## 6 Discussion
### Origin of the #50 Phlogopite Zone
#### 6.1.4 Origin of the #50 Phlogoptite Rinds  
The rinds are extremely narrow and challenging to discern. X-ray elemental maps initially revealed the presence of the #50 phlogoptite rinds in the outermost layer of certain phlogoptite crystals (Figure 3). An obvious feature of the rinds is the re-enrichment of Mg. Based on the origin of olivine finds [34], crystallization from residual kimberlite melt under high oxygen fugacity conditions is suggested [33,70,72,73]. Notably, the rinds of the #50 phlogoptite exhibit two distinct compositional ranges, namely, low-Ti-Cr finds (designated rind I) and high-Ti-Cr rinds (designated rind II). We propose that the formation of rind I (low-Ti-Cr rinds) is attributed to melting depletion during the late stages of magmatic evolution. The differentiation of crystallization should be considered the primary factor leading to the depletion of residual melt.  
Rind II with high-Ti-Cr component characteristics appears somewhat abruptly in crystals with rinds. However, we posit that the presence of this composition is not random and that it substantiates the longstanding conjecture regarding a hydrothermal (H\({}_{2}\)O-rich fluids) superimposition event during the late magmatic evolution of the No. 50 pipe kimberlite. The overlapping of late hydrothermal processes involving H\({}_{2}\)O-rich fluids may account for the abnormal increase in Cr concentrations observed in some rinds II. The hydrothermal fluid infiltrated the phlogoptite cores through cleavage planes, causing metasomatism in the xenocrystic cores and resulting in the formation of metasomatic minerals such as serpentine and talc (part 6.1 above; Figures 3 and 4). The diverse range of rinds illustrates the complex nature of the environment during the later stages of magmatic evolution. In addition, we hypothesize that another factor contributing to maintaining a high Ti concentration was an increase in oxygen fugacity during the crystallization process in near-surface environments. This led to a significant influx of Ti from the melt into rind II, resulting in an anomalously enriched Ti concentration [74]. Similar to the origin of low-Cr/Cr-poor phlogoptite, this explanation is also consistent with the Mg enrichment of the rinds.  
A plausible conjecture here is that rind I might not have formed simultaneously with rind II, albeit direct evidence to substantiate this claim is lacking. It is speculated that under high oxygen fugacity conditions, a large amount of Ti in the residual magma entered rind  
Figure 8: Evolutionary trends in the composition of groundmass micas are indicated for lamproite (brown shaded fields and labeled continuous black lines), Group II kimberlites (orangeites; dashed box and light gray lines), and Group I kimberlite (labeled box and continuous black lines) (modified from [PERSON] et al., 2006) [23]. (**A**) Al\({}_{2}\)O\({}_{3}\) (wt%) vs. TiO\({}_{2}\)(wt%); (**B**) Al\({}_{2}\)O\({}_{3}\) (wt%) vs. FeO(wt%).  
II, which led to further loss of Ti from the residual magma; subsequently, rind I may have formed in this environment. Rind II may have crystallized simultaneously or slightly later than the low-Cr/Cr-poor outer rims and before rind I.