# Phlogopite-Forming Reactions as Indicators of Metasomatism in the Lithospheric Mantle
## 3 Phlogopite-Forming Reactions in the Upper-Mantle Rocks
### Reactions in Peridotites
**(b)** The pseudosection for the representative harzburgite JSL266 from the Lethlakane kimberlite pipe [46] for 4.5 GPa and 1000 \({}^{\circ}\)C; white isopleths—100 Cr/(A1 + Cr) in garnet, blue isopleths—100 Ca/(Ca + Mg + Fe + Mn) in garnet, magenta isopleths—Cr\({}_{2}\)O\({}_{3}\)/Al\({}_{2}\)O\({}_{3}\) ratio in spinel. **(c)** The pseudosection for the spinel lherzolite from the Hamar-Daban Range, southern Siberia (sample 520-9 from [52]) at 1.8 GPa and 950 \({}^{\circ}\)C; white isopleths—the Al\({}_{2}\)O\({}_{3}\) (wt. %) content in orthopyroxene, magenta isopleths – Cr\({}_{2}\)O\({}_{3}\)/Al\({}_{2}\)O\({}_{3}\) ratio in spinel. 1—phase field Cpx + Opx + OI + Spl + Phl + Rt, where a presence of Na-phlogopite component in mica is predicted. All mineral abbreviations are from [48]. The slopes of phlogopite-forming reactions show that a high H\({}_{2}\)O activity is favorable for peridotite transformation at lower K\({}_{2}\)O activity (Figure 2a). This conclusion is qualitatively consistent with the conclusion by [PERSON] et al. [53; 54] that K\({}_{2}\)O concentration is negatively correlated with the H\({}_{2}\)O/(H\({}_{2}\)O + CO\({}_{2}\)) ratio that is required for phlogopite stabilization in peridotite. Within a narrow log(\(a_{\text{K${}_{2}$O}}\)), all associations of metasomatized garnet peridotite are stable, from unaltered Iberzolite Opx + OI + Grt + Cpx + Ru (rutile appears since no Ti substitutions are accounted in model pyroxene, garnet and olivine solid solutions) through phlogopite-bearing garnet peridotite Grt + Opx + PhI + OI + Cpx to Cr-spinel-bearing phlogoptite wehrlite PhI + OI + Cpx + Spl. Spinel forms via decomposition of Cr-bearing garnet, whereas mode of clinopyroxene increases via phlogopite-forming reactions. New clinopyroxene along with phlogopite forms in the rocks, which initially did not contain clinopyroxene, as well. To illustrate this feature, the log(\(a_{\text{H${}_{2}$O}}\)) - log(\(a_{\text{K${}_{2}$O}}\)) pseudosection for 4.5 GPa and 1000 \({}^{\circ}\)C (Figure 2b) is computed for the representative harzburge JSL266 from the Lethlakane kimberlite pipe [46], which has not contained clinopyroxene before metasomatis. Transformation of this rock via increase of K\({}_{2}\)O activity results in gradual disappearance of Cr-bearing garnet with partial preservation of orthopyroxene, which is a dominant phase in the initial rock. As a result, new clinopyroxene and Cr-bearing spinel form consistently with petrographic observations in the series of metasomatized xenoliths [46]. However, in some cases, new clinopyroxene does not form during garnet decomposition to phlogopite [44; 47]. The pseudosection in Figure 2b demonstrates that such cases would correspond to lower log(\(a_{\text{H${}_{2}$O}}\)) and log(\(a_{\text{K\[1/2\mathrm{Prp}+3/2\mathrm{En}+[1/2\mathrm{K_{2}O}+\mathrm{H_{2}O}]=\mathrm{Phl} \tag{2}\]0O}}\)) values. Such phlogopite-bearing reaction textures are usually accompanied by stronger zoning of garnet grain relics (see below). Reaction (4) describes a formation of phlogopite directly after orthopyroxene. Phlogopite-bearing reaction textures on orthopyroxene are common feature of the metasomatized peridotite (Figure 1b). As a rule, phlogopite in them is present as inclusions and ingrowths in \"spongy\" clinopyroxene surrounding orthopyroxene and commonly is accompanied by secondary olivine (Figure 1b). Formation of clinopyroxene and olivine could be related to the reaction of orthopyroxene with a Ca-component of the alkaliic fluid, for example, CaCO\({}_{3}\), so that the total reaction of the phlogopite formation after orthopyroxene can be written as  
\[1/2\mathrm{Prp}+3/2\mathrm{En}+[1/2\mathrm{K_{2}O}+\mathrm{H_{2}O}]=\mathrm{Phl} \tag{2}\]1  
However, it cannot be excluded that clinopyroxene \"exsolves\" from initial orthopyroxene after replacement of the enstatite component by phlogopite. Reaction textures on orthopyroxene are characteristic for both garnet and spinel-facies peridotite.