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Boron coordination and B/Si ordering controls over equilibrium boron isotope fractionation among minerals, melts, and fluids

Boron coordination and B/Si ordering controls over equilibrium boron isotope fractionation among minerals, melts, and fluids
Boron coordination and B/Si ordering controls over equilibrium boron isotope fractionation among minerals, melts, and fluids

The high mobility of boron during fluid-rock interaction makes it an effective tracer for the sources of magmatic and metamorphic fluids, as recorded in minerals such as tourmaline and muscovite. Although advances have been made in quantifying the fractionation of boron isotopes among different phases, boron isotope fractionation in complex silicate melts remains poorly understood. Here, we propose appropriate models for the BO 3 and BO 4 units in silicate melts covering a wide range of chemical compositions and boron coordination structures in silicate magmas, and report the results of a theoretical investigation of boron isotope fractionation among silicate melt, minerals and fluids using a first principles theoretical approach. A comparison of measured and calculated α factors in mineral-melt and fluid-melt systems shows good agreement, suggesting the applicability of a simplified treatment of boron coordination structures in silicate melt. The results of this study show that the proportion of trigonal/tetrahedral coordinated boron and the B/Si ordering in silicate tetrahedral layers control the boron isotope fractionation among different phases, and that the effect of chemical composition is minor (less than 2‰ at 600 K). The temperature-dependent boron isotope fractionations are described as 1000lnα mica-basic fluid = 0.8–2.4 × (1000/T) - 0.8 × (1000/T) 2, 1000lnα mica-acidic fluid = 7.0–14.0 × (1000/T) - 1.2 × (1000/T) 2 and 1000lnα mica-tur = 1.9–5.4 × (1000/T) -3.4 × (1000/T) 2 (T is temperature in Kelvins). At magmatic temperatures, ∆ 11B values between mineral/fluid and melt also vary with the proportion of the BO 4 unit in the melt. This study underpins the applicability of the white mica-tourmaline geothermometers and boron isotopes for fluid source identification, and also offers an explanation of boron isotope fractionation in systems that contain complex silicate melts.

Aqueous liquid, Boron isotope fractionation, Boron isotope-geothermometer, Boron-bearing minerals, Silicate melt
0009-2541
Li, Yin-chuan
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Wei, Hai-zhen
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Palmer, Martin R.
d2e60e81-5d6e-4ddb-a243-602537286080
Jiang, Shao-yong
dc929006-69ac-4366-bf2b-dc97527602d3
Liu, Xi
9589a507-97cc-4672-845c-134499274338
Williams-jones, Anthony E.
62e80214-47d8-41b6-8838-910c1f2f7384
Ma, Jing
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Lu, Jian-jun
a5102598-bc25-4892-b6c4-7ac803412055
Lin, Yi-bo
19d2c485-0f70-4619-ae88-ce68003f31a9
Dong, Ge
0766e617-6852-4720-a253-13d0becd6d4d
Li, Yin-chuan
5f9307cf-80a5-4ac8-a6bb-bd678b5206fe
Wei, Hai-zhen
60e6342b-b6d2-4ac6-a4d5-a1dec963b1a7
Palmer, Martin R.
d2e60e81-5d6e-4ddb-a243-602537286080
Jiang, Shao-yong
dc929006-69ac-4366-bf2b-dc97527602d3
Liu, Xi
9589a507-97cc-4672-845c-134499274338
Williams-jones, Anthony E.
62e80214-47d8-41b6-8838-910c1f2f7384
Ma, Jing
3495e3ca-6f13-42ae-8b6a-7d8d4e3166fe
Lu, Jian-jun
a5102598-bc25-4892-b6c4-7ac803412055
Lin, Yi-bo
19d2c485-0f70-4619-ae88-ce68003f31a9
Dong, Ge
0766e617-6852-4720-a253-13d0becd6d4d

Li, Yin-chuan, Wei, Hai-zhen, Palmer, Martin R., Jiang, Shao-yong, Liu, Xi, Williams-jones, Anthony E., Ma, Jing, Lu, Jian-jun, Lin, Yi-bo and Dong, Ge (2021) Boron coordination and B/Si ordering controls over equilibrium boron isotope fractionation among minerals, melts, and fluids. Chemical Geology, 561, [120030]. (doi:10.1016/j.chemgeo.2020.120030).

Record type: Article

Abstract

The high mobility of boron during fluid-rock interaction makes it an effective tracer for the sources of magmatic and metamorphic fluids, as recorded in minerals such as tourmaline and muscovite. Although advances have been made in quantifying the fractionation of boron isotopes among different phases, boron isotope fractionation in complex silicate melts remains poorly understood. Here, we propose appropriate models for the BO 3 and BO 4 units in silicate melts covering a wide range of chemical compositions and boron coordination structures in silicate magmas, and report the results of a theoretical investigation of boron isotope fractionation among silicate melt, minerals and fluids using a first principles theoretical approach. A comparison of measured and calculated α factors in mineral-melt and fluid-melt systems shows good agreement, suggesting the applicability of a simplified treatment of boron coordination structures in silicate melt. The results of this study show that the proportion of trigonal/tetrahedral coordinated boron and the B/Si ordering in silicate tetrahedral layers control the boron isotope fractionation among different phases, and that the effect of chemical composition is minor (less than 2‰ at 600 K). The temperature-dependent boron isotope fractionations are described as 1000lnα mica-basic fluid = 0.8–2.4 × (1000/T) - 0.8 × (1000/T) 2, 1000lnα mica-acidic fluid = 7.0–14.0 × (1000/T) - 1.2 × (1000/T) 2 and 1000lnα mica-tur = 1.9–5.4 × (1000/T) -3.4 × (1000/T) 2 (T is temperature in Kelvins). At magmatic temperatures, ∆ 11B values between mineral/fluid and melt also vary with the proportion of the BO 4 unit in the melt. This study underpins the applicability of the white mica-tourmaline geothermometers and boron isotopes for fluid source identification, and also offers an explanation of boron isotope fractionation in systems that contain complex silicate melts.

Text
Li et al 2021 pre publication - Accepted Manuscript
Restricted to Repository staff only until 26 December 2021.
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More information

Accepted/In Press date: 14 December 2020
e-pub ahead of print date: 26 December 2020
Published date: 5 February 2021
Keywords: Aqueous liquid, Boron isotope fractionation, Boron isotope-geothermometer, Boron-bearing minerals, Silicate melt

Identifiers

Local EPrints ID: 446946
URI: http://eprints.soton.ac.uk/id/eprint/446946
ISSN: 0009-2541
PURE UUID: d2b0e250-d42a-499f-9a08-32a26bb55a6b

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Date deposited: 26 Feb 2021 17:33
Last modified: 26 Feb 2021 17:33

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Contributors

Author: Yin-chuan Li
Author: Hai-zhen Wei
Author: Shao-yong Jiang
Author: Xi Liu
Author: Anthony E. Williams-jones
Author: Jing Ma
Author: Jian-jun Lu
Author: Yi-bo Lin
Author: Ge Dong

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