The Stability of Hydrous Potassic Phases in Lherzolitic Mantle--an Experimental Study to 9.5 GPa in Simplified and Natural Bulk Compositions

To investigate the pressure stability limit of phlogopite and the INTRODUCTIONpressure-temperature stability field of its breakdown product K-Hydrous potassic phases are potential hosts for alkalis richterite, experiments were conducted from 4•0 to 9•5 GPa and and H 2 O in metasomatized lherzolitic mantle to depths between 800°C and 1400°C in a subalkaline system K 2 Onear the transition zone (Trønnes et al., 1988; Trønnes, Na 2 O-CaO-MgO-Al 2 O 3 -SiO 2 -H 2 O (KNCMASH) and in nat-1990) and are likely to be source components for deepural phlogopite and K-richterite-doped lherzolite systems.In seated generation of potassium-rich magmas (e.g.Wend-KNCMASH, phlogopite breaks down between 6•0 and 6•5 GPa landt & Eggler, 1980;Lloyd et al., 1985; Foley, 1992; at 800°C and between 6•5 and 7•0 GPa at 1100°C to form Thibault et al., 1992;Mitchell, 1995;Luhr, 1997).The potassic amphibole by the reaction phlogopite + clinopyroxene + hydrous phases phlogopite and K-rich richteritic amorthopyroxene = K-richterite + garnet + olivine + H 2 O.In phibole are potentially stable in natural peridotitic bulk the natural system, the stability field of amphibole is shifted towards compositions and carry significant amounts of potassium lower pressures by ~0•5 GPa.The high-temperature stability limit and water.Phlogopite-spinel peridotites are common in of K-richterite in KNCMASH was located between 1300 and xenolith suites of metasomatized shallow upper mantle 1400°C at 8•0 GPa and at <1300°C at 7•0 GPa.Thus, Ksampled by basaltic volcanism (Nixon, 1987, and refrichterite can be stable in the mantle wedge above subduction zones erences therein), whereas phlogopite-garnet peridotites below a depth of ~180-200 km.Because of the small difference are typically part of xenolith suites from deep continental in K/OH ratios between phlogopite and K-richterite, only a small lithosphere.In rare cases K-richterite-bearing peridotites amount of aqueous fluid is likely to be produced during phlogopite are found as xenoliths in kimberlites (e.g.Erlank et al., breakdown in an average mantle lherzolite bulk composition.This 1987;Tallarico & Leonardos, 1995).Although phlogopite fluid might be trapped by nominally anhydrous minerals before it peridotites typically show well-equilibrated textures, texcan migrate to hotter portions of the mantle wedge.Phlogopite tures of K-richterite-bearing peridotites indicate that the breakdown therefore is unlikely to be a factor in inducing significant amphibole is not in equilibrium with the host lherzolite melting of the wedge leading to arc magmatism.(Erlank et al., 1987).The occurrence of diamond-bearing phlogopite peridotites (Dawson & Smith, 1975;Waters & Erlank, 1988) testifies that phlogopite is stable above 4 GPa in peridotitic bulk compositions under cool lithospheric P-T conditions.With respect to T, phlogopite is KEY WORDS: experimental study; high pressure; K-richterite; lherzolite; stable in lherzolite to <1250°C at 3•0 GPa under waterabsent conditions and to <1150°C at the same pressure phlogopite