TY - JOUR A1 - Klemme, Stephan A1 - Feldhaus, Michael A1 - Potapkin, Vasily A1 - Wilke, Max A1 - Borchert, Manuela A1 - Louvel, Marion A1 - Loges, Anselm A1 - Rohrbach, Arno A1 - Weitkamp, Petra A1 - Welter, Edmund A1 - Kokh, Maria A. A1 - Schmidt, Christian A1 - Testemale, Denis T1 - A hydrothermal apparatus for x-ray absorption spectroscopy of hydrothermal fluids at DESY JF - Review of scientific instruments : a monthly journal devoted to scientific instruments, apparatus, and techniques N2 - We present a new autoclave that enables in situ characterization of hydrothermal fluids at high pressures and high temperatures at synchrotron x-ray radiation sources. The autoclave has been specifically designed to enable x-ray absorption spectroscopy in fluids with applications to mineral solubility and element speciation analysis in hydrothermal fluids in complex compositions. However, other applications, such as Raman spectroscopy, in high-pressure fluids are also possible with the autoclave. First experiments were run at pressures between 100 and 600 bars and at temperatures between 25 degrees C and 550 degrees C, and preliminary results on scheelite dissolution in fluids of different compositions show that the autoclave is well suited to study the behavior of ore-forming metals at P-T conditions relevant to the Earth's crust. Y1 - 2021 U6 - https://doi.org/10.1063/5.0044767 SN - 0034-6748 SN - 1089-7623 VL - 92 IS - 6 PB - AIP Publishing CY - Melville ER - TY - JOUR A1 - Klaver, Martijn A1 - Steenstra, Edgar S. A1 - Borchert, Manuela A1 - Welter, Edmund A1 - Wilke, Max A1 - Berndt, Jasper A1 - Klemme, Stephan T1 - The effect of alkalinity on Ni-O bond length in silicate glasses BT - implications for Ni isotope geochemistry JF - Chemical geology : official journal of the European Association for Geochemistry N2 - Equilibrium mass-dependent ("stable") isotopic fractionation of an element during magmatic processes is driven by a contrast in bonding environment between minerals and silicate melt, which is expressed as an isotopic fractionation factor. A quantitative understanding of such isotopic fractionation factors is vital to interpret observed isotopic variations in magmatic rocks. It is well known that the local environment and the bond strength of an element dictate the sign and magnitude of isotopic fractionation between minerals, but it is uncertain how the structure and chemical composition of a silicate melt can affect mineral-melt isotopic fractionation factors. To explore this, we studied the coordination environment of nickel (Ni) in different silicate glasses using extended X-ray absorption fine structure (EXAFS) measurements at the German synchrotron X-ray source (DESY). We determined -Ni-O bond lengths in a suite of synthetic but near-natural silicate glasses using EXAFS and found that the former vary systematically with melt alkalinity, which is best described by the parameter ln[1 + (Na + K)/Ca]. With increasing melt alkalinity, Ni occupies more IV-fold coordinated sites, which are associated with a shorter -Ni-O bond length. Next, we use the ionic model, which allows to predict isotopic fractionation factors based on the difference in bond length between two phases. We find that more alkaline melts have a stronger preference for the heavier isotopes of Ni than less alkaline melts. This implies that the magnitude of mineral-melt Ni isotope fractionation factors, for instance between olivine and melt, will depend on the alkalinity of the melt. At magmatic temperatures, however, the variation in fractionation factors caused by melt alkalinity will rarely exceed 0.05 parts per thousand and is thus mostly negligible, in particular in the realm of basaltic melt compositions. Nevertheless, the relationship between melt alkalinity and fractionation factor reported here can be used to extrapolate empirical data for mineral-melt Ni isotope fractionation factors, once such data become available, to the full range of magma compositions on Earth and other Solar System bodies. KW - EXAFS KW - Ni-O bond length KW - silicate glasses KW - Ni isotopes KW - isotopic fractionation factors Y1 - 2022 U6 - https://doi.org/10.1016/j.chemgeo.2022.121070 SN - 0009-2541 SN - 1872-6836 VL - 610 PB - Elsevier CY - Amsterdam ER -