Journal of Petrology Advance Access originally published online on January 7, 2005
Journal of Petrology 2005 46(4):749-781; doi:10.1093/petrology/egh096
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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
1 INSTITUT FÜR MINERALOGIE UND PETROGRAPHIE, UNIVERSITÄT INNSBRUCK, INNRAIN 52, A-6020 INNSBRUCK, AUSTRIA
2 DEPARTMENT OF MECHANICAL AND MATERIALS ENGINEERING, FLORIDA INTERNATIONAL UNIVERSITY, 10555 WEST FLAGLER STREET, MIAMI, FL 33175, USA
3 BAYERISCHES GEOINSTITUT, UNIVERSITÄT BAYREUTH, UNIVERSITÄTSSTRASSE 30, D-95447 BAYREUTH, GERMANY
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.
KEY WORDS: crichtonite; magnetoplumbite; high-P–T experiments; phase relations; upper mantle
* Corresponding author. Telephone: +43-(0)512-507-5506. Fax: +43-(0)512-507-2926. E-mail: juergen.konzett{at}uibk.ac.at