ORIGINAL PAPER Alkali in phlogopite and amphibole and their effects on phase relations in metasomatized peridotites: a high-pressure study P. Fumagalli Æ S. Zanchetta Æ S. Poli Received: 2 December 2008 / Accepted: 20 April 2009 / Published online: 8 May 2009 Ó Springer-Verlag 2009 Abstract 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 680–1,000°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] 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 A ˚ phase, Mg 3 Si 4 O 10 (OH) 2 nH 2 O, are envisaged. Phlogo- pite 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-bear- ing 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. Keywords Metasomatism  Phlogopite  Ca-amphibole  10 A ˚ phase  Subduction zones  K-doped lherzolites Introduction Global chemical recycling via lithospheric subduction strongly controls the earth’s mantle processes. Mantle metasomatism testifies to the efficiency of mass transport: when alkali-rich fluids interact with ultramafics, elements such as hydrogen and potassium are hosted in mantle rocks and phases such as phlogopite and amphibole develop. The resulting hybridized mantle wedge is down dragged by corner flow and undergoes further deeper transformations, following a widely accepted multi-stage model as proposed by Wyllie and Sekine (1982). Phlogopite–spinel peridotites (Nixon 1987), phlogopite–garnet peridotites, phlogopite– K-richterite peridotite xenoliths, and ‘‘orogenic’’ phlogo- pite peridotites of UHP terrains (Ulten peridotite: Rampone and Morten 2001; Bardane peridotite, Norway: van Roermund et al. 2002; Sulu garnet peridotite, China: Zhang et al. 2007) document that such a process might occur both within the mantle wedge and at the slab–mantle interface, i.e., at relatively low temperatures. Several experimental studies have been devoted to determining the stability field of phlogopite, starting from simple systems of phlogopite alone (Kushiro et al. 1967), phlogopite ? forsterite (Kushiro 1969; Trønnes 2002), phlogopite ? enstatite and phlogopite ? diopside (Kushiro 1970; Modreski and Boettcher 1973; Luth 1997), phlogo- pite ? diopside ? enstatite (Sudo and Tatsumi 1990), to Communicated by J. Hoefs. Electronic supplementary material The online version of this article (doi:10.1007/s00410-009-0407-4) contains supplementary material, which is available to authorized users. P. Fumagalli (&)  S. Poli Dipartimento di Scienze della Terra, Universita ` degli Studi di Milano, Via Botticelli 23, 20133 Milan, Italy e-mail: patrizia.fumagalli@unimi.it S. Zanchetta Dipartimento di Scienze Geologiche e Geotecnologie, Universita ` degli Studi di Milano Bicocca, Piazza della Scienza 4, 20126 Milan, Italy 123 Contrib Mineral Petrol (2009) 158:723–737 DOI 10.1007/s00410-009-0407-4