## 4 Discussion
### Applications to Mantle Metasomatism  
#### 4.3.1 Composition of Phlogopites Formed by Melt/Rock Reaction  
The main aim of this study was to assess whether the phlogopites in cratonic garnet peridotite were formed by reaction with a passing potassic silicate melt. The phlogopites in the lamproite section reflect magmatic crystallization from the lamproite melt, whereas the phlogopites occurring intersitially in the peridotite are the product of a reaction between melt and peridotite as the melt moves through the garnet peridotite and into the extraction trap. This is formed by the reaction of lamproitic melt primarily with garnet: the garnet reacts with the alkali- and water-rich melt and the high Cr\({}_{2}\)O\({}_{3}\) content of the garnet (Table 2) is incorporated into the newly formed phlogopite. This conclusion is backed up by the observation that the interstitial phlogopite is xenomorphic (Figure 2) and has a much higher Cr\({}_{2}\)O\({}_{3}\) content than the original lamproite (0.03 wt%; Table 2). At 2 GPa, the reaction also consumes Opx, whereas at 3 GPa Opx survives, indicating increased stability of Opx towards higher pressures, as is typical in peridotite [45]. Phlogopites in cratonic peridotite have been studied for several decades. [PERSON] [46] defined primary-textured micas as being single large crystals that have regular boundaries with silicate and oxide minerals, whereas secondary-textured micas usually form reaction rims around other silicate minerals, particularly orthopyroxene and garnet, and often occur as veinlets in the peridotite. [PERSON] et al. [4] showed that the compositions of these phlogopites show lower TiO\({}_{2}\) and higher Cr\({}_{2}\)O\({}_{3}\) contents in later minerals, as judged by textures, and interpreted the high Cr\({}_{2}\)O\({}_{3}\) micas to be the products of metamsomatis by K- and H\({}_{2}\)O-rich fluids in which the garnet reacts with the fluid (or melt). Later studies of other localities confirmed the higher Cr\({}_{2}\)O\({}_{3}\) contents but found that the behaviour of TiO\({}_{2}\) was inconsistent, often also increasing in late micas (Figure 8; [47; 48; 49; 50]). Micas in kelphritic rims around garnets have particularly high Cr\({}_{2}\)O\({}_{3}\), extending up to 3 wt% in examples from ilmenite-rutile-phlogopite-sulphide series xenoliths despite lower concentrations of <0.75 wt% in primary-textured micas [51]. Figure 8: Comparison of phlogopites in peridotite (green squares) and lamproite (yellow squares) sections of the current experiments with phlogopites in natural mantle rocks. Data sources: Metasomatised Kaapvusal peridotite [4; 50]; Lashaine peridotite [47]; Jagersfontein peridotite (1 = primary, 2 = secondary; [50]); IRPS suite [51]; Bulfatonia peridotite close to vein, and polymict brecias [48]; MARID and PIC xenoliths [52]; experiments reacting carbonate-silicate melt with peridotite (“cbt reaction expts”; [53]). [PERSON] et al. [5] describe the metasomatic progression from garnet peridotites (GP), through garnet phlogopite peridotites (GPP) and phlogopite peridotites (PP) to phlogopite K-richterite peridotites (PKP), resulting from increasing metasomatic alteration by alkali-, volatile- and incompatible element-rich fluids, which were re-interpreted as melts by [6]. The experimental results presented here are the equivalent of phlogopite peridotite, where metasomatic alteration has removed the garnet completely. The phlogopites occurring interstitially in the current experiments have similar trends of decreasing TiO\({}_{2}\) and increasing Cr\({}_{2}\)O\({}_{3}\) relative to phlogopites in the lamproite section and plot towards the upper end of Cr\({}_{2}\)O\({}_{3}\) contents of secondary-textured micas in the natural peridotites (Figure 8). They may be equivalent to those in kelyphitic rims, but garnet was completely replaced by phlogopites under these experimental conditions, as a result of the relatively high temperature and low pressure [54].