The effect of halogens (F, Cl) on the near-liquidus crystallinity of a hydrous trachyte melt

Abstract

The effect of F and Cl on the liquidus temperature of a hydrous (~3.5–4 wt% H2O) trachytic melt (~66 wt% SiO2) at 925 to 990 °C and at 100 MPa has been experimentally investigated. We employed a novel disequilibrium approach involving diffusion couple experiments with the two-diffusion couple end-members differing solely in halogen concentrations. A shift of the liquidus temperature by ~50 °C was observed between a halogen-poor and halogen-enriched melt. Each experiment spanned the entire range of F and Cl concentrations between the two end-member compositions. We determined the halogen concentrations at the transition from crystal-bearing to crystal-free melt. These concentrations correspond to the liquidus halogen concentrations of the melt at each experimental temperature. We demonstrate that there is a limiting halogen concentration (~0.19–0.52 wt% F; ~0.07–0.24 wt% Cl), below which the melt crystallizes spherulitic clinopyroxene during heating to the run temperature. At high temperatures, upon diffusion of F and Cl into the halogen-poor melt, those crystals dissolve, leaving behind a dissolution front parallel to the diffusion interface. We propose that the dissolution is a consequence of F and Cl complexing with some of the main cationic components of clinopyroxene (Mg, Fe, Ca), thereby destabilizing this phase. Thus, the experimental dissolution of clinopyroxene is a manifestation of a liquidus depression caused by increased halogen content. Our results show that the liquidus shifts at a rate of ~1575(379) K/mol% of F and Cl in the melt, which is a minimum estimate, assuming both halogens equally drive dissolution. This liquidus depression is valid for a range of halogen concentrations (~0.06–0.87 wt% F; ~0.06–0.36 wt% Cl) and the experimental temperatures. Our findings illustrate that the degassing of halogens during or prior to an eruption can enhance crystallization in the melt and therefore influence magma physical properties that may ultimately affect eruption style.

Diffusion of halogens (F, Cl, Br, I) in silicic melt

“…Synthesis and experimental methods utilizing the Hekla material ("HX" series -"Hekla experiments") are described in detail in the study of Feisel et al (2019). Synthesis of starting glass derived from Cordón Caulle obsidian lava ("CCX" series -"Cordón Caulle experiments") is described in detail in Feisel et al (2022). The general approach used for both melts is briefly described below.…”

“…Hydrous experiments were carried out using CCX starting material only and are denoted by "CCX-H". The capsule preparation followed the procedure described in Feisel et al (2022) using powdered starting glass and Pt-tube with a diameter of 4 mm and a wall thickness of 0.2 mm as capsule material. After welding the capsule shut on the bottom and tamping into a cylindrical shape using a tight-fitting rod, ground glass powder of the halogen-enriched samples was introduced until the capsule was filled about halfway.…”

“…For example, F is known to reduce the viscosity of silicic melts to a similar degree as added H2O Dingwell and Hess, 1998;Zimova and Webb, 2007;Giordano et al, 2008;Baasner et al, 2013), and can depress the isobaric liquidus temperature in basalt (e.g., Filiberto et al, 2012) and felsic melts (e.g., Dolejš and Baker, 2007b;Dolejš and Baker, 2007a). Even though more complex, substantial effects on the liquidus temperature have also been reported for Cl-bearing natural melts of both basaltic (e.g., Filiberto et al, 2014;Farcy et al, 2016) and silicic systems (Feisel et al, 2022). The physico-chemical effects of added Br and I to a melt are, by contrast, far less understood.…”

Heavy halogen compositions of lamprophyres derived from metasomatized lithospheric mantle beneath eastern North China Craton

“…The serpentinized lithospheric mantle is considered to be the main carrier of the deep halogen cycle (e.g., Kendrick et al, 2017;Kendrick et al, 2013;Pagé and Hattori, 2019). The subducted halogens play an important role in controlling the diffusion, viscosity, density, and phase equilibrium of silicate melts (e.g., Baasner et al, 2013;Feisel et al, 2022;Filiberto and Treiman, 2009a;Filiberto and Treiman, 2009b;Thomas and Wood, 2021;Zimova and Webb, 2006). On a mole by mole basis, chlorine has twice the effect of water on lowering the liquidus temperature of basalt (Filiberto and Treiman, 2009a).…”