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THE COMPOSITION OF ANORTHOCLASE AND NEPHELINE IN MOUNT KENYA PHONOLITE AND KILIMANJARO TRACHYTE, AND CRYSTAL–GLASS PARTITIONING OF ELEMENTS

J. Barry Dawson^1,, Richard W. Hinton¹ and Ian M. Steele²

¹ Grant Institute of Geosciences, King’s Buildings, University of Edinburgh, Edinburgh EH9 3JW, U.K.
² Department of the Geophysical Sciences, University of Chicago, Chicago, Illinois 60637, U.S.A.

E-mail address: jbdawson{at}glg.ed.ac.uk

Compositional data (major, minor and trace elements) are given for anorthoclase and glass in phonolite from Mount Kenya and trachyte from the Kibo cone of Kilimanjaro, and for nepheline in the Mount Kenya specimen. With the exception of Ba and Sr in anorthoclase, Eu in one of the anorthoclase crystals, and V and Ga in nepheline, the other trace elements (Li, Be, B, F, Mg, P, Cl, Ti, Cr, Fe, Mn, Rb, Y, Zr, Mo, the REE, Pb, Th and U) are more concentrated in the glass. Although differing in detail, the results broadly agree with element partitioning found for anorthoclase from Antarctic phonolite and for sanidine in rhyolites from Pantelleria and the western U.S.A. In the Mount Kenya phonolite, nepheline concentrates Li, Be, Mg, P, V, Cr, Fe, Mn, Ga and Rb relative to feldspar. In the Kibo specimen, anorthoclase fractionation has led to less Si, resulting in normative nepheline in the residual glass, and increased Zr and P concentrations; this is a potentially important factor in the formation of Zr-and P-rich phonolites such as those extruded later in the overall sequence of lavas at Kibo.

Keywords: anorthoclase, nepheline, glass, trace-element partitioning, Mount Kenya, Kilimanjaro, East Africa.