# Phlogopite-Forming Reactions as Indicators of Metasomatism in the Lithospheric Mantle
## 3 Phlogopite-Forming Reactions in the Upper-Mantle Rocks  
### Reactions in Peridotites  
Phlogopite extensively forms in mantle periodtites transforming initial harzburgites and lherzolites to phlogopite wehrlites both in garnet and spinel-facies [37; 38; 39; 40; 41; 42; 43; 44; 45; 46; 47]. Xenoliths of metasomatized peridotite show various specific textures indicating major reactions forming phlogopite via interaction of the rocks with potassic fluids and melts (Figure 1a-c). [PERSON] and [PERSON] [49] and [PERSON] [50] firstly proposed the reaction:  
\[5\mathrm{En}+\mathrm{Grt}+[\mathrm{K_{2}O}+2\mathrm{H_{2}O}]=\mathrm{Phl}+\mathrm{ Di} \tag{1}\]  
(all mineral abbreviations in the text are from [50]), where Grt--pyrope-grossular garnet CaMg\({}_{2}\)A1\({}_{2}\)Si\({}_{3}\)O\({}_{12}\), as the major mechanism for phlogopite formation in garnet-facies peridotites. The reaction is a combination of two end-member reactions:  
\[1/2\mathrm{Prp}+3/2\mathrm{En}+[1/2\mathrm{K_{2}O}+\mathrm{H_{2}O}]=\mathrm{Phl} \tag{2}\]  
\[1/2\mathrm{Grs}+9/2\mathrm{En}+[1/2\mathrm{K_{2}O}+\mathrm{H_{2}O}]=\mathrm{Phl }+3/2\mathrm{Di} \tag{3}\]  
which demonstrate a leading role of orthopyroxene in the phlogopite formation. Presence of Al\({}_{2}\)O\({}_{3}\) as Mg-Tschernack molecule in orthopyroxene suggests the additional reaction:  
\[5\mathrm{En}+\mathrm{Mg}\mathrm{-Ts}+[\mathrm{K_{2}O}+2\mathrm{H_{2}O}]=2 \mathrm{Phl} \tag{4}\]  
Figure 1: Phlogopite-bearing reaction textures in the metasomatized peridotite xenoliths in kimberlies of the Udachnaya pipe, Yakutia. (**a**) Wide phlogopite corona containing relics of orthopyroxene around garnet from the garnet peridotite; later kelibite zone is developed between phlogopite and garnet relic. (**b**) Reaction zone composed of phlogopite, clinopyroxene, olivine and calcite at the contacts of orthopyroxene and Cr-spinel from the spinel harzburtige. (**c**) The reaction zone of “spongy” clinopyroxene, phlogopite and olivine developed on orthopyroxene in garnet Iberzolite (photo courtesy of [PERSON], Institute of the Earth’s Crust, Siberian Branch of RAS, Irkutsk). All mineral abbreviations are from [48]. In mantle periodtites, reactions (2) and (3) form phlogopite-bearing reaction textures between orthopyroxene and garnet (Figure 1a), which commonly accompanied by clinopyroxene and Cr-spinel. [PERSON] [49] concluded that phlogopite-rich xenoliths from kimberlites were formed from initial garnet harzburgites or orthopyroxene-rich lherzolites with increasing degree of metasomatism. For example, in xenoliths from the Lethlakane pipe, Botswana [46], this process is expressed in gradual replacement of garnet in harzburgites with the assemblage clinopyroxene + phlogopite \(\pm\) Cr-spinel and decrease of the orthopyroxene mode in accordance with the reactions (1)-(3). The pseudosection \(\log(a_{\mathrm{H_{2}O}})-\log(a_{\mathrm{K_{2}O}})\) in Figure 2a represents stages of metasomatic transformation exemplified by a representative composition of a garnet lherzolite corresponding to the primitive mantle composition [51] at 2 GPa and 900 \({}^{\circ}\)C (i.e., parameters near the continental geotherm). Figure 2: The \(\log(a_{\mathrm{H_{2}O}})-\log(a_{\mathrm{K_{2}O}})\) pseudosections for various periodtites illustrating reactions of phlogopite formation and regularities of compositional variation of coexisting minerals in these reactions. **(a)** The pseudosection for the garnet lherzolite corresponding to the primitive mantle composition [51] at 2 GPa and 900 \({}^{\circ}\)C; white isopleths—100 Cr/(A1 + Cr) in garnet, blue isopleths—100 Ca/(Ca + Mg + Fe + Mn) in garnet. 1—phase field Amp + Crpx + OI + Spl + Phl, where a presence of Na-phlogopite component in mica is predicted, 2—phase field Cpx + OI + Spl + Phl, where a presence of Na-phlogopite component in mica is predicted.