Phase Relations and Stability of Magnetoplumbite- and Crichtonite-Series Phases under Upper-Mantle P-T Conditions: an Experimental Study to 15 GPa with Implications for LILE Metasomatism in the Lithospheric Mantle

High-pressure–high-temperature experiments were performedin the range 7–15 GPa and 1300–1600°C to investigatethe stability and phase relations of the K- and Ba-dominantmembers of the crichtonite and magnetoplumbite series of phasesin simplified bulk compositions in the systems TiO₂–ZrO₂–Cr₂O₃–Fe₂O₃–BaO–K₂Oand TiO₂–Cr₂O₃–Fe₂O₃–BaO–K₂O. Both seriesof phases occur as inclusions in diamond and/or as constituentsof metasomatized peridotite mantle xenoliths sampled by kimberlitesor alkaline lamprophyres. They can accommodate large ion lithophileelements (LILE) and high field strength elements (HFSE) on awt % level and, hence, can critically influence the LILE andHFSE budget of a metasomatized peridotite even if present onlyin trace amounts. The Ba and K end-members of the crichtoniteseries, lindsleyite and mathiasite, are stable to 11 GPa and1500–1600°C. Between 11 and 12 GPa, lindsleyite breaksdown to form two Ba–Cr-titanates of unknown structurethat persist to at least 13 GPa. The high-pressure breakdownproduct of mathiasite is a K–Cr-titanate with an idealizedformula KM₇O₁₂, where M = Ti, Cr, Mg, Fe. This phase possessesspace group P6₃/m with a = 9·175(2) Å, c = 2·879(1)Å, V = 209·9(1) ų. Towards high temperatures,lindsleyite persists to 1600°C, whereas mathiasite breaksdown between 1500 and 1600°C to form a number of complexTi–Cr-oxides. Ba and K end-members of the magnetoplumbiteseries, hawthorneite and yimengite, are stable in runs at 7,10 and 15 GPa between 1300 and 1400°C coexisting with anumber of Ti–Cr-oxides. Molar mixtures (1:1) of lindsleyite–mathiasiteand hawthorneite–yimengite were studied at 7–10GPa and 1300–1400°C, and 9–15 GPa and 1150–1400°C,respectively. In the system lindsleyite–mathiasite, onehomogeneous Ba–K phase is stable, which shows a systematicincrease in the K/(K + Ba) ratio with increasing pressure. Inthe system hawthorneite–yimengite, two coexisting Ba–Kphases appear, which are Ba rich and Ba poor, respectively.The data obtained from this study suggest that Ba- and K-dominantmembers of the crichtonite and magnetoplumbite series of phasesare potentially stable not only throughout the entire subcontinentallithosphere but also under conditions of an average present-daymantle adiabat in the underlying asthenosphere to a depth ofup to 450 km. At still higher pressures, both K and Ba may remainstored in alkali titanates that would also be eminently suitablefor the transport of other ions with large ionic radii. KEY WORDS: crichtonite; magnetoplumbite; high-P–T experiments; phase relations; upper mantle