# 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  
The above regularities in the phlogopite-forming reactions are supported by experiments on interaction of peridotite assemblages with potassic fluids of different anionic composition. [PERSON] and [PERSON] [60] studied interaction of a model peridotite in the Na\({}_{2}\)O-bearing CMAS system at 1.9 \(\Gamma\Pi\)a and 900-1200 \({}^{\circ}\)C with H\({}_{2}\)O-KCl fluids with variable H\({}_{2}\)O/KCl ratio. They revealed that formation of phlogopite, clinopyroxene and, probably, olivine actively proceeded after decomposition of aluminous orthopyroxene, garnet, spinel and pargasite amphibole, encompassing all spectrum of the above reactions. Authors mention that instability of orthopyroxene with Al-rich phases in presence of potassic fluids is a major factor of phlogopite formation within a wide range of fluid composition. [PERSON] and [PERSON] [61] and [PERSON] and [PERSON] [62] noted that phlogopite growth was accompanied by regular decrease of the orthopyroxene/clinopyroxene and orthopyroxene/olivine ratios, rapid disappearance of pargasite and spinel at 2 GPa and garnet at 3 GPa during interaction of a model peridotite with the H\({}_{2}\)O-K\({}_{2}\)CO\({}_{3}\) fluids containing up to 80 wt. % carbonate. Phlogopite is found to be the only K-bearing crystalline phase in these experiments. However, the situation is more complex at higher pressures. [PERSON] and [PERSON] [63] revealed that phlogopite was unstable in presence of hydrous carbonate-silicate liquid simulating Group II kimberlites at \(>\)4 GPa and \(>\)1150 \({}^{\circ}\)C and suggested reactions:  
\[2\text{Phl}+\text{CO}_{2}=3\text{En}+\text{Prp}+(\text{K}_{2}\text{CO}_{3}+2 \text{H}_{2}\text{O}) \tag{9}\]  
\[2\text{Phl}+\text{Ms}=\text{Fo}+2\text{En}+\text{Prp}+(\text{K}_{2}\text{CO}_{3} +2\text{H}_{2}\text{O}) \tag{10}\]  
as limiting phlogopite-out mechanisms in CO\({}_{2}\) and carbonate-bearing systems. [PERSON] et al. [52; 53] added a reaction  
\[2\text{Phl}+\text{Cal}=\text{Fo}+\text{Di}+\text{Prp}+(\text{K}_{2}\text{CO}_{3} +2\text{H}_{2}\text{O}) \tag{11}\]as a mechanism of disappearance of phlogopite at 5.5-7.0 GPa at 1200-1350 \({}^{\circ}\)C. They showed that at constant H\({}_{2}\)O/(CO\({}_{2}\) + H\({}_{2}\)O) ratio 0.35-0.40, phlogopite formed in the system with a bulk K\({}_{2}\)O content above 11 wt. %, whereas K-carbonates were stable at K\({}_{2}\)O 3-4 wt. %. Along with other data [63; 64; 65; 66], these results indicate that stabilization of phlogopite with respect to K-carbonate in systems with high \(\chi_{\text{CO}_{2}}\) requires higher K\({}_{2}\)O. This conclusion can be generalized: The lower H\({}_{2}\)O activity in a fluid equilibrated with peridotite, the higher K\({}_{2}\)O activities are necessary for the phlogopite formation. This conclusion is visualized in \(\log(a_{\text{H}_{2}}\text{O})-\log(a_{\text{K}_{2}}\text{O})\) pseudosections for peridotite, where the phlogopite-in reaction lines show a negative slope (Figure 2a-c). Pseudosection modeling shows that olivine mode remains constant or just slightly decreases (by 3-4 vol. %) in the course of the phlogopite-forming reactions. Thus, olivine is not actively consumed during the formation of phlogopite in the peridotite assemblages. Phlogopite-bearing reaction textures in mantle xenoliths indicate a passive role of primary olivine and formation of new generations of olivine via reaction like (5) and (6). Passive role of olivine in the phlogopite-forming reactions is proved by the experiments [60; 61; 62]. Nevertheless, olivine can be a reactant phase in the phlogopite-forming reaction during interaction of peridotite with alkalic silicate melts. It has been recently demonstrated in the experiments on interaction of dunite with alkalic sediment-derived melts and hydrous basanitic melts, modeling processes a fore-arc subduction and postcollisional rifting environments, respectively [10].