Experimental constraints on phlogopite chemistry in lamproites: 2. Effect of pressure-temperature variations

The chemistry of micas from high-pressure (5-40 kbar) experiments on leucite lamproite and olivine lamproite show much less variation as a function of pressure and temperature at fixed fluid composition in comparison to the variation due to fluid composition. Micas at high pressures tend to have higher Al₂O₃ and lower TiO₂, K/Al and Cr₂O₃. Decreasing temperature produces decreasing Mg/(Mg + Fe) and Al₂O₃, and increasing SiO₂, K₂O and F in micas, but these trends may be a function of the degree of crystallization and may differ in natural lamproites where fractional crystallization occurs. Mica has higher TiO₂ where it coexists with titanate minerals. Strongly olivine-normative lamproitic melts are likely agents of mantle enrichment in the sub-continental mantle at depths of 100-200 km. The sub-liquidus experimental mica compositions can be used to constrain models of vein chemistry in inhomogeneous mantle compositions. -from Author