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 Kand 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 highpressure breakdown product of mathiasite is a K–Cr-titanate with an idealized formula KM7O12, where M 1⁄4 Ti, Cr, Mg, Fe. This phase possesses space group P63/m with a 1⁄4 9 175(2) A, c 1⁄4 2 879(1) A, V 1⁄4 209 9(1) A. 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 þ 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 Baand 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.