Li–Na interdiffusion and diffusion-driven lithium isotope fractionation in pegmatitic melts
<p>In this study, we investigate the diffusion of Li and its stable isotopes (<span class="inline-formula"><sup>6</sup></span>Li and <span class="inline-formula"><sup>7</sup></span>Li) in flux-rich (1.8 % Li<span class=&quo...
| Main Authors: | , , , , , |
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| Format: | Article |
| Language: | English |
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Copernicus Publications
2023-11-01
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| Series: | European Journal of Mineralogy |
| Online Access: | https://ejm.copernicus.org/articles/35/1009/2023/ejm-35-1009-2023.pdf |
| _version_ | 1797626688899645440 |
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| author | C. R. Singer H. Behrens I. Horn M. Oeser R. Dohmen S. Weyer |
| author_facet | C. R. Singer H. Behrens I. Horn M. Oeser R. Dohmen S. Weyer |
| author_sort | C. R. Singer |
| collection | DOAJ |
| description | <p>In this study, we investigate the diffusion of Li and its stable isotopes (<span class="inline-formula"><sup>6</sup></span>Li and <span class="inline-formula"><sup>7</sup></span>Li) in flux-rich (1.8 % Li<span class="inline-formula"><sub>2</sub></span>O, 2.6 % B<span class="inline-formula"><sub>2</sub></span>O<span class="inline-formula"><sub>3</sub></span>, 2.3 % P<span class="inline-formula"><sub>2</sub></span>O<span class="inline-formula"><sub>5</sub></span> and 3 % F) pegmatitic melts in order to contribute to the understanding of Li enrichment in such systems. Two glasses were synthesized with a model pegmatitic composition, one of which is highly enriched in Li (<span class="inline-formula"><i>></i></span> 1 wt %, PEG2-blue) and the other one essentially Li-free (PEG2-Li-free). Diffusion couple experiments were performed to determine the chemical diffusivity of Li in dry pegmatitic melts. Experiments were conducted using rapid-heat and rapid-quench cold-seal pressure vessels in a temperature range of 650–940 <span class="inline-formula"><sup>∘</sup></span>C at 100 MPa with Ar as the pressure medium. We observed rapidly formed diffusion profiles, driven by an interdiffusive exchange of the monovalent alkalis Li and Na, while the other elements are immobile on the timescale of experiments (1–30 min). From these experiments, activation energies for Li–Na interdiffusion were determined as 99 <span class="inline-formula">±</span> 7 kJ mol<span class="inline-formula"><sup>−1</sup></span> with a pre-exponential factor of log <span class="inline-formula"><i>D</i><sub>0</sub></span> <span class="inline-formula">=</span> <span class="inline-formula">−</span>5.05 <span class="inline-formula">±</span> 0.33 (<span class="inline-formula"><i>D</i><sub>0</sub></span> in m<span class="inline-formula"><sup>2</sup></span> s<span class="inline-formula"><sup>−1</sup></span>). Li and Na partitioning between the stronger depolymerized PEG2-blue and the less depolymerized PEG2-Li-free leads to a concentration jump at the interface; i.e. Na is enriched in the more depolymerized PEG2-blue. Li–Na interdiffusion coefficients in the studied melt composition are in a similar range as Li and Na tracer diffusivities in other dry aluminosilicate melts, confirming little to no effect of aluminosilicate melt composition on Li diffusivity. Thus, added fluxes do not enhance the Li diffusivity in the same way as observed for H<span class="inline-formula"><sub>2</sub></span>O (Holycross et al., 2018; Spallanzani et al., 2022). Using melt viscosity as a proxy for the polymerization of the melt shows that water has a stronger potential to depolymerize a melt compared to other fluxing elements. Faster diffusion of <span class="inline-formula"><sup>6</sup></span>Li compared to <span class="inline-formula"><sup>7</sup></span>Li leads to a strong Li isotope fractionation along the diffusion profile, resulting in <span class="inline-formula"><i>δ</i><sup>7</sup></span>Li as low as <span class="inline-formula">−</span>80 ‰ relative to the diffusion-unaffected regions. This diffusive isotope fractionation can be quantified with an empirical isotope fractionation factor (<span class="inline-formula"><i>β</i></span>) of 0.20 <span class="inline-formula">±</span> 0.04, similar to previously observed <span class="inline-formula"><i>β</i></span> values for Li diffusion in melts. This suggests in accordance with previously published data that a <span class="inline-formula"><i>β</i></span> value of ca. 0.2 seems to be universally applicable to diffusive Li isotope fractionation in aluminosilicate melts.</p> |
| first_indexed | 2024-03-11T10:13:46Z |
| format | Article |
| id | doaj.art-5d98c4d081cb46b19bff32e8c98064ca |
| institution | Directory Open Access Journal |
| issn | 0935-1221 1617-4011 |
| language | English |
| last_indexed | 2024-03-11T10:13:46Z |
| publishDate | 2023-11-01 |
| publisher | Copernicus Publications |
| record_format | Article |
| series | European Journal of Mineralogy |
| spelling | doaj.art-5d98c4d081cb46b19bff32e8c98064ca2023-11-16T10:57:15ZengCopernicus PublicationsEuropean Journal of Mineralogy0935-12211617-40112023-11-01351009102610.5194/ejm-35-1009-2023Li–Na interdiffusion and diffusion-driven lithium isotope fractionation in pegmatitic meltsC. R. Singer0H. Behrens1I. Horn2M. Oeser3R. Dohmen4S. Weyer5Institut für Mineralogie, Leibniz Universität Hannover, 30167 Hanover, GermanyInstitut für Mineralogie, Leibniz Universität Hannover, 30167 Hanover, GermanyInstitut für Mineralogie, Leibniz Universität Hannover, 30167 Hanover, GermanyInstitut für Mineralogie, Leibniz Universität Hannover, 30167 Hanover, GermanyInstitut für Geologie, Mineralogie und Geophysik, Ruhr Universität Bochum, 44780 Bochum, GermanyInstitut für Mineralogie, Leibniz Universität Hannover, 30167 Hanover, Germany<p>In this study, we investigate the diffusion of Li and its stable isotopes (<span class="inline-formula"><sup>6</sup></span>Li and <span class="inline-formula"><sup>7</sup></span>Li) in flux-rich (1.8 % Li<span class="inline-formula"><sub>2</sub></span>O, 2.6 % B<span class="inline-formula"><sub>2</sub></span>O<span class="inline-formula"><sub>3</sub></span>, 2.3 % P<span class="inline-formula"><sub>2</sub></span>O<span class="inline-formula"><sub>5</sub></span> and 3 % F) pegmatitic melts in order to contribute to the understanding of Li enrichment in such systems. Two glasses were synthesized with a model pegmatitic composition, one of which is highly enriched in Li (<span class="inline-formula"><i>></i></span> 1 wt %, PEG2-blue) and the other one essentially Li-free (PEG2-Li-free). Diffusion couple experiments were performed to determine the chemical diffusivity of Li in dry pegmatitic melts. Experiments were conducted using rapid-heat and rapid-quench cold-seal pressure vessels in a temperature range of 650–940 <span class="inline-formula"><sup>∘</sup></span>C at 100 MPa with Ar as the pressure medium. We observed rapidly formed diffusion profiles, driven by an interdiffusive exchange of the monovalent alkalis Li and Na, while the other elements are immobile on the timescale of experiments (1–30 min). From these experiments, activation energies for Li–Na interdiffusion were determined as 99 <span class="inline-formula">±</span> 7 kJ mol<span class="inline-formula"><sup>−1</sup></span> with a pre-exponential factor of log <span class="inline-formula"><i>D</i><sub>0</sub></span> <span class="inline-formula">=</span> <span class="inline-formula">−</span>5.05 <span class="inline-formula">±</span> 0.33 (<span class="inline-formula"><i>D</i><sub>0</sub></span> in m<span class="inline-formula"><sup>2</sup></span> s<span class="inline-formula"><sup>−1</sup></span>). Li and Na partitioning between the stronger depolymerized PEG2-blue and the less depolymerized PEG2-Li-free leads to a concentration jump at the interface; i.e. Na is enriched in the more depolymerized PEG2-blue. Li–Na interdiffusion coefficients in the studied melt composition are in a similar range as Li and Na tracer diffusivities in other dry aluminosilicate melts, confirming little to no effect of aluminosilicate melt composition on Li diffusivity. Thus, added fluxes do not enhance the Li diffusivity in the same way as observed for H<span class="inline-formula"><sub>2</sub></span>O (Holycross et al., 2018; Spallanzani et al., 2022). Using melt viscosity as a proxy for the polymerization of the melt shows that water has a stronger potential to depolymerize a melt compared to other fluxing elements. Faster diffusion of <span class="inline-formula"><sup>6</sup></span>Li compared to <span class="inline-formula"><sup>7</sup></span>Li leads to a strong Li isotope fractionation along the diffusion profile, resulting in <span class="inline-formula"><i>δ</i><sup>7</sup></span>Li as low as <span class="inline-formula">−</span>80 ‰ relative to the diffusion-unaffected regions. This diffusive isotope fractionation can be quantified with an empirical isotope fractionation factor (<span class="inline-formula"><i>β</i></span>) of 0.20 <span class="inline-formula">±</span> 0.04, similar to previously observed <span class="inline-formula"><i>β</i></span> values for Li diffusion in melts. This suggests in accordance with previously published data that a <span class="inline-formula"><i>β</i></span> value of ca. 0.2 seems to be universally applicable to diffusive Li isotope fractionation in aluminosilicate melts.</p>https://ejm.copernicus.org/articles/35/1009/2023/ejm-35-1009-2023.pdf |
| spellingShingle | C. R. Singer H. Behrens I. Horn M. Oeser R. Dohmen S. Weyer Li–Na interdiffusion and diffusion-driven lithium isotope fractionation in pegmatitic melts European Journal of Mineralogy |
| title | Li–Na interdiffusion and diffusion-driven lithium isotope fractionation in pegmatitic melts |
| title_full | Li–Na interdiffusion and diffusion-driven lithium isotope fractionation in pegmatitic melts |
| title_fullStr | Li–Na interdiffusion and diffusion-driven lithium isotope fractionation in pegmatitic melts |
| title_full_unstemmed | Li–Na interdiffusion and diffusion-driven lithium isotope fractionation in pegmatitic melts |
| title_short | Li–Na interdiffusion and diffusion-driven lithium isotope fractionation in pegmatitic melts |
| title_sort | li na interdiffusion and diffusion driven lithium isotope fractionation in pegmatitic melts |
| url | https://ejm.copernicus.org/articles/35/1009/2023/ejm-35-1009-2023.pdf |
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