| Abstract
| - High-pressure-high-temperature experiments were performed in the range 7-15 GPa and 1300-1600°C to investigate the stability and phase relations of the K- and Ba-dominant members of the crichtonite and magnetoplumbite series of phases in simplified bulk compositions in the systems TiO2-ZrO2-Cr2O3-Fe2O3-BaO-K2O and TiO2-Cr2O3-Fe2O3-BaO-K2O. Both series of phases occur as inclusions in diamond and/or as constituents of metasomatized peridotite mantle xenoliths sampled by kimberlites or alkaline lamprophyres. They can accommodate large ion lithophile elements (LILE) and high field strength elements (HFSE) on a wt % level and, hence, can critically influence the LILE and HFSE budget of a metasomatized peridotite even if present only in trace amounts. The Ba and K end-members of the crichtonite series, lindsleyite and mathiasite, are stable to 11 GPa and 1500-1600°C. Between 11 and 12 GPa, lindsleyite breaks down to form two Ba-Cr-titanates of unknown structure that persist to at least 13 GPa. The high-pressure breakdown product of mathiasite is a K-Cr-titanate with an idealized formula KM7O12, where M = Ti, Cr, Mg, Fe. This phase possesses space group P63/m with a = 9·175(2) Å, c = 2·879(1) Å, V = 209·9(1) Å3. Towards high temperatures, lindsleyite persists to 1600°C, whereas mathiasite breaks down between 1500 and 1600°C to form a number of complex Ti-Cr-oxides. Ba and K end-members of the magnetoplumbite series, hawthorneite and yimengite, are stable in runs at 7, 10 and 15 GPa between 1300 and 1400°C coexisting with a number of Ti-Cr-oxides. Molar mixtures (1:1) of lindsleyite-mathiasite and hawthorneite-yimengite were studied at 7-10 GPa and 1300-1400°C, and 9-15 GPa and 1150-1400°C, respectively. In the system lindsleyite-mathiasite, one homogeneous Ba-K phase is stable, which shows a systematic increase in the K/(K + Ba) ratio with increasing pressure. In the system hawthorneite-yimengite, two coexisting Ba-K phases appear, which are Ba rich and Ba poor, respectively. The data obtained from this study suggest that Ba- and K-dominant members of the crichtonite and magnetoplumbite series of phases are potentially stable not only throughout the entire subcontinental lithosphere but also under conditions of an average present-day mantle adiabat in the underlying asthenosphere to a depth of up to 450 km. At still higher pressures, both K and Ba may remain stored in alkali titanates that would also be eminently suitable for the transport of other ions with large ionic radii.
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