Alkali in phlogopite and amphibole and their effects on phase relations in metasomatized peridotites. A high pressure study

Subsolidus phase relations for a K-doped lherzolite are investigated in the model system K2O–Na2O–CaO–FeO–MgO–Al2O3–SiO2–H2O at 1.5–6.0 GPa and 680–1,000 C. Phlogopite is ubiquitous and coexists with Ca-amphibole up to 3.2 GPa and 900 C. High-pressure phlogopites show a peculiar mineral chemistry dependent on pressure: e.g., at 5.5 GPa and 680 C, excess of Si (up to 3.4 apfu) coupled with deficiency in Al (as low as 0.58 apfu) and K ? Na (as low as 0.97 apfu), suggest a significant amount of a talc/10 A ° phase component ([v]XIISi1K-1Al-1 IV , where [v]XII is interlayer vacancy). Mixed layering or solid solution relations between high-pressure phlogopites and the 10 A ° phase, Mg3Si4O10(OH)2 nH2O, are envisaged. Phlogopite modal abundance, derived by weighted least squares, is maximum at high-pressure and relative low-temperature conditions and therefore along the slab–mantle interface (10.3 ± 0.7 wt.%, at 4.8 GPa, 680 C). In phlogopite-bearing systems, Ca-amphibole breaks down between 2.5 and 3.0 GPa, and 1,000 C, through the water conservative reaction 5(pa ? 0.2 KNa-1) ? 17en ? 15phl = (10di ? 4jd) ? 5py ? 12fo ? 20(phl ? 0.2 talc), governed by bulk composition and pressure-dependent variations of K/OH in K-bearing phases and as a result, it does not necessarily imply a release of fluid.