ALBERT

Description: The assemblage strontium anorthite, quartz, and kyanite was reacted with H 2 O+CaCl 2 solutions at 500 °C and pressures between 460 and ~1300 MPa using a hydrothermal diamond-anvil cell. Information on the kinetics was obtained in situ based on time-resolved synchrotron-radiation X-ray fluorescence analyses of the Sr concentration in the fluid. The reaction products (anorthite or zoisite) were studied using transmission electron microscopy to obtain information on the reaction mechanism and mineral-fluid partitioning of strontium. The time required for equilibration was primarily controlled by the reaction mechanism, but not discernibly affected by pressure or chloride concentration. Nucleation and growth of zoisite at the expense of strontium anorthite was much faster than the Sr-Ca exchange reaction of strontium anorthite to anorthite, and resulted in chemically homogeneous crystals. The anorthite had developed a high nanoporosity during the reaction, which is indicative of coupled dissolution-precipitation. A zoisite-fluid exchange coefficient \[ {K}_{D(\hbox{ Sr }-\hbox{ Ca })}^{\hbox{ zoisite }-\hbox{ fluid }}=\frac{{X}_{\hbox{ Sr }}^{\hbox{ zoisite }}}{{X}_{\hbox{ Ca }}^{\hbox{ zoisite }}}/\frac{{X}_{\hbox{ Sr }}^{\hbox{ fluid }}}{{X}_{\hbox{ Ca }}^{\hbox{ fluid }}}=0.42 \] was obtained for the Sr-Ca fractionation at 500 °C and ~1300 MPa. At low bulk Sr/Ca, this value is in very good agreement with literature data, which are based on zoisite syntheses from oxide and hydroxide mixtures in chloridic fluids at 600 °C, 2 GPa and analyses after quench. This suggests that the Ca-Sr ratios in fluid and zoisite were not affected by back reactions during quenching. The constrained anorthite-fluid Sr partition coefficient for 500 °C, 460 MPa is, likewise, consistent with literature data, but determination of mineral-fluid partition and exchange coefficients can be hampered by quench phases in nanopores if coupled dissolution-precipitation acted as reaction mechanism.

Description: Mid- and Far-infrared spectra of a natural ilvaite sample were collected in situ in a diamond-anvil cell (DAC) as a function of pressure. The composition of the natural material is Ca 2+ 0.94 (Fe 2+ 0.61 Mn 2+ 0.40 )(Fe 3+ 1.01 Fe 2+ 0.96 Al 3+ 0.02 Mg 2+ 0.01 )[Si 2.03 O 7 /O/ (OH)] as determined by electron microprobe. One series of DAC experiments was performed at GFZ in the Mid-IR using argon as pressure medium and a Globar as light source. Four pressure series were performed at Bessy II in the Far-IR region using petroleum jelly as pressure medium and synchrotron light. For the Far-IR region we used a custom-made vacuum microscope adapted to an FTIR spectrometer. Pressure-induced changes in the Mid- and Far-IR spectra were analysed via the autocorrelation method for all five pressure series. All five series confirm the monoclinic to orthorhombic phase transition at about 2.3 GPa already known from X-ray diffraction studies. Our results show that incorporation of Mn 2+ does not lower the transition pressure as reported previously. However, the magnitude of the initial monoclinic β angle and the concentration of impurities seem to control the transition pressure. The first evidence of a second pressure-induced phase transition at much higher pressure (between 10 and 11 GPa) is reported here. Both transitions are clearly visible throughout the whole spectral ranges: in the OH-stretching region, the Mid-IR region from 1400 to 400 cm –1 and in the Far-IR region down to 50 cm –1 . The structural change at 10.5 GPa observed via infrared spectroscopy may reflect a pressure-induced suppression of the Jahn–Teller effect of the strongly distorted M2 octahedron. Above 11 GPa and up to 20 GPa no further discontinuities could be detected. One series going in pressure up to 31 GPa may indicate an additional structural change above 20 GPa.

Description: Ringwoodite [(Mg,Fe) 2 SiO 4 ] is the high-pressure polymorph of olivine stable in the upper mantle between ~525 to 660 km. Information on its temperature-dependent water content and Fe-oxidation state bears important implications on the hydrogen cycle and oxidation state of the Earth’s interior. We conducted several multi-anvil experiments to synthesize iron-bearing (0.11 ≤ x Fe ≤ 0.24) hydrous ringwoodite under oxidizing and reducing conditions. The experiments were performed at 1200 °C and pressures between 16.5 and 18.3 GPa. The incorporation of hydrogen and iron in ringwoodite was studied using Fourier transform infrared (FTIR), Mössbauer (MB), ultraviolet-visible (UV-VIS), and electron energy loss (EEL) spectroscopy. For MB spectroscopy, ringwoodite enriched in 57 Fe was synthesized. The IR spectra of ringwoodite show a broad OH band around 3150 cm –1 and two shoulders on the high-energy side: one intense at 3680 cm –1 and one weak at around 3420 cm –1 . The water content of the samples was determined using FTIR spectroscopy to have a maximum value of 1.9(3) wt% H 2 O. UV-VIS spectra display a broad band around 12 700 cm –1 and a shoulder at 9900 cm –1 representing the spin-allowed dd-transitions of VI Fe 2+ . The weaker band around 18 200 cm –1 is a distinct feature of Fe 2+ -Fe 3+ intervalence charge transfer indicating the presence of Fe 3+ in the samples. EEL spectra yield Fe 3+ fractions ranging from 6(3)% at reducing conditions to 12(3)% at oxidizing conditions. We performed heating experiments up to 600 °C in combination with in situ FTIR spectroscopy to evaluate the temperature-dependent behavior of ringwoodite, especially with respect to hydrogen incorporation. We observed a color change of ringwoodite from blue to green to brown. The heat-treated samples displayed hydrogen loss, an irreversible rearrangement of part of the hydrogen atoms (FTIR), as well as oxidation of Fe 2+ to Fe 3+ evidenced by the appearance of the spin-forbidden dd-transition band for Fe 3+ and the ligand-metal (O 2– -Fe 3+ ) transition band in the optical spectra. An increased Fe 3+ fraction was also revealed by EEL and MB spectroscopy (up to 16% Fe 3+ /Fe). Analyses of MB data revealed the possibility of tetrahedral Fe 3+ in the annealed ringwoodite. These results lead to a reinterpretation of the broad OH band, which is a combination of several bands, mainly [V Mg (OH) 2 ] x ), a weaker high-energy band at 3680 cm –1 ([V Si (OH) 4 ] x ) and a shoulder at 3420 cm –1 ([(Mg/Fe) Si (OH) 2 ] x ).

Description: This PhD project about the effect of water incorporation on the structure and phase stability of wadsleyite and ringwoodite is integrated in the DFG priority program 1236 "Strukturen und Eigenschaften von Kristallen bei extrem hohen Drücken und Temperaturen". The corresponding research work was conducted at the Helmholtz Centre Potsdam, German Research Centre for Geosciences, GFZ, Department 3.3 Chemistry and Physics of Earth Materials. Seismic profiles of the Earth’s inner structure reveal discontinuities in the p- and s-wave velocities at certain depths. The discontinuities at 410, 520, and 660 km depth are assigned to the transformations of the (Mg,Fe)2SiO4-polymorphs olivine (a-form) to wadsleyite (ß) and ringwoodite (?). However, the observed depths and widths of these velocity jumps are not constant but vary on a global scale, especially in the case of d520. The reason for that has been subject matter of various discussions. The main issue of this PhD work is to evaluate the effect of hydrogen on the structure and phase stabilities of wadsleyite and ringwoodite and to investigate if the incorporation of water in these minerals might be responsible for the observed depths variations of d520. The experimental approach included the performance of high-pressure syntheses (varied parameters: pressure, water content, composition, oxygen fugacity) in a multi-anvil apparatus and an extensive investigation of the synthesized material. X-ray diffraction techniques and electron microprobe analyses were applied to identify the samples and define their structure and composition in detail. However, the main focus of the work was to localize and quantify hydrogen (and iron (Fe)) within the samples by spectroscopic techniques and to evaluate the effect of the incorporated species on the structure and phase stabilities of wadsleyite and ringwoodite. In the first part of this study the pressure-depending behavior of dry vs. hydrous iron free wadsleyite was investigated by IR spectroscopy. Thereby we could show that the incorporation of hydrogen shifts the phase transition from orthorhombic to monoclinic wadsleyite about 1.6 GPa to lower pressures. In the second part, the location of hydrogen in ringwoodite was investigated. We found that the main mechanisms are related to octahedral vacancies [VMg(OH)2]x and the hydrogarnet substitution [VSi(OH)4]x. The third part of the work was related to the effect of non-hydrostatic pressure conditions on the structure of hydrous ringwoodite. Thereby we could show that such conditions lead to a stress-induced proton disorder. In the fourth part we investigated the direct effect of hydrogen, iron and different oxygen fugacities on the phase relations between wadsleyite and ringwoodite. Thus we could prove that the incorporation of hydrogen expands the stability field of wadsleyite towards higher pressure and that this effect is even enhanced at oxidizing compared to reducing conditions in the experiment. The last part of the work consisted of calorimetric measurements on dry wadsleyite. The collected data present the basis for future action on the thermodynamic modeling of dry and hydrous wadsleyite and ringwoodite. On basis of the results of this study, we could prove that there is an effect of water on the phase relations between wadsleyite and ringwoodite that possibly can explain the observed variations in the shape of the 520km-discontinuity., Die vorliegende Arbeit behandelt den Einfluss des Wassereinbaus auf die Struktur und Phasenstabilitäten von Wadsleyit und Ringwoodit und ist eingegliedert in das DFG Schwerpunktprogramm 1236 mit dem Titel “Strukturen und Eigenschaften von Kristallen bei extrem hohen Drücken und Temperaturen”. Die dazugehörigen Forschungsarbeiten wurden am Helmholtz-Zentrum Potsdam, Deutsches GeoForschungsZentrum, GFZ, Department 3.3 Chemie und Physik der Geomaterialien durchgeführt. Seismische Profile, die die Struktur des Erdinneren abbilden, weisen in bestimmten Erdtiefen Unstetigkeiten in den p- und s-Wellengeschwindigkeiten auf. Die Diskontinuitä en in 410, 520 und 660 km Tiefe werden den Phasenumwandlungen der (Mg,Fe)2SiO4-Polymorphe Olivin (a-Polymorph) zu Wadsleyit (ß) und Ringwoodit (?) zugeordnet. Jedoch treten diese Unstetigkeiten global gesehen nicht exakt in der gleichen Tiefe auf, sondern weisen mitunter große Schwankungen auf. Dafür werden verschiedenste Gründe angeführt. Die vorliegende Arbeit soll in diesem Zusammenhang den Einfluss von Wasserstoff auf die Struktur und Phasenstabilitäten von Wadsleyit und Ringwoodit untersuchen sowie klären, ob der Einbau von Wasser in diese Minerale als Erklärung für die beobachteten Tiefenschwankungen der 520km-Diskontinuität herangezogen werden kann. Die experimentelle Herangehensweise an das Thema beinhaltete die Durchführung von Hochdrucksynthesen (veränderliche Parameter: Druck, Wassergehalt, chemische Zusammensetzung, Sauerstofffugazität) in einer Multi-Anvil-Apparatur sowie die anschließende ausführliche Untersuchung des synthetisierten Materials. Röntgenbeugungsanalysen sowie Mikrosondenuntersuchungen wurden angewendet, um die Phasen zu identifizieren und deren Struktur und chemische Zusammensetzung im Detail zu bestimmen. Das Hauptaugenmerk dieser Arbeit lag jedoch darin, Wasserstoff (und Eisen (Fe)) in der Struktur mit Hilfe von spektroskopischen Methoden zu lokalisieren, zu quantifizieren und den Einfluss der eingebauten Spezies auf die Struktur und Phasenstabilitäten von Wadsleyit und Ringwoodit zu bewerten. Im ersten Teil der Studie wurde daher das druckabhängige Verhalten von trockenem gegenüber wasserhaltigem Mg-Wadsleyit mittels IR-Spektroskopie untersucht. Dabei konnten wir zeigen, dass der Einbau von Wasserstoff die Phasenumwandlung vom orthorhombischen in den monoklinen Wadsleyit um etwa 1.6 GPa zu geringeren Drücken verschiebt. Im zweiten Teil wurde die genaue Positionierung von Wasserstoff innerhalb der Ringwooditstruktur untersucht. Der Hauptmechanismus konnte dabei oktaedrischen Leerstellen zugeordnet werden ([VMg(OH)2]x), der zweite wichtige Einbaumechanismus wird über die Hydrogranat-Substitution ([VSi(OH)4]x) vollzogen. Der dritte Teil der Arbeit befasste sich mit dem Effekt von nicht hydrostatischen Druckbedingungen auf die Struktur von wasserhaltigem Ringwoodit. Dabei konnten wir zeigen, dass derartige Bedingungen zu einer stress-induzierten Unordnung des Wasserstoffes führen kann. Im vierten Teil der Arbeit untersuchten wir den direkten Einfluss von Wasserstoff, Eisen und unterschiedlichen Sauerstofffugazitäten auf die Phasenbeziehungen zwischen Wadsleyit und Ringwoodit. Auf diese Weise war es uns möglich, zu beweisen, dass der Einbau von Wasserstoff das Stabilitätsfeld von Wadsleyit zu höheren Drücken hin erweitert und dass darüberhinaus dieser Effekt noch verstärkt auftritt, wenn die Bedingungen im Experiment eher oxidierend als reduzierend sind. Im letzten Teil der Studie wurden kalorimetrische Messungen am trockenen Wadsleyit durchgeführt. Die daraus gewonnenen Daten stellen die Grundlage für weitere Studien zum thermodynamischen Modellieren der Eigenschaften von trockenem und wasserhaltigem Wadsleyit und Ringwoodit dar. Anhand unserer Untersuchungen konnten wir zeigen, dass es tatsächlich einen Einfluss von eingebautem Wasser auf die Phasenbeziehungen zwischen Wadsleyit und Ringwoodit gibt, welcher die beobachteten Schwankungen bezüglich der Tiefe und Breite der 520km-Diskontinuität erklären kann.