The effect of alkalinity on Ni-O bond length in silicate glasses
- 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 systematicallyEquilibrium 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.…
Author details: | Martijn Klaver, Edgar S. Steenstra, Manuela Borchert, Edmund Welter, Max WilkeORCiDGND, Jasper Berndt, Stephan Klemme |
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DOI: | https://doi.org/10.1016/j.chemgeo.2022.121070 |
ISSN: | 0009-2541 |
ISSN: | 1872-6836 |
Title of parent work (English): | Chemical geology : official journal of the European Association for Geochemistry |
Subtitle (English): | implications for Ni isotope geochemistry |
Publisher: | Elsevier |
Place of publishing: | Amsterdam |
Publication type: | Article |
Language: | English |
Date of first publication: | 2022/08/20 |
Publication year: | 2022 |
Release date: | 2024/09/02 |
Tag: | EXAFS; Ni isotopes; Ni-O bond length; isotopic fractionation factors; silicate glasses |
Volume: | 610 |
Article number: | 121070 |
Number of pages: | 7 |
Funding institution: | Humboldt Foundation postdoctoral fellowship; Marie Sklodowska-Curie; Postdoctoral Fellowship |
Organizational units: | Mathematisch-Naturwissenschaftliche Fakultät / Institut für Geowissenschaften |
DDC classification: | 5 Naturwissenschaften und Mathematik / 55 Geowissenschaften, Geologie / 550 Geowissenschaften |
Peer review: | Referiert |