# 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  
Reaction textures indicating instability of phlogopite with respect to other K-bearing phases are rarely reported in both peridotite and eclogite xenoliths. However, in the metasomatized xenoliths, phlogopite coexists or is associated with a number of other potassic phases, which can be competing to phlogopite as indicators of potassium and water activities during mantle metasomatism. K-richterite, (K, Na)\({}^{\text{A}}\)Na\({}^{\text{M}}\)Ca\({}^{\text{M}}\)Mg\({}_{5}\)Si\({}_{8}\)O\({}_{22}\)(OH)\({}_{2}\), is a typical amphibole coexisting with phlogopite in xenoliths of the MARID (mica-amphibole-rutile-ilmenite-diopside) suite [67], strongly metasomatized phlogopite-K-richterite peridotite, so called PKP [68], and garnet-free phlogopite-diopside veins in peridotite xenoliths [39; 40]. According to experimental studies [19; 69; 70; 71; 72], the assemblage garnet + K-richterite is unstable at pressures below 6-7 GPa with respect to the assemblage clinopyroxene + phlogopite \(\pm\) orthopyroxene because of reaction:  
\[\text{K-Rich}+\text{Prp}=\text{Phl}+\text{Di}+\text{Jd}+2\text{En} \tag{12}\]  
suggesting that the K-richterite forms at depths greater than \(\sim\)200 km. However, the leading factor that stabilizes K-richterite is a potassium content (activity) in the mantle systems. K-richterite along with phlogopite easily crystallizes in the peralkaline, i.e., with molar (Na\({}_{2}\)O+K\({}_{2}\)O)/Al\({}_{2}\)O\({}_{3}\)\(>\) 1, systems [19; 71; 72]. The MARID suite is the best example of peralkaline mantle assemblages. The peralkaline assemblages related to metasomatism are PKP, i.e., phlogopite-K-richterite peridotite [39; 46], which are products of a progressive replacement of garnet by phlogopite and eventually by K-richterite so that garnet periodtites are succeeded by garnet-phlogopite peridotite (GPP), garnet-free phlogopite peridotite (PP) and, ultimately, by PKP. These observations indicate that K-richterite is a product of higher degree of metasomatism corresponding to the higher K (and Na) activities than it is needed for the phlogopite. At these conditions, Al is predominantly accommodated by phlogopite, and K starts to be bound to Ca and Si in amphibole. However, there is no reports for phlogopite replacement by K-richterite in metasomatized peridotites. Both minerals form jointly at advanced stages of metasomatism at the expense of pyroxenes, Cr-spinel and other minerals, which survived initial stages of metasomatism. Another competing phase for phlogopite in mantle assemblages can be sanidine, KAlSi\({}_{3}\)O\({}_{8}\), since its stability extends to pressures 5.5-7 GPa at 1000-1600 \({}^{\circ}\)C [73; 74]. There are numerous reports on sanidine inclusions in diamonds, which belong to eclogite paragenesis [75; 76; 77; 78; 79; 80; 81]. The close association of sanidine with eclogite paragenes is supported by findings of this phase in eclogite xenoliths including coesite and kyanite varieties and grosspidites [82; 83; 84; 85], as well as in products of metasomatic modifications of eclogites [86; 87; 88]. Findings of sanidine in peridotite assemblages are unique. \"Mixed assemblage\" of sanidine with Fe-enriched olivine is reported only in diamond from the Shengli-1 kimberlite pipe, China [89]. Both in H\({}_{2}\)O-free conditions and in presence of H\({}_{2}\)O or H\({}_{2}\)O-CO\({}_{2}\) fluids, sanidine is not stable with Mg-rich olivine at pressures above 1.5-2.5 GPa but can coexist with phlogopite at pressures below 2.5 GPa [90] compatible to off-cratonic spinel-facies peridotite only. As an example, feldspar with 50-60 mol. % of sanidine was described in spinel peridotite xenoliths from number of alkali basalt occurrences (see references in [91]). In all rocks, feldspar forms fine-grained aggregates with olivine, Ti-oxides and Cr-spinel between corroded primary spinel and orthopyroxene suggesting a reaction, which is similar to phlogopite-forming reaction (6).