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 K(2)O-Na(2)O-CaO-FeO-MgO-Al(2)O(3)-SiO(2)-H(2)O at 1.5-6.0 GPa and 680-1,000A degrees C. Phlogopite is ubiquitous and coexists with Ca-amphibole up to 3.2 GPa and 900A degrees C. High-pressure phlogopites show a peculiar mineral chemistry dependent on pressure: e.g., at 5.5 GPa and 680A degrees 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 phase component ([v](XII)Si(1)K(-1)Al (-1) (IV) , where [v](XII) is interlayer vacancy). Mixed layering or solid solution relations between high-pressure phlogopites and the 10 phase, Mg(3)Si(4)O(10)(OH)(2) nH(2)O, 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 +/- A 0.7 wt.%, at 4.8 GPa, 680A degrees C). In phlogopite-bearing systems, Ca-amphibole breaks down between 2.5 and 3.0 GPa, and 1,000A degrees 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.