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.
Highlights
Owing to increasing ionic radii from F to I, halogen elements display a range from mildly incompatible (F) to highly incompatible (I) behaviour within silicate minerals
Experimental runs produced phase assemblages consisting of mixtures of HGM or HGM plus forsterite or enstatite, along with an aqueous fluid
Our experiments demonstrate that partition coefficients for fluorine between HGM and aqueous fluid are high (Table 3) and that fluorine solubility in all HGM is at the several wt %
Summary
Owing to increasing ionic radii from F to I, halogen elements display a range from mildly incompatible (F) to highly incompatible (I) behaviour within silicate minerals. This incompatible nature, coupled with their volatile behaviour, implies that halogen elements will preferentially enter silicate melts and aqueous fluids during geological processes such as melting, degassing, and dehydration reactions. The incompatible nature of halogen elements suggests that they should not be fractionated from one another during these processes, and they make for ideal tracers of volatile transport between major geological reservoirs (Schilling et al 1980; Ito et al 1983; Jambon et al 1995)
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