@article{PohlenzRosaMathonetal.2018, author = {Pohlenz, Julia and Rosa, A. D. and Mathon, O. and Pascarelli, S. and Belin, S. and Landrot, G. and Murzin, V. and Veligzhanin, A. and Shiryaev, A. and Irifune, T. and Wilke, Max}, title = {Structural controls of CO2 on Y, La and Sr incorporation in sodium-rich silicate - carbonate melts by in-situ high P-T EXAFS}, series = {Chemical geology : official journal of the European Association for Geochemistry}, volume = {486}, journal = {Chemical geology : official journal of the European Association for Geochemistry}, publisher = {Elsevier}, address = {Amsterdam}, issn = {0009-2541}, doi = {10.1016/j.chemgeo.2017.12.023}, pages = {1 -- 15}, year = {2018}, abstract = {Carbonate-rich silicate and carbonate melts play a crucial role in deep Earth magmatic processes and their melt structure is a key parameter, as it controls physical and transport properties. Carbon-rich melts can be strongly enriched in trace elements, but the structural incorporation mechanisms of these elements are difficult to study because such melts generally cannot be quenched to glasses. In this contribution we investigate the influence of CO2 on the local environments of trace elements contained in silicate glasses with variable CO2 concentrations and in silicate and carbonate melts. The melts were studied in-situ at high pressure and temperature conditions using the Paris-Edinburgh press (2.2 to 2.6 GPa and 1200 to 1500 degrees C). The compositions studied include sodium-rich peralkaline silicate melts and glasses and carbonate melts similar to those occurring naturally at Oldoinyo Lengai volcano. The local environments of yttrium (Y), lanthanum (La) and strontium (Sr) were investigated using extended X-ray absorption fine structure (EXAFS) spectroscopy. Main findings of the study suggest: (1) In peralkaline silicate glasses the local structure of Y is unaffected by the CO2 content. Contrary, a slight increase of oxygen bond lengths of Sr and La is inferred with increasing CO2 content in peralkaline glasses, while they remain constant in glasses of even higher peralkalinity independent of the CO2 content. (2) In silicate melts of different CO2 contents Y-O bond lengths are constant, while a slight increase within carbonate melt compositions is deduced. On the other hand, a steady bond lengths increase over the whole compositional range is inferred for La-O and Sr-O. This may well be explained by distinct preferences of these elements for specific local environments. Based on these new data, we suggest potential mechanisms for the structural incorporation of these elements, a key step towards understanding their partitioning behavior in natural magmatic systems.}, language = {en} } @article{RosaPohlenzdeGrouchyetal.2016, author = {Rosa, A. D. and Pohlenz, Julia and de Grouchy, C. and Cochain, B. and Kono, Y. and Pasternak, S. and Mathon, O. and Irifune, T. and Wilke, Max}, title = {In situ characterization of liquid network structures at high pressure and temperature using X-ray absorption spectroscopy coupled with the Paris-Edinburgh press}, series = {High pressure research}, volume = {36}, journal = {High pressure research}, publisher = {American Geophysical Union}, address = {Abingdon}, issn = {0895-7959}, doi = {10.1080/08957959.2016.1199693}, pages = {332 -- 347}, year = {2016}, abstract = {We review recent progress in studying structural properties of liquids using X-ray absorption spectroscopy coupled with the Paris-Edinburgh press at third-generation synchrotron facilities. This experimental method allows for detecting subtle changes in atomic arrangements of melts over a wide pressure-temperature range. It has been also employed to monitor variations of the local coordination environment of diluted species contained in glasses, liquids and crystalline phases as a function of the pressure and temperature. Such information is of great importance for gaining deeper insights into the physico-chemical properties of liquids at extreme condition, including the understanding of such phenomena as liquid-liquid phase transitions, viscosity drops and various transport properties of geological melts. Here, we describe the experimental approach and discuss its potential in structural characterization on selected scientific highlights. Finally, the current ongoing instrumental developments and future scientific opportunities are discussed.}, language = {en} } @phdthesis{Pohlenz2019, author = {Pohlenz, Julia}, title = {Structural insights into sodium-rich silicate - carbonate glasses and melts}, doi = {10.25932/publishup-42382}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-423826}, school = {Universit{\"a}t Potsdam}, pages = {XXII, 117}, year = {2019}, abstract = {Carbonate-rich silicate and carbonate melts play a crucial role in deep Earth magmatic processes and their melt structure is a key parameter, as it controls physical and chemical properties. Carbonate-rich melts can be strongly enriched in geochemically important trace elements. The structural incorporation mechanisms of these elements are difficult to study because such melts generally cannot be quenched to glasses, which are usually employed for structural investigations. This thesis investigates the influence of CO2 on the local environments of trace elements contained in silicate glasses with variable CO2 concentrations as well as in silicate and carbonate melts. The compositions studied include sodium-rich peralkaline silicate melts and glasses and carbonate melts similar to those occurring naturally at Oldoinyo Lengai volcano, Tanzania. The local environments of the three elements yttrium (Y), lanthanum (La) and strontium (Sr) were investigated in synthesized glasses and melts using X-ray absorption fine structure (XAFS) spectroscopy. Especially extended X-ray absorption fine structure spectroscopy (EXAFS) provides element specific information on local structure, such as bond lengths, coordination numbers and the degree of disorder. To cope with the enhanced structural disorder present in glasses and melts, EXAFS analysis was based on fitting approaches using an asymmetric distribution function as well as a correlation model according to bond valence theory. Firstly, silicate glasses quenched from high pressure/temperature melts with up to 7.6 wt \% CO2 were investigated. In strongly and extremely peralkaline glasses the local structure of Y is unaffected by the CO2 content (with oxygen bond lengths of ~ 2.29 {\AA}). Contrary, the bond lengths for Sr-O and La-O increase with increasing CO2 content in the strongly peralkaline glasses from ~ 2.53 to ~ 2.57 {\AA} and from ~ 2.52 to ~ 2.54 {\AA}, respectively, while they remain constant in extremely peralkaline glasses (at ~ 2.55 {\AA} and 2.54 {\AA}, respectively). Furthermore, silicate and unquenchable carbonate melts were investigated in-situ at high pressure/temperature conditions (2.2 to 2.6 GPa, 1200 to 1500 °C) using a Paris-Edinburgh press. A novel design of the pressure medium assembly for this press was developed, which features increased mechanical stability as well as enhanced transmittance at relevant energies to allow for low content element EXAFS in transmission. Compared to glasses the bond lengths of Y-O, La-O and Sr-O are elongated by up to + 3 \% in the melt and exhibit higher asymmetric pair distributions. For all investigated silicate melt compositions Y-O bond lengths were found constant at ~ 2.37 {\AA}, while in the carbonate melt the Y-O length increases slightly to 2.41 {\AA}. The La-O bond lengths in turn, increase systematically over the whole silicate - carbonate melt joint from 2.55 to 2.60 {\AA}. Sr-O bond lengths in melts increase from ~ 2.60 to 2.64 {\AA} from pure silicate to silicate-bearing carbonate composition with constant elevated bond length within the carbonate region. For comparison and deeper insight, glass and melt structures of Y and Sr bearing sodium-rich silicate to carbonate compositions were simulated in an explorative ab initio molecular dynamics (MD) study. The simulations confirm observed patterns of CO2-dependent local changes around Y and Sr and additionally provide further insights into detailed incorporation mechanisms of the trace elements and CO2. Principle findings include that in sodium-rich silicate compositions carbon either is mainly incorporated as a free carbonate-group or shares one oxygen with a network former (Si or [4]Al) to form a non-bridging carbonate. Of minor importance are bridging carbonates between two network formers. Here, a clear preference for two [4]Al as adjacent network formers occurs, compared to what a statistical distribution would suggest. In C-bearing silicate melts minor amounts of molecular CO2 are present, which is almost totally dissolved as carbonate in the quenched glasses. The combination of experiment and simulation provides extraordinary insights into glass and melt structures. The new data is interpreted on the basis of bond valence theory and is used to deduce potential mechanisms for structural incorporation of investigated elements, which allow for prediction on their partitioning behavior in natural melts. Furthermore, it provides unique insights into the dissolution mechanisms of CO2 in silicate melts and into the carbonate melt structure. For the latter, a structural model is suggested, which is based on planar CO3-groups linking 7- to 9-fold cation polyhedra, in accordance to structural units as found in the Na-Ca carbonate nyerereite. Ultimately, the outcome of this study contributes to rationalize the unique physical properties and geological phenomena related to carbonated silicate-carbonate melts.}, language = {en} }