Fluorine partitioning between humite-group minerals and aqueous fluids: implications for volatile storage in the upper mantle

Lewis Hughes, Alison Pawley

Research output: Contribution to journalArticlepeer-review

Abstract

Phase equilibrium experiments were performed in the MgO-SiO2-H2O system, along with fluorine (MSH+F), at conditions between 800-1000°C and 2.0-2.5 GPa to constrain the solubility of fluorine in humite-group minerals and to determine fluorine partitioning between humite-group minerals and aqueous fluid. Fluorine solubility in humite-group minerals ranges between ~1 and 11 wt% F and is dependent on the salinity of the fluid (~0.2-3.5 wt% F), indicating that humite-group minerals have exceedingly high saturation limits for F, and that a full solid solution between F- and OH- is possible within the crystal structure. Raman spectroscopy reveals the preferential ordering of F, promoting the formation of a stable OH-F bond. Mineral-fluid partition coefficients are always greater than unity, with average coefficients of DFclinohumite/fluid = 3, DFhumite/fluid = 2, DFchondrodite/fluid = 4 and DFnorbergite/fluid = 4. Partition coefficients are independent of pressure or temperature but decrease with increasing fluid salinity. Fluorine is, therefore, highly soluble and compatible within this group of ultramafic mantle minerals. High solubility and mineral-fluid partition coefficients, together with wide stability fields in pressure and temperature space demonstrate that humite-group minerals are potential storage sites for F, and by extension H2O, during subduction. Upon the breakdown of less stable hydrous and/or fluorine-rich phases during lower grades of subduction zone metamorphism, fluorine may redistribute into phases such as humite-group minerals and be transported beyond the volcanic arc and to the upper mantle.
Original languageEnglish
JournalContributions to Mineralogy and Petrology
DOIs
Publication statusPublished - 29 Aug 2019

Keywords

  • Fluorine
  • Humite-group minerals
  • Partitioning
  • Subduction
  • Volatiles
  • Aqueous fluid

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