cl− and H+. At high H+ and Cl− activity, the results are consistent with View the MathML sourceCrCl3-4(H2O)0-2-1-0 complexes as major Cr(III) species, the Cr coordination number of which increases with pressure by becoming more aquated. At low Cl− activity, i.e. in our study at high-temperature low-pressure conditions, the data indicate mixed View the MathML sourceCrClx(H2O)y(OH)z3-x-z complexes with Cl–Cr ratios less than three

Studies on the thermal decomposition of lanthanum(III) valerate and lanthanum(III) caproate in argon

The decomposition of La-valerate (La(C$_{4}$H$_{9}$CO$_{2}$)$_{3}$·xH$_{2}$O (x ≈ 0.45)) and La-caproate (La(C$_{5}$H$_{11}$CO$_{2}$)$_{3}$·xH$_{2}$O (x ≈ 0.30)) was studied upon heating at 5 °C/min in a flow of argon. Using a variety of techniques including simultaneous TG-DTA, FTIR, X-ray diffraction with both laboratory Cu Kα and synchrotron sources as well as hot-stage microscopy, it was found that both compounds melt prior to decomposition and that the main decomposition stage from the molten, anhydrous state leads to the formation of La-dioxycarbonate (La$_{2}$O$_{2C}$O$_{3}$) via an unstable intermediate product and release of symmetrical ketones. Final decomposition to La$_{2}$O$_{3}$ takes place with release of CO$_{2}$

Influence of the octahedral cationic-site occupancies on the framework vibrations of Li-free tourmalines, with implications for estimating temperature and oxygen fugacity in host rocks

Tourmalines, XY3Z6T6O18(BO3)3V3W, are excellent petrogenetic indicators as they capture the signature of the host-rock bulk composition. Raman spectra of tourmalines can be used as fingerprints for species identification and crystal-chemical analysis. While Li-bearing species are directly distinguishable by the shape of the OH-stretching vibrations, the discrimination of Mg- and Fe-dominant species can be hindered by the coexistence of at least two types of octahedrally coordinated Rn+ cations. Thirty Li-free tourmaline samples comprising 14 different species were studied by Raman spectroscopy and electron microprobe. All nine Fe3+-bearing samples were also analyzed by single-crystal X‑ray diffraction and Mössbauer spectroscopy. The Raman scattering analysis shows that Mg-dominant species can be immediately distinguished from Fe-dominant species by the shape of the vibrational modes at ~200–240 cm–1 arising from the YO6 vibrations. Trivalent Fe can be observed and quantified by shifts of the framework vibrations toward lower wavenumbers. The position of the main ZO6 vibrational mode (275–375 cm–1) can be used to determine the ZFe3+ content, while the YFe3+ content can be inferred from the position of the peak at ~315 cm–1. Fits to the data points indicate that the homovalent substitution of Fe3+ for Al3+ leads to a considerably larger downward shift of the ZO6 vibrational mode than the heterovalent substitution Mg2+ for Al3+. The intensity ratio of the two major YO6 vibrational modes (200–240 cm–1) of the fully characterized Fe3+-bearing samples reflects the amount of Y-site Mg and thus can be used to deduce the site-occupancy disorder of Mg over the Y and Z site for tourmaline species with Mg ≤2 apfu. By combining the information from framework and OH-stretching vibrations, Raman spectroscopy alone can be used as a micrometer-scale sensitive non-destructive method for the analysis of tourmaline crystal chemistry including trivalent Fe, which is the major tracer for oxygen fugacity and central for intersite geothermometry

Nickel and platinum in high-temperature H2O + HCl fluids: Implications for hydrothermal mobilization

International audienc

The effect of chrysotile nanotubes on the serpentine-fluid Li-isotopic fractionation

International audienceWe determined the lithium isotope fractionation between synthetic Li-bearing serpentine phases lizardite, chrysotile, antigorite, and aqueous fluid in the P, T range 0.2-4.0 GPa, 200-500°C. For experiments in the systems lizardite-fluid and antigorite-fluid, 7Li preferentially partitioned into the fluid and Δ7Li values followed the T-dependent fractionation of Li-bearing mica-fluid (Wunder et al. 2007). By contrast, for chrysotile-fluid experiments, 7Li weakly partitioned into chrysotile. This contrasting behavior might be due to different Li environments in the three serpentine varieties: in lizardite and antigorite lithium is sixfold coordinated, whereas in chrysotile lithium is incorporated in two ways, octahedrally and as Li-bearing water cluster filling the nanotube cores. Low-temperature IR spectroscopic measurements of chrysotile showed significant amounts of water, whose freezing point was suppressed due to the Li contents and the confined geometry of the fluid within the tubes. The small inverse Li-isotopic fractionation for chrysotile-fluid results from intra-crystalline Li isotope fractionation of octahedral Li[6] with preference to 6Li and lithium within the channels (Li[Ch]) of chrysotile, favoring 7Li. The nanotubes of chrysotile possibly serve as important carrier of Li and perhaps also of other fluid-mobile elements in serpentinized oceanic crust. This might explain higher Li abundances for low- T chrysotile-bearing serpentinites relative to high- T serpentinites. Isotopically heavy Li-bearing fluids of chrysotile nanotubes could be released at relatively shallow depths during subduction, prior to complete chrysotile reactions to form antigorite. During further subduction, fluids produced during breakdown of serpentine phases will be depleted in 7Li. This behavior might explain some of the Li-isotopic heterogeneities observed for serpentinized peridotites

Cu and Ni solubility in high-temperature aqueous fluids

Copper and nickel are generally associated in magmatic sulfide ores formed byimmiscibility in mafic and ultramafic magmas. In contrast, hydrothermal Cu-Ni deposits are uncommon andthese elements usually occur in separate Cu-Fe-sulfide and Ni-Co-Ag-Bi-As-S mineralizations. Among theporphyry-type deposits formed at high temperatures to about 700 °C, there are many copper but no nickeldeposits [1], pointing to a higher solubility of Cu relative to Ni in aqueous fluids at such conditions. The aim ofthis study is to measure the solubilities of Cu and Ni sulfides in high-temperature hydrothermal fluids in-situusing synchrotron-radiation micro-X-ray fluorescence spectrometry.Synthetic CuS or NiS crystals were partly dissolved in aqueous NaCl, NaCl+HCl, or CaCl2 solutions attemperatures of 400 to 600 °C and pressures between 70 and 900 MPa using a modified hydrothermaldiamond-anvil cell with a recess in one diamond [2]. Consecutive XRF spectra of the fluid in the recess werecollected in a confocal mode to exclude signal contributions from the crystals in the sample chamber [3].Equilibrium was assumed if the determined concentrations of the dissolved metals indicated that a steadystate was attained.The measured dissolved Cu concentrations ranged between 22 ppm at 70 MPa, 500 °C and 235 ppm at 306MPa, 600 °C in 0.5 to 1.6 m NaCl solutions. We observed a decrease in Cu concentration with increasingpressure at constant temperature, and for 1.6 m NaCl an increase by a factor of two along an isochore from120 MPa, 500 °C to 306 MPa, 600 °C. Higher Cu solubilities were determined in more concentrated solutions.A preliminary run with a more acidic NaCl+HCl solution (pH ~1) revealed a dramatic increase in the dissolvedCu concentration to 7898 ppm at 170 MPa, 500 °C.The measured dissolved Ni concentrations ranged between 3 ppm at 200 MPa, 500 °C in a 1 m NaCl solutionand 33 ppm at 411 MPa, 500 °C in a 0.75 m CaCl2 solution. A solubility maximum at 500 °C along anisochore was observed for both solutions. The Ni solubility increased with pressure at constant temperature.Experiments with aqueous CaCl2 solutions resulted in higher dissolved Ni concentrations compared to NaClsolutions at similar pressure-temperature conditions.Our experiments suggest that the solubility of Cu and Ni in aqueous fluids is mainly governed by fluidcomposition. For both elements, solubility increased in more chlorine-rich fluids, which could reflect metalchlorinecomplexation. Preliminary results for Cu indicate a strong dependence of the solubility on the pH ofthe fluid. A contrasting solubility behavior of Cu and Ni was observed with increasing pressure, which might beone reason for the difference in hydrothermal ore deposit formation.[1] Barnes (1979) Geochemistry of hydrothermal ore deposits, Wiley. [2] Schmidt and Rickers (2003) Am.Mineral. 88, 288-292. [3] Wilke el al. (2010) J. Synchrotron Rad. 17, 669-675

Copper complexation and solubility in high-temperature hydrothermal fluids: A combined study by Raman, X-ray fluorescence, and X-ray absorption spectroscopies and ab initio molecular dynamics simulations

Data for the solubility of CuS (reacting to Cu2S), Cu, and bornite + chalcopyrite + pyrite (reacting to Cu-Fe-S solid solution) in H2O + NaCl fluids were determined in situ using synchrotron-radiation X-ray fluorescence (SRXRF) spectroscopy. The aqueous Cu concentrations ranged between 25 ppm at 500 degrees C, 320 MPa, 0.5 m NaCl and 760 ppm at 500 degrees C, 310 MPa, 4.78 m NaCl, increased with temperature along an isochore and with NaCl molality, and decreased with pressure. The X-ray absorption near edge structure (XANES) spectra of the fluid from dissolution of CuS or Cu in H2O + NaCl at 500 degrees C are nearly identical with published spectra of CuCl2- (aq), but differ significantly from reported spectra of Cu(I) in Cl-free hydrosulfide solutions. Raman spectra of H2O + HCl +/- NaCl fluids reacted with CuS or, for comparison, metallic Cu were measured at temperatures to 600 degrees C and pressures to 2 GPa to test if this technique can provide additional information on the complexation of Cu(I) and on the solubility of copper in hydrothermal fluids. These spectra showed that CuCl2- (aq) was the most abundant Cu(I) species at all conditions. In addition, CuCl32-(aq) was observed at high HC1 concentrations, but the Raman spectra provided no convincing evidence for Cu(I) complexation with H2S or S-3(-) (HS-(aq) was below detection in these acidic fluids). Generally, there was an increase in the sum of the integrated intensities of the bands assigned to Cu complexes with increasing HCl concentration, and a decrease if sulfide was present. At all fluid compositions, the intensity of the Raman bands from Cu-Cl stretching vibrations decreased with increasing pressure at constant temperature for single-phase fluids, without formation of additional bands. Based on ab initio modeling, the complexes CuCl2-(aq), Cu(HS)(2)(-)(aq), and Cu(HS)Cl (aq) are not distinguishable by Raman spectroscopy, but the stretching vibration of Cu(I) complexes with H2S should occur at significantly lower wavenumbers. Overall, the results indicate that Cu(I) is transported predominantly as CuCl2-(aq) in reducing sulfur-free or H2S +/- S-3(-)-bearing chloridic hydrothermal fluids. The decrease in the Cu solubility in H2O + HCl +/- NaCl +/- H2S fluids with increasing pressure without a detectable change in speciation is caused by decrease in the formation constant of CuCl2-(aq) with pressure. Changes in the copper speciation and depressurization can be ruled out as causes for hydrothermal copper ore formation at high fluid densities above the critical density. At this condition, copper ore may precipitate by dilution, cooling in the presence of H2S, increase in pH, and/or an increase in the H2S activity

Thermal decomposition of lanthanum(III) butyrate in argon atmosphere

The thermal decomposition of La(C3H7CO2)3·xH2O (x ≈ 0.82) was studied in argon during heating at 5 K/min. After the loss of bound H2O, the anhydrous butyrate presents at 135 °C a phase transition to a mesophase, which turns to an isotropic liquid at 180 °C. The decomposition of the anhydrous butyrate is associated to a solidification process. The final decomposition to La2O3 takes place via two intermediate products: La2O(C3H7CO2)4 and La2O2CO3 with release of CO2 and the symmetrical ketone C3H7COC3H7