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Hibonite, Ca(Al,Cr,Ti,Si,Mg,Fe²⁺)₁₂O₁₉, in granulite xenoliths from the Chyulu Hills volcanic field, Kenya

¹ Institut de Géologie, Université de Neuchtel, Rue Emile-Argand 11, CH-2007 Neuchtel, Switzerland
² Institut für Isotopengeologie und Mineralische Rohstoffe, ETH Zentrum, Sonneggstrasse 5, CH-8092 Zürich, Switzerland

^* E-mail: alexey.ulyanov{at}unine.ch

In basanites of the Chyulu Hills volcanic field, Kenya, hibonite is found in two xenoliths of mafic meta-igneous granulites dominated by clinopyroxene and plagioclase. It occurs as small grains in the inner parts of complex corona textures where it forms intergrowths with spinel and sapphirine and shows reaction relationships with later mullite and sillimanite. Hibonite and whole-rock samples were analyzed for major and trace elements by electron microprobe, XRF and LA-ICPMS. Chemically, the analyzed hibonite grains are close to the idealized formula Ca(Al,Cr,Ti,Si,Mg,Fe²⁺)₁₂O₁₉ and do not contain other major components. They are similar to terrestrial hibonite in their (Fe+Mg) contents but show the elevated Al and Ca abundances as well as the relative depletion in Ti and REE typical of meteoritic hibonite. Silica contents are high and exceed those in any other terrestrial and meteoritic hibonite. In order to evaluate the possibility of magmatic crystallization of hibonite in the igneous protoliths of the studied rocks, we compare some of the measured element abundances with those expected from element partitioning data for hibonite and Ca,Al-rich silicate melt. Based on this comparison, formation of low-Ti hibonite that is relatively rich in LREE appears consistent with magmatic crystallization, whereas hibonite with elevated Ti contents, low in LREE, is obviously the result of diffusion re-equilibration in the course of subsolidus cooling.

Key-words: hibonite, mafic granulite, trace elements, East African Rift, LA-ICPMS.