# Phlogopite-Forming Reactions as Indicators of Metasomatism in the Lithospheric Mantle
## 3 Phlogopite-Forming Reactions in the Upper-Mantle Rocks
### Experimental Studies of Phlogopite-Forming Reactions in Peridotites
None of the above experimental studies reported variations of compositions of phases coexisting with phlogopite. In order to do it, experiments on the systems enstatite-pyrope-grossular and enstatite-pyrope-knorringite in presence of H\({}_{2}\)O-KCl fluids with variable H\({}_{2}\)O/KCl ratio were performed at 5 GPa. These experiments were aimed at the tracing of variations of grossular and knorringite contents in garnet with variations of the KCl content in the fluid. Study of the reactions (2) and (3) in the system enstatite-pyrope-grossular at 5 GPa and 1000 \({}^{\circ}\)C shows that phlogopite actively forms at the KCl content 5-10 mol. % in the fluid. At these conditions, phlogopite coexists with both pyroxenes and garnet. Increase of the KCl content in the fluid results in gradual disappearance of garnet and orthopyroxene and stronger domination of phlogopite and clinopyroxene (Figure 3). Figure 3: BSE images of the run samples in the system enstatite–pyrope–grossular illustrating a gradual transformation of the garnet-two pyroxene assemblage (\(\chi_{\text{KCl}}\) = 0) to clinopyroxene–phlogopite assemblage (\(\chi_{\text{KCl}}\) = 0.4) with the increasing KCl content in the aqueous-salt fluid at 5 GPa and 1000 \({}^{\circ}\)C. The presence of silicate melt in the run products is not excluded. Therefore, at 40 mol. % of KCl in the fluid, phlogopite and clinopyroxene only are present in the run products. Phase compositions regularly vary with the increase of the salt content in the fluid (Figure 4a,b). The Al-content of orthopyroxene decreases in correspondence with the reaction (4) (Figure 4a). Grossular content in garnet increases with the increase of the KCl concentration in the fluid up to 10 mol. % demonstrating a domination of the reaction (2) over the reaction (3) (Figure 4b). However, further increase of the KCl concentration up to 20 mol. %, the grossular content in garnet decreases reflecting simultaneous operation of these reactions (Figure 4b). In the system enstatite-pyrope-knorringite at the KCl content in the fluid 0-10 mol. %, garnet contains 3.0-3.6 wt. % Cr2O3 (8-9 mol. % of knorringite). Cr-bearing phlogopite (about 2 wt. % Cr\({}_{2}\)O\({}_{3}\)) appears in this system at 10 mol. % KCl in the fluid, and its formation results in tendency for a slight increase of the knorringite content in garnet (Figure 4c). Cr-bearing spinel does not form in the system enstatite-pyrope-knorringite because of relatively high SiO\({}_{2}\) bulk content in comparison to the typical peridotite. Nevertheless, at 20 mol. % KCl in the fluid, kyanite with up to 7 wt. % Cr2O3 appears in the run products resulting in a decrease of the Cr2O3 content in garnet down to 1.7-1.8 wt. %, i.e., 3-5 mol. % of knorringite (Figure 4c). The Cr2O3 content in the coexisting phlogopite also decreases by about 1 wt. %. Thus, the above experiments allowed reproduction of some characteristic regularities in variations of garnet and pyroxene compositions in the course of phlogopite formation in peridotite. These effects  
Figure 4: Dependence of phase compositions in the systems enstatite–pyrope–grossular–H\({}_{2}\)O–KCl and enstatite–pyrope–knorringite–H\({}_{2}\)O–KCl on the salt concentration in a fluid at 5 GPa. (**a**) Al in orthopyroxene (atoms per formula unit, apfu); (**b**) grossular in garnet (mol. %); (**c**) knorringite in garnet (mol. %). can be applied for the quantitative and qualitative estimates of variations in K activity during the modal mantle metasomatism as illustrated by the \(\log(a_{\text{K}_{2}\text{O}})-\log(a_{\text{H}_{2}\text{O}})\) pseudosections (Figure 2a-c).