# Phlogopite-Forming Reactions as Indicators of Metasomatism in the Lithospheric Mantle
## 3 Phlogopite-Forming Reactions in the Upper-Mantle Rocks
### Reactions in Peridotites
In spinel-facies peridotite, phlogopite-forming reactions are also result in decrease of modal contents of orthopyroxene and spinel according to reaction:  
\[1/2\mathrm{Prp}+3/2\mathrm{En}+[1/2\mathrm{K_{2}O}+\mathrm{H_{2}O}]=\mathrm{Phl} \tag{2}\]2  
Figure 1c demonstrates the reaction texture corresponding to the reaction (6) in spinel harzburge from the Udachanya kimberlite pipe, Yakutia. Again, phlogopite in this reaction texture is accompanied by clinopyroxene and olivine, which were formed either because of Ca-carbonate component in the fluid (calcite is present in the reaction texture) or extraction of diopside component from the starting orthopyroxene initially containing up to 0.6 wt. % CaO. Figure 2c shows mineral assemblages forming during the metasomatic transformation of spinel lherzolite at 1.8 GPa and 950 \({}^{\circ}\)C exemplified by a xenolith from the Hamar-Daban range, southern Siberia (sample 520-9 from [52]). In this rock all metasomatic reactions proceed after orthopyroxene. At log(\(a_{\text{H\[1/2\mathrm{Prp}+3/2\mathrm{En}+[1/2\mathrm{K_{2}O}+\mathrm{H_{2}O}]=\mathrm{Phl} \tag{2}\]3O}}\)) \(>\) -1, phlogopite forms at the expense of orthopyroxene and Al-component of spinel via reaction (6) Although orthopyroxene remains stable with phlogopite, its Al-content significantly decreases with a progress of the phlogopite-forming reactions in correspondence with the reaction (4). [PERSON] [55] and [PERSON] et al. [56] specially paid attention that the clinopyroxene-phlogopite assemblage in peridotite xenoliths formed via break-up of primary pargasitic amphibole reflecting the following reaction:  
\[1/2\mathrm{Prp}+3/2\mathrm{En}+[1/2\mathrm{K_{2}O}+\mathrm{H_{2}O}]=\mathrm{Phl} \tag{2}\]4  
The reaction (7) implies that relations between phlogopite and Ca-Na-amphibole are regulated by the ratio of K and Na activities in fluids, which is probably determined by fractionation of alkalis during fluid migration through the mantle rock column (e.g., [57]). The pseudosections (Figure 2a,c) demonstrate that the coexistence of phlogopite and pargasite amphibole characterizes relatively high water activity, whereas increase of potassium activity results in replacement of Ca-Na amphibole with phlogopite. Both pseudosections predict that phlogopite forming from decomposition of amphibole would be enriched in Na, i.e., in aspidolite component, Asp--NaMg\({}_{3}\)AlSi\({}_{3}\)O\({}_{10}\)(OH)\({}_{2}\). Therefore, the reaction (7) can be re-written as:  
\[1/2\mathrm{Prp}+3/2\mathrm{En}+[1/2\mathrm{K_{2}O}+\mathrm{H_{2}O}]=\mathrm{Phl} \tag{2}\]5  
The Na\({}_{2}\)O content in phlogopite from spinel-facies metasomatized peridotites usually varies within 0.1-0.7 wt. % (e.g., [46; 47; 49; 57]), but in some cases reaches 1.5 wt. %. Various authors [55; 58] explain this compositional feature of phlogopite by metasomatic processes induced by Na-rich fluid. In this case, the K/Na ratio of phlogopite serves as an indicator of the K/Na ratio of the fluid. In addition, this ratio can be an indicator of initial Na content in the reacting fluid/melt, as well as temperature, since the phlogopite/melt (and, probably, fluid) Na partitioning coefficient decreases with cooling [10]. However, present calculations show that Na-component in phlogopite can be produced from transformation of hydrous amphibole-bearing peridotite at pressures below 2.5 GPa, i.e., below the probable baric stability limit of aspidolite [59]. Phlogopite-forming reactions in peridotite are commonly accompanied by compositional changes of primary minerals. Various authors [37; 41; 42; 43; 44; 45; 56; 49; 55] note re-crystallization of clinopyroxenes and olivine with formation of new generations of these minerals along with phlogopite. New clinopyroxene is usually depleted in Na\({}_{2}\)O and Al\({}_{2}\)O\({}_{3}\) implying a participation of jadeite and Ca-Tschernack components in the phlogopite-forming reactions. There are various tendencies in variation of garnet composition in a progress of phlogopite-forming reactions.