Ti and Cr in High-Pressure Mica: Experimental Study and Application to the Mantle Assemblages

Experiments aimed at the synthesis of Cr- and Ti-bearing phlogopite in the silicate-carbonate systems peridotite--K2CO3 + H2O and basalt--K2CO3 + H2O at 7 GPa and 900-1200°? were carried out. It is shown that the crystallization of titanium-bearing phlogopite requires subducted crustal material at mantle depths. However, the mantle peridotite should predominate over basalt for Ti-phlogopite crystallization; otherwise, dioctahedral mica (aluminoceladonite) with (Mg + Fe)/VIAl > 1 is formed via the scheme 2VIAl = VITi4+ + VI(Mg + Fe). The competitive behavior of Ti and Cr upon incorporation into phlogopite is considered. It is shown that the presence of >1.3 wt % TiO2 introduces a limitation on the high concentrations of Cr2O3 via the scheme VI(Mg2+) + IV(Si4+) = VI(Cr3+) + IV(Al3+). This can explain the compositional patterns of phlogopite from inclusions in natural diamonds, in which the Ti content is much higher than that of Cr. The results obtained support the original idea that the composition of phlogopite may be applied to distinguish the paragenetic associations of diamond.