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Boron isotope composition of coexisting tourmaline and hambergite in alkaline and granitic pegmatites

Authors

Sunde,  Øyvind
External Organizations;
GFZ SIMS Publications, Deutsches GeoForschungsZentrum;

Friis,  Henrik
External Organizations;
GFZ SIMS Publications, Deutsches GeoForschungsZentrum;

Andersen,  Tom
External Organizations;
GFZ SIMS Publications, Deutsches GeoForschungsZentrum;

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Trumbull,  R.
3.1 Inorganic and Isotope Geochemistry, 3.0 Geochemistry, Departments, GFZ Publication Database, Deutsches GeoForschungsZentrum;
GFZ SIMS Publications, Deutsches GeoForschungsZentrum;

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Wiedenbeck,  Michael
3.1 Inorganic and Isotope Geochemistry, 3.0 Geochemistry, Departments, GFZ Publication Database, Deutsches GeoForschungsZentrum;
GFZ SIMS Publications, Deutsches GeoForschungsZentrum;

Lyckberg,  Peter
External Organizations;
GFZ SIMS Publications, Deutsches GeoForschungsZentrum;

Agostini,  Samuele
External Organizations;
GFZ SIMS Publications, Deutsches GeoForschungsZentrum;

Casey,  William H.
External Organizations;
GFZ SIMS Publications, Deutsches GeoForschungsZentrum;

Yu,  Ping
External Organizations;
GFZ SIMS Publications, Deutsches GeoForschungsZentrum;

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5000238.pdf
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Citation

Sunde, Ø., Friis, H., Andersen, T., Trumbull, R., Wiedenbeck, M., Lyckberg, P., Agostini, S., Casey, W. H., Yu, P. (2020): Boron isotope composition of coexisting tourmaline and hambergite in alkaline and granitic pegmatites. - Lithos, 352-353, 105293.
https://doi.org/10.1016/j.lithos.2019.105293


Cite as: https://gfzpublic.gfz-potsdam.de/pubman/item/item_5000238
Abstract
The boron isotopic composition of tourmaline and hambergite (Be2BO3[OH,F]) from peraluminous (n = 12), peralkaline (n = 1), and peralkaline nepheline syenite (n = 16) pegmatites has been measured by secondary ion mass spectrometry, for which a new hambergite reference material was developed. The focus of this study is on nepheline syenite pegmatites from the Larvik Plutonic Complex (Norway) and one peralkaline pegmatite related to the nearby Eikeren-Skrim Complex (Norway), where we investigate the source of boron as being from magmatic vs. external fluids. Tourmaline-hambergite mineral pairs were also analysed from peraluminous pegmatite localities (Russia, Tajikistan, and Pakistan) to test for systematic B-isotope fractionation between these two minerals. Tourmaline and hambergite from peraluminous granitic pegmatites have light boron ratios (δ11B = −12.9to −1.0‰) associated with S-type granites, whereas peralkaline granitic and nepheline syenite pegmatites have boron ratios (δ11B = −1.7 to 11.8‰), which we interpret is a result of heavy‑boron enrichment from external fluids. Our data show that hambergite tracks isotope variations of its geochemical setting and could therefore be used as a proxy mineral in place of tourmaline when geochemical stability favours hambergite. The results suggest a slight but consistent partitioning of B-isotopes between tourmaline and hambergite, with Δ11B = δ11Btourmaline−δ11Bhambergite in the range of approximately −3‰ to −5‰. Boron is in trigonal coordination with oxygen in both of these mineral phases as verified by NMR. Single crystal XRD analyses of tourmaline and hambergite reveal consistent longer <B-O > distances of tourmaline relative to hambergite. We attribute the boron isotopic fractionation to the longer <B-O > bond-lengths in tourmaline compared with hambergite.