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
T2  - 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
UR  - https://publishup.uni-potsdam.de/frontdoor/index/index/docId/65506
SN  - 0009-2541
SN  - 1872-6836
VL  - 610
PB  - Elsevier
CY  - Amsterdam
ER  -