Lithium-Isotopenfraktionierung in magmatischen Systemen: Erkenntnisse aus In Situ-Analysen magmatischer Minerale mittels Femtosekunden-Laserablation-MC-ICP-MS
Zusammenfassung der Projektergebnisse
In this project, an analytical method was successfully developed to perform in situ Li concentration and isotope analyses of silicate reference glasses and magmatic crystals (olivine) by femtosecond-laser ablation-MC-ICP-MS. The method was applied to natural olivine crystals from various volcanic settings in order to investigate the diffusion of Li in these minerals and the fractionation of Li isotopes during magma evolution. The analytical technique was tested by analyzing a series of reference glasses in situ and cross calibrating them with published values acquired by other methods (solution nebulization MC- ICP-MS, SIMS and TIMS). The results of this methodical investigation have been published in Journal of Analytical Atomic Spectrometry (Steinmann et al., 2019) and show that precise and accurate in situ measurements of δ7Li with ~2‰ (2 σ) analytical uncertainty can be performed by fs-LA-MC-ICP-MS. The developed technique was then applied to chemically zoned olivines from a continental intra-plate setting (Massif Central, France). Intra-mineral variations in the Mg-Fe and Li isotopic compositions revealed a diffusive origin of the chemical zoning, and Li gives insight into a second diffusive event, which remains hidden in Mg-Fe. Profiles of the first diffusive event were modeled with fixed time scales, based on Mg-Fe inter-diffusion, acquired by Oeser et al. (2015), for the residence time in a magma reservoir. A second, relatively short-lived, diffusive re-equilibration episode was assumed to be caused by the degassing of the magma and concomitant Li decrease in the melt with no effect on Mg-Fe distribution. This multi-stage magma evolution history could be reconstructed by combining the information from the observed Li chemical and isotopic zoning with the results of a detailed diffusion modeling. A systems analysis for two volcanic plumbing systems of distinct geotectonic setting was performed in order to retrace the passage of olivine crystals through various melt reservoirs and investigate the behavior and diffusivity of Li in these crystals. The melt reservoirs of the plumbing systems, as inferred from intra-mineral forsterite variations, appear to be mirrored by Li contents as well. Diffusion of Li in the investigated crystals was modeled to be 1.4–2.5 times faster than that of the Mg-Fe diffusion couple, which is significantly less than the diffusivity determined in experimental studies for olivine. The analytical method developed in this project was also applied to plagioclase crystals in the Mesa Falls Tuff of the Yellowstone volcanic system in order to expose significant Li isotopic zoning coupled to Li chemical zoning in these crystals, which have consequently been used to model decompression time scales for these silicic magmas.
Projektbezogene Publikationen (Auswahl)
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(2020): Lithium isotope fractionation in magmatic systems: Constraints from in situ δ7Li determinations on magmatic minerals by femtosecond-laser ablation-MC-ICP-MS. Dissertation, TIB/UB Hannover, 130 Seiten
Steinmann L.
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(2019): In situ high-precision lithium isotope analyses at low concentration levels with femtosecond-LA-MC-ICP-MS. Journal of Analytical Atomic Spectrometry, v. 34, 1447-1558
Steinmann L.K., Oeser M., Horn I., Seitz H.-M., Weyer S.
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(2020): Multi-Stage Magma Evolution in Intra-Plate Volcanoes: Insights From Combined in situ Li and Mg–Fe Chemical and Isotopic Diffusion Profiles in Olivine. Frontiers in Earth Science, v. 8, 201
Steinmann L.K., Oeser M., Horn I., Weyer S.
