The Formation of K-Cymrite in Subduction Zones and its Potential in the Transport of Potassium, Water and Nitrogen into the Mantle
- Authors: Sokol A.G.1, Korsakov A.V.1, Kruk A.N.1
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Affiliations:
- V.S. Sobolev Institute of Geology and Mineralogy of the Siberian Branch of the Russian Academy of Sciences
- Issue: Vol 69, No 12 (2024)
- Pages: 1139-1150
- Section: Articles
- URL: https://journals.rcsi.science/0016-7525/article/view/277663
- DOI: https://doi.org/10.31857/S0016752524120023
- EDN: https://elibrary.ru/IDSRPI
- ID: 277663
Cite item
Abstract
The conditions for the formation of K-cymrite in volatile-rich pelite and partially devolatilized quartz–muscovite–chlorite schist were experimentally investigated at pressures of 5.5, 6.3, and 7.8 GPa and temperatures ranging from 900 to 1090°C. The experimental samples at these P-T conditions formed an eclogite-like assemblage of solid phases (Grt + Coe + Phe + Cpx + Ky, with accessory Po + Ru + Zrn ± Mnz) and water-enriched supercritical fluid-melt. Analysis of the obtained data indicates that the stability of phengite and its potential replacement by K-cymrite in the eclogite-like residue depends on the P-T conditions and the amount of volatiles in the metasediment. In samples of volatile-rich pelite and schist at 5.5 GPa and 900°C, as well as at 6.3 GPa and 1000°C, phengite remains stable in equilibrium with the fluid-melt (3–13 wt%). For the first time, phase assemblage with phengite and K-cymrite (± kokchetavite) was identified using Raman mapping in samples of pelite and schist obtained at 7.8 GPa and 1070°C. It was concluded that the most effective transport of volatiles (primarily water) in the metasediment to depths exceeding 240 km may occur during its partial and early (before the formation of supercritical fluid-melt) devolatilization. In this case, almost all phengite may reach depths of 240 km during subduction of the metasediment and then transform into water-bearing K-cymrite. Furthermore, in the presence of nitrogen in the metasediment, nitrogen-bearing K-cymrite can facilitate the further transport of LILE (large-ion lithophile elements), water, and nitrogen. However, the formation of a significant portion of supercritical fluid-melt leads to the complete dissolution of phengite with increasing P-T parameters, making further transport of LILE, water, and nitrogen impossible. During deep multi-stage devolatilization, phengite remains stable up to depths of 240 km; however, during further subduction, it likely transforms into an anhydrous mineral such as K-hollandite.
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About the authors
A. G. Sokol
V.S. Sobolev Institute of Geology and Mineralogy of the Siberian Branch of the Russian Academy of Sciences
Author for correspondence.
Email: sokola@igm.nsc.ru
Russian Federation, Novosibirsk
A. V. Korsakov
V.S. Sobolev Institute of Geology and Mineralogy of the Siberian Branch of the Russian Academy of Sciences
Email: korsakov@igm.nsc.ru
Russian Federation, Novosibirsk
A. N. Kruk
V.S. Sobolev Institute of Geology and Mineralogy of the Siberian Branch of the Russian Academy of Sciences
Email: krukan@igm.nsc.ru
Russian Federation, Novosibirsk
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