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,CrTi,Si,Mg,Fe2+)(12)O-19 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.