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°С 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 2^VIAl = ^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.