TY - JOUR

T1 - Experiments reveal enrichment of 11B in granitic melt resulting from tourmaline crystallisation

AU - Cheng, L.

AU - Zhang, C.

AU - Zhou, Y.

AU - Horn, I.

AU - Weyer, S.

AU - Holtz, F.

N1 - Funding Information: We thank U. Kroll, D. Qi, X. Li for their help of experiment and D. Wang for his assistance of EMPA analysis. The stay of L. Cheng at Hannover was supported by the Basic Research Fund of the Institute of Geology, China Earthquake Administration (IGCEA2018 and IGCEA1914). Laboratory costs were supported by the German Science Foundation (DFG, project HO1337/49). This paper has been greatly improved by the constructive comments from Horst R. Marschall, Vincent van Hinsberg, and an anonymous reviewer. We are also grateful to Horst R. Marschall for his editorial handling.

PY - 2022/2/24

Y1 - 2022/2/24

N2 - Tourmaline is the most common boron-rich mineral in magmatic systems. In this study, we determined experimentally the fractionation of boron isotopes between granitic melt and tourmaline for the first time. Our crystallisation experiments were performed using a boron-rich granitic glass (B2O3 ≈ 8 wt. %) at 660−800 °C, 300 MPa, and aH2O = 1, in which tourmaline occurs as the only boron-hosting mineral. Our experimental results at four different temperatures show a small and temperature-dependent boron isotope fractionation between granitic melt and tourmaline (Δ11Bmelt–Tur = þ0.90 ± 0.05 ‰ at 660 °C and þ0.23 ± 0.12 ‰ at 800 °C), and the temperature dependence can be defined as Δ11Bmelt–Tur = 4.51 × (1000/T [K]) − 3.94 (R2 = 0.96). Using these boron isotope fractionation factors, tourmaline can serve as a tracer to quantitatively interpret boron isotopic ratios in evolved magmatic systems. Our observation that 11B is enriched in granitic melt relative to tourmaline suggests that the δ11B of late-magmatic tourmaline should be higher than tourmaline that crystallised at an early stage, if B isotope fractionation is not affected by other processes, such as fluid loss.

AB - Tourmaline is the most common boron-rich mineral in magmatic systems. In this study, we determined experimentally the fractionation of boron isotopes between granitic melt and tourmaline for the first time. Our crystallisation experiments were performed using a boron-rich granitic glass (B2O3 ≈ 8 wt. %) at 660−800 °C, 300 MPa, and aH2O = 1, in which tourmaline occurs as the only boron-hosting mineral. Our experimental results at four different temperatures show a small and temperature-dependent boron isotope fractionation between granitic melt and tourmaline (Δ11Bmelt–Tur = þ0.90 ± 0.05 ‰ at 660 °C and þ0.23 ± 0.12 ‰ at 800 °C), and the temperature dependence can be defined as Δ11Bmelt–Tur = 4.51 × (1000/T [K]) − 3.94 (R2 = 0.96). Using these boron isotope fractionation factors, tourmaline can serve as a tracer to quantitatively interpret boron isotopic ratios in evolved magmatic systems. Our observation that 11B is enriched in granitic melt relative to tourmaline suggests that the δ11B of late-magmatic tourmaline should be higher than tourmaline that crystallised at an early stage, if B isotope fractionation is not affected by other processes, such as fluid loss.

UR - http://www.scopus.com/inward/record.url?scp=85126435173&partnerID=8YFLogxK

U2 - 10.7185/GEOCHEMLET.2206

DO - 10.7185/GEOCHEMLET.2206

M3 - Article

AN - SCOPUS:85126435173

VL - 20

SP - 37

EP - 42

JO - Geochemical Perspectives Letters

JF - Geochemical Perspectives Letters

SN - 2410-339X

ER -