# Phlogopite-Forming Reactions as Indicators of Metasomatism in the Lithospheric Mantle
## 3 Phlogopite-Forming Reactions in the Upper-Mantle Rocks
### Relation of Phlogopite with Other Potassic Phases in the Metasomatized Upper-Mantle Rocks
However, in many cases, these reaction textures are not accompanied by phlogopite (or amphibole). Moreover, there are textural evidences for replacement of phlogopite by feldspar-bearing aggregates [91]. These textures features suggest an extremely low H\({}_{2}\)O activity in a fluid, which interacted with peridotite at temperatures 950-1000 \({}^{\circ}\)C and pressures below 2 GPa. The pseudosection \(\log(a_{\text{H}_{2}\text{O}})-\log(a_{\text{K}_{2}\text{O}})\) for the spinel lherzolite from the Hamar-Daban Range, southern Siberia (sample 520-9 in [52]), demonstratesthat log\(n_{\mathrm{H_{2}O}}\) \(<\) -1.5 is needed to produce feldspar-bearing assemblage in this peridotite (Figure 2c). This result is roughly consistent with the experimental data on the peridotite-H\({}_{2}\)O-CO\({}_{2}\) systems at pressures below 1.5 GPa, showing a possibility of subsolidus phlogopite decomposition to form K-feldspar at X\({}_{\mathrm{CO2}}\) above 0.8 [92]. The pseudosection for Iberzolite (Figure 2c) demonstrates that K-feldspar generally forms at the K\({}_{2}\)O-activities higher than it is needed for phlogopite formation. Thus, appearance of K-feldspar instead of phlogopite in perodotite assemblages manifests an unusual condition of extremely low water activity, which can be attributed, probably, to water-poor alkaline carbonatite or carbonate-salt liquids interacting with mantle rocks. In strongly metasomatized peridotite xenoliths from kimberlites, phlogopite commonly coexists with various rare K-Ba-bearing phases, such as hawthorneite-yimengite, HAWYI (K, Ba)(Cr, Ti, Mg, Fe, Al)\({}_{12}\)O\({}_{19}\), lindsleyite-mathiasite, LIMA (K, Ba)(Cr, Ti, Mg, Fe, Al)\({}_{21}\)O\({}_{38}\), priderite, (K, Ba)\({}_{1-2}\)(Cr, Ti, Mg, Fe, Al)\({}_{6-7}\)O\({}_{16}\) and other more rare phases [93]. Experimental data on synthesis of K-Ba-titanates demonstrate that there are virtually no restrictions on their stability in the upper-mantle [94; 95] confirming the possibility of coexistence of these phases with diamond in the subcontinent upper mantle in the regions of generation of kimberlites and lamproites. It is evident that specific chemical conditions are responsible for the stability of K-Ba-titanates along with phlogopite. Phlogopite is not only active container of K and other LILE, but also is able to accommodate Cr and Ti. Thus, phlogopite acts as contender for K-Ba-titanates. Thus, these minerals form when the ability to concentrate K and LILE in phlogopite (and K-richterite) is exhausted manifesting the highest degree of metasomatic changes in the conditions of high activity of alkaline components in fluids (melts) with low SiO\({}_{2}\) activity [96; 97]. Figure 5 shows coexisting phlogopite and K-titanates (mathiasite and priderite) in the products of experiments on interaction of chromite + ilmenite (rutile) with H\({}_{2}\)O-CO\({}_{2}\)-K\({}_{2}\)CO\({}_{3}\) fluids at 5 GPa and 1200 \({}^{\circ}\)C [98]. Phlogopite was formed in these experiments due to a presence of silicate phases included or intergrown with starting natural ilmenite and chromite. It contains 0.6-1.9 wt. % TiO\({}_{2}\) and 1.8-2.7 wt. % Cr\({}_{2}\)O\({}_{3}\), reflecting formation via the reactions involving both chromite and Ti-phases. The highest concentrations of Ti and Cr are detected in phlogopites forming in the reactions with fluids with high K\({}_{2}\)CO\({}_{3}\)/(H\({}_{2}\)O + CO\({}_{2}\)) ratios suggesting more active involvement of chromite and ilmenite with the potassium-rich fluids forming both Cr-bearing K-titanates and phlogopite. However, phlogopite is present even in run products, which do not contain titanates. These experiments are characterized by the lowest K\({}_{2}\)CO\({}_{3}\)/(H\({}_{2}\)O + CO\({}_{2}\)) ratios in the starting fluid mixture.