# Phlogopite-Forming Reactions as Indicators of Metasomatism in the Lithospheric Mantle
## 3 Phlogopite-Forming Reactions in the Upper-Mantle Rocks
### Reactions in Peridotites
[PERSON] [43] and [PERSON] et al. [44] noted that formation of phlogopite reaction rims around garnet resulted in their zoning with respect to both major and trace elements. In garnets from harzburgite xenoliths from the Wesselton kimberlites, South Africa [44], CaO increases, MgO and Cr\({}_{2}\)O\({}_{3}\) decrease from cores to rims, contacting to phlogopite-Cr-spinel reaction textures. Garnet rims are enriched in Y, Zr, Ti suggesting extensive metasomatic influence. Overall, composition of garnets changes from harzburgitic cores to lherzolitic rims. In order to characterize compositional variations in minerals in the course of the phlogopite-forming reactions, isopleths in the pseudosections were plotted (Figure 2a,b). In both cases (primitive lherzolite and depleted harzburgite), the pseudosections predict a significant increase of grossular content in garnet. The rimward increase of the grossular in garnet at the phlogopite reaction rims is consistent with observation by [43; 44] and implies a domination of the reaction (2) over reaction (3). In fact, secondary clinopyroxene has not been described in the phlogopite reaction rims around garnet from the xenoliths studied by [PERSON] et al. [44]. Following to Figure 2b, metastatic transformations in these xenoliths seemed to occur within the garnet stability field (Ol + Spl + Grt + Opx + Phl) in terms of K\({}_{2}\)O and H\({}_{2}\)O activities. The pseudosection also demonstrates a concomitant increase of Cr\({}_{2}\)O\({}_{3}\)-content in garnet in the course of phlogopite formation. In contrast, [PERSON] et al. [44] noted a slight, just about 1 wt. %, decrease of Cr\({}_{2}\)O\({}_{3}\) during the phlogopite-forming reactions in number of harzburgite xenoliths from the Wesselton kimberlites. The inconsistency between observations from the natural assemblages and modeling results can be explained by the following reasons. The phlogopite solid solution model, which is used in the present calculations,does not account for the Cr incorporation. In natural systems, phlogopite usually contains Cr\({}_{2}\)O\({}_{3}\) (0.3-1 wt. %), probably, influencing on the Cr-content in coexisting garnet. In addition, garnet at contacts with phlogopite reaction textures shows a significant (up to 0.5 wt. %) increase of TiO\({}_{2}\) substituting Cr\({}_{2}\)O\({}_{3}\). However, solubility of Ti in garnet is not accounted in the garnet solid solution model in the present calculations, as well. In contrast to the observations by [PERSON] [43] and [PERSON] et al. [44] on garnet zoning, [PERSON] et al. [46] found that garnets from the phlogopite-bearing harzburgites from the Lethlakane pipe were generally homogeneous in composition, and zoning in most cases is absent. Both Cr-spinel and clinopyroxene are present in the phlogopite reaction textures in these xenoliths. Taking these observations into account and following to Figure 2b, metasomatic transformations in these rocks proceeded beyond the garnet stability field at higher K\({}_{2}\)O and/or H\({}_{2}\)O activities, i.e., in the field Cpx + Ol + Spl + Opx + Phl. Formation of phlogopite after Cr-bearing garnet produces Cr-rich spinel as illustrated in the diagrams of Figure 2a,b. The position of isopleths shows that the Cr\({}_{2}\)O\({}_{3}\)/Al\({}_{2}\)O\({}_{3}\) ratio of newly formed spinel rapidly increases along with its mode with a progress of phlogopite formation via increase of log(\(a_{\text{K}_{2}\text{O}}\)) and/or log(\(a_{\text{H}_{2}\text{O}}\)) (Figure 2b). As an illustration for this rule, [PERSON] et al. [46] noted that phlogopite rewhlites, which represent the highest degree of metasomatic transformation of garnet harzburgites, contain the most Cr-rich spinel. Similar increase of the Cr\({}_{2}\)O\({}_{3}\)/Al\({}_{2}\)O\({}_{3}\) ratio of spinel occurs via phlogopite-formation on the spinel-facies Iberzolite as well (Figure 2c).