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

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 TiO 2 –ZrO 2 –Cr 2 O 3 –Fe 2 O 3 –BaO–K 2 O and TiO 2 –Cr 2 O 3 –Fe 2 O 3 –BaO–K 2 O. 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 KM 7 O 12 , where M = Ti, Cr, Mg, Fe. This phase possesses space group P 6 3 / 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.