dc.contributor.author
Loges, Anselm
dc.contributor.author
Manni, Marco
dc.contributor.author
Louvel, Marion
dc.contributor.author
Wilke, Max
dc.contributor.author
Jahn, Sandro
dc.contributor.author
Welter, Edmund
dc.contributor.author
Borchert, Manuela
dc.contributor.author
Qiao, Shilei
dc.contributor.author
Keller, Bettina G.
dc.contributor.author
John, Timm
dc.date.accessioned
2024-03-21T11:58:54Z
dc.date.available
2024-03-21T11:58:54Z
dc.identifier.uri
https://refubium.fu-berlin.de/handle/fub188/42961
dc.identifier.uri
http://dx.doi.org/10.17169/refubium-42675
dc.description.abstract
Zirconium and hafnium behave nearly identically in most geological processes due to their identical nominal ionic charge and similar radius. Some of the most pronounced exceptions from this rule are observed in fluoride-rich aqueous systems, suggesting that aqueous fluoride complexation may be involved in Zr/Hf fractionation. To understand the mechanisms causing this phenomenon, we investigated complexation of Zr4+ and Hf4+ in fluoride-rich (1.0 mol/kg HF) aqueous solutions at 40 MPa and 100–400 °C, using synchrotron X-ray absorption spectroscopy (X-ray absorption near edge structure and extended X-ray absorption fine structure) combined with classical and ab initio molecular dynamics simulations. The dominant experimentally observed complexes are [Zr(F,OH)4·2H2O]0 and [Hf(F,OH)4·2H2O]0, respectively. The first coordination shell comprises a distorted octahedron, with fluoride and hydroxide ligands at a similar mean radial distance (1.9–2.0 Å) from the central cation, and H2O ligands at a slightly greater distance (>2.1 Å). With increasing temperature, the H2O ligands move further out, causing first an increasing distortion of the octahedron and subsequently a partial transition to less hydrated complexes as a certain fraction of the H2O molecules move to the second shell at > 3 Å. As a consequence, the radial distance of the F- and OH– anions from the central cation, as well as the overall average radial distance of the first shell decreases due to decreased steric repulsion from the H2O ligands. Both experiments and simulations agree in that Hf forms slightly shorter bonds to its nearest neighbors than Zr. The results suggest two hypotheses for the mechanism of Zr/Hf fractionation during precipitation of minerals from fluoride-rich hydrothermal solutions: 1) The heavy twin (Hf) prefers the lower coordination (shorter bonds) and is thus less likely to enter into the higher coordination found in the solids. This mechanism would be analogous to equilibrium isotope fractionation. 2) The change of Hf into a higher coordination environment (e.g., from solution to solid) is slower because it forms stronger ligand-bonds than Zr. This would be analogous to reactive kinetic isotope fractionation. In either case mass dependent fractionation qualitatively matches the observations but mass independent effects on bond strength may also be significant. Quantitative investigations of these effects are needed and may also shed light on the currently still somewhat enigmatic fractionation behavior of Zr isotopes.
en
dc.format.extent
15 Seiten
dc.rights.uri
https://creativecommons.org/licenses/by/4.0/
dc.subject
Aqueous speciation
en
dc.subject
Hydrothermal fluoride complexation
en
dc.subject
Geochemical twin fractionation
en
dc.subject.ddc
500 Naturwissenschaften und Mathematik::550 Geowissenschaften, Geologie::550 Geowissenschaften
dc.title
Complexation of Zr and Hf in fluoride-rich hydrothermal aqueous fluids and its significance for high field strength element fractionation
dc.type
Wissenschaftlicher Artikel
dcterms.bibliographicCitation.doi
10.1016/j.gca.2023.12.013
dcterms.bibliographicCitation.journaltitle
Geochimica et Cosmochimica Acta
dcterms.bibliographicCitation.pagestart
167
dcterms.bibliographicCitation.pageend
181
dcterms.bibliographicCitation.volume
366
dcterms.bibliographicCitation.url
https://doi.org/10.1016/j.gca.2023.12.013
refubium.affiliation
Geowissenschaften
refubium.affiliation
Biologie, Chemie, Pharmazie
refubium.affiliation.other
Institut für Geologische Wissenschaften / Fachrichtung Geochemie, Hydrogeologie, Mineralogie
refubium.affiliation.other
Institut für Chemie und Biochemie
refubium.resourceType.isindependentpub
no
dcterms.accessRights.openaire
open access
dcterms.isPartOf.eissn
1872-9533
refubium.resourceType.provider
WoS-Alert