Impact of fluorine on the thermal stability of phlogopite

Abstract

Knowledge of volatile cycling is vital to understand evolution of the planet and the life it supports. Although it is gradually accepted that the mantle is a vast store house of H 2 O and other volatiles, the impact of co-existing volatiles on the thermal stabilities of OH and the lattice of the host mineral is still poorly understood.Phlogopite is one of the few hydrous minerals capable of carrying both water and halogens into the mantle. Previous observations from both experiments and textural relationships in natural samples have indicated that F-rich phlogopite can be stable under ultra-high-temperature conditions. Here, the impact of F on the thermal stability of phlogopite was investigated via XRD, Raman, and IR spectroscopy from room temperature to 1000 to 1200 °C. Based on the experimental results from a F-poor and a F-rich natural phlogopites, we show that about 4 wt% F can increase the breakdown temperature of phlogopite by 100 °C under ambient pressure. The impact mechanism mainly involves preventing OH and lattice softening at high temperatures. This study reveals the links between F and the behavior of OH and phlogopite lattice, which is important for constraining volatile cycling as well as F's role in the physical and chemical properties of the upper mantle.

Thermal Stability of F‐Rich Phlogopite and K‐Richterite During Partial Melting of Metasomatized Mantle Peridotite With Implications for Deep Earth Volatile Cycles

“…Assuming that the effects of F on thermal stability are approximately linear, consistent with in situ observations at ambient pressures reported by Sun et al. (2022), the thermal stability could be increased by as much as ∼222 and ∼445°C for phlogopite containing 4 and 8 wt.%, respectively. Note that an approximately linear dependency of phlogopite thermal stability with F content is also consistent with the observations for simplistic natural systems obtained at 0.7–2.8 GPa (Dooley & Patiño Douce, 1996; Foley et al., 1986a; Hensen & Osanai, 1994).…”

“…Previous studies have shown that phlogopite from peridotitic xenoliths can contain up to 8 wt.% F (Liu et al, 2011), with examples from Sicily, Italy (Gianfagna et al, 2007) and Namalulu, Tanzania (Feneyrol et al, 2012) (see also Figure S1 in Supporting Information S1). Assuming that the effects of F on thermal stability are approximately linear, consistent with in situ observations at ambient pressures reported by Sun et al (2022), the thermal stability could be increased by as much as ∼222 and ∼445°C for phlogopite containing 4 and 8 wt.%, respectively. Note that an approximately linear dependency of phlogopite thermal stability with F content is also consistent with the observations for simplistic natural systems obtained at 0.7-2.8 GPa (Dooley & Patiño Douce, 1996;Foley et al, 1986a;Hensen & Osanai, 1994).…”

“… Thermal stabilities of phlogopite as a function of X F , defined as F/(F/OH), at different pressures, based on data from this study and previous work. Included for comparison purposes are 1 bar data (Hammouda et al., 1995; Prost & Laperche, 1990; Sun et al., 2022) and high‐pressure data (0.7–1.5 GPa, Dooley & Patiño Douce, 1996; 0.9 GPa, Hensen & Osanai, 1994). Dashed lines represent fits to thermal stabilities that were derived for a single study at a constant pressure or at 2 and 5 GPa (this study, for comparative purposes).…”

“…Figures 7 and 8 show the modeled stabilities of K-richterite and phlogopite in a F-absent and F-rich system, with variable F contents, along different geothermal gradients (continental vs. oceanic). (Hammouda et al, 1995;Prost & Laperche, 1990;Sun et al, 2022) and high-pressure data (0.7-1.5 GPa, Dooley & Patiño Douce, 1996; 0.9 GPa, Hensen & Osanai, 1994). Dashed lines represent fits to thermal stabilities that were derived for a single study at a constant pressure or at 2 and 5 GPa (this study, for comparative purposes).…”

“…They proposed that O‐H bonds and lattice modes are stiffer for F‐rich phlogopite compared to F‐poor phlogopite, resulting in greater stability of F‐rich phlogopite at high temperatures. Previous data were thus mostly obtained in ''simple'' chemical systems, of which it is unclear if it can be applied to natural systems at all, or experiments were run in non‐equilibrium, ambient pressure conditions (Sun et al., 2022). Here, we assess the effects of F on the thermal stability of phlogopite and K‐richterite in complex compositions at P‐T conditions directly relevant for partial melting of metasomatized mantle peridotite.…”