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

Feisel, Yves; Castro, Jonathan M.; Helo, Christoph und Dingwell, Donald B. (2022): The effect of halogens (F, Cl) on the near-liquidus crystallinity of a hydrous trachyte melt. In: American Mineralogist, Bd. 107, Nr. 6: S. 1007-1017

Abstract

The effect of F and Cl on the liquidus temperature of a hydrous (similar to 3.5-4 wt% H2O) trachytic melt (similar to 66 wt% SiO2) at 925 to 990 degrees 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 similar to 50 degrees 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 (similar to 0.19-0.52 wt% F;similar to 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 similar to 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 (similar to 0.06-0.87 wt% F;similar to 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.

• Dokumententyp: Zeitschriftenartikel
• Fakultät: Geowissenschaften > Department für Geo- und Umweltwissenschaften
• Themengebiete: 500 Naturwissenschaften und Mathematik > 550 Geowissenschaften, Geologie
• ISSN: 0003-004X
• Sprache: Englisch