Details
Originalsprache | Englisch |
---|---|
Fachzeitschrift | Geochimica et cosmochimica acta |
Frühes Online-Datum | 26 Nov. 2024 |
Publikationsstatus | Elektronisch veröffentlicht (E-Pub) - 26 Nov. 2024 |
Abstract
In this study, we experimentally investigate the diffusion of B in flux-rich (1.7 % Li 2O, 2.5 % B 2O 3, 2.7 % P 2O 5 and 3.5 % F) pegmatite forming melts in order to assess the transport mechanisms of B in rare-element pegmatite deposits. Glasses were synthesized with either different B isotope compositions or with different B concentrations (up to 2.45 wt% B 2O 3). In order to explore the effect of water on transport properties, nominally dry and hydrous glasses (3.1–3.9 wt% H 2O) were investigated. The produced glasses were combined to diffusion couples to study self-diffusion and chemical diffusion of B in the same chemical system. Experiments were conducted using an internally heated pressure vessel and a rapid-heat and rapid-quench cold-seal pressure vessel at 100 MPa Ar pressure in the temperature range of 850–1250 °C for 1.6–97.3 h. In chemical diffusion experiments, the flow of B is compensated by a combined opposing flow of all other cations and F, without changing their element ratios. Thus, effective binary diffusion coefficients are obtained. Similar activation energies were determined for B self-diffusion and chemical diffusion under nominally dry conditions (196 ± 6 kJ/mol and 200 ± 17kJ/mol, respectively). Many hydrous experiments show a tilted to strongly deformed interface after the run, which was probably formed by advective flow of the low-viscosity melts during heating under pressure. Based on the successful experiments, an activation energy of 138 ± 8 kJ/mol was estimated for chemical diffusion of B in hydrous melts. We show that B diffusivity correlates with the Eyring diffusivity and, therefore, with the melt viscosity. The stable isotopes of B fractionate kinetically along the diffusion profiles due to the faster diffusivity of 10B over 11B. This isotope fractionation can be quantified with an empirical isotope fractionation factor (β) of 0.032 ± 0.002. This effect is small, but significant and was previously not observed in experimental studies. While it is insensitive to the water contents used in our study, it seems to be strongly dependent on the experimental boundary conditions, such as the ratios of B between the diffusion couple halves and the B enrichment in the melt. Solid-state NMR experiments reveal that the majority of B is in trigonal coordination in our melts, which effectively weakens the melt structure. This further suggests that coordination differences are unlikely to be the driving force for B isotope fractionation between melt and fluid during late-stage fluid exsolution in pegmatitic systems.
ASJC Scopus Sachgebiete
- Erdkunde und Planetologie (insg.)
- Geochemie und Petrologie
Fachgebiet (basierend auf ÖFOS 2012)
- NATURWISSENSCHAFTEN
- Geowissenschaften
- Geologie, Mineralogie
- Geochemie
- NATURWISSENSCHAFTEN
- Geowissenschaften
- Geologie, Mineralogie
- Petrologie
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in: Geochimica et cosmochimica acta, 26.11.2024.
Publikation: Beitrag in Fachzeitschrift › Artikel › Forschung › Peer-Review
}
TY - JOUR
T1 - Boron diffusion, related isotope fractionation and the structural role of B in pegmatite forming melts
AU - Singer, Christian R.
AU - Behrens, Harald
AU - Horn, Ingo
AU - Fechtelkord, Michael
AU - Weyer, Stefan
N1 - Publisher Copyright: © 2024 The Authors
PY - 2024/11/26
Y1 - 2024/11/26
N2 - In this study, we experimentally investigate the diffusion of B in flux-rich (1.7 % Li 2O, 2.5 % B 2O 3, 2.7 % P 2O 5 and 3.5 % F) pegmatite forming melts in order to assess the transport mechanisms of B in rare-element pegmatite deposits. Glasses were synthesized with either different B isotope compositions or with different B concentrations (up to 2.45 wt% B 2O 3). In order to explore the effect of water on transport properties, nominally dry and hydrous glasses (3.1–3.9 wt% H 2O) were investigated. The produced glasses were combined to diffusion couples to study self-diffusion and chemical diffusion of B in the same chemical system. Experiments were conducted using an internally heated pressure vessel and a rapid-heat and rapid-quench cold-seal pressure vessel at 100 MPa Ar pressure in the temperature range of 850–1250 °C for 1.6–97.3 h. In chemical diffusion experiments, the flow of B is compensated by a combined opposing flow of all other cations and F, without changing their element ratios. Thus, effective binary diffusion coefficients are obtained. Similar activation energies were determined for B self-diffusion and chemical diffusion under nominally dry conditions (196 ± 6 kJ/mol and 200 ± 17kJ/mol, respectively). Many hydrous experiments show a tilted to strongly deformed interface after the run, which was probably formed by advective flow of the low-viscosity melts during heating under pressure. Based on the successful experiments, an activation energy of 138 ± 8 kJ/mol was estimated for chemical diffusion of B in hydrous melts. We show that B diffusivity correlates with the Eyring diffusivity and, therefore, with the melt viscosity. The stable isotopes of B fractionate kinetically along the diffusion profiles due to the faster diffusivity of 10B over 11B. This isotope fractionation can be quantified with an empirical isotope fractionation factor (β) of 0.032 ± 0.002. This effect is small, but significant and was previously not observed in experimental studies. While it is insensitive to the water contents used in our study, it seems to be strongly dependent on the experimental boundary conditions, such as the ratios of B between the diffusion couple halves and the B enrichment in the melt. Solid-state NMR experiments reveal that the majority of B is in trigonal coordination in our melts, which effectively weakens the melt structure. This further suggests that coordination differences are unlikely to be the driving force for B isotope fractionation between melt and fluid during late-stage fluid exsolution in pegmatitic systems.
AB - In this study, we experimentally investigate the diffusion of B in flux-rich (1.7 % Li 2O, 2.5 % B 2O 3, 2.7 % P 2O 5 and 3.5 % F) pegmatite forming melts in order to assess the transport mechanisms of B in rare-element pegmatite deposits. Glasses were synthesized with either different B isotope compositions or with different B concentrations (up to 2.45 wt% B 2O 3). In order to explore the effect of water on transport properties, nominally dry and hydrous glasses (3.1–3.9 wt% H 2O) were investigated. The produced glasses were combined to diffusion couples to study self-diffusion and chemical diffusion of B in the same chemical system. Experiments were conducted using an internally heated pressure vessel and a rapid-heat and rapid-quench cold-seal pressure vessel at 100 MPa Ar pressure in the temperature range of 850–1250 °C for 1.6–97.3 h. In chemical diffusion experiments, the flow of B is compensated by a combined opposing flow of all other cations and F, without changing their element ratios. Thus, effective binary diffusion coefficients are obtained. Similar activation energies were determined for B self-diffusion and chemical diffusion under nominally dry conditions (196 ± 6 kJ/mol and 200 ± 17kJ/mol, respectively). Many hydrous experiments show a tilted to strongly deformed interface after the run, which was probably formed by advective flow of the low-viscosity melts during heating under pressure. Based on the successful experiments, an activation energy of 138 ± 8 kJ/mol was estimated for chemical diffusion of B in hydrous melts. We show that B diffusivity correlates with the Eyring diffusivity and, therefore, with the melt viscosity. The stable isotopes of B fractionate kinetically along the diffusion profiles due to the faster diffusivity of 10B over 11B. This isotope fractionation can be quantified with an empirical isotope fractionation factor (β) of 0.032 ± 0.002. This effect is small, but significant and was previously not observed in experimental studies. While it is insensitive to the water contents used in our study, it seems to be strongly dependent on the experimental boundary conditions, such as the ratios of B between the diffusion couple halves and the B enrichment in the melt. Solid-state NMR experiments reveal that the majority of B is in trigonal coordination in our melts, which effectively weakens the melt structure. This further suggests that coordination differences are unlikely to be the driving force for B isotope fractionation between melt and fluid during late-stage fluid exsolution in pegmatitic systems.
KW - Boron isotopes
KW - Solid-state NMR spectroscopy
KW - Pegmatite forming melts
KW - Diffusion
UR - http://www.scopus.com/inward/record.url?scp=85211050307&partnerID=8YFLogxK
U2 - 10.1016/j.gca.2024.11.023
DO - 10.1016/j.gca.2024.11.023
M3 - Article
JO - Geochimica et cosmochimica acta
JF - Geochimica et cosmochimica acta
SN - 0016-7037
ER -