Chemistry of Ti-poor spinels, ilmenites and rutiles from peridotite and eclogite xenoliths

Abstract

Electron microprobe analyses are given for Ti-poor spinels from ten lherzolite and six harzburgite nodules. One of the spinels in a lherzolite from Lashaine volcano falls into the iron-rich trend of the alkali pyroxenite suite. All the others, plus about thirty analyses from the literature, fall in a fairly narrow band from (Mg 2Fe 6 2)(Al 14Cr 2) to (Mg 3Fe 5 2)(Fe 1 3Al 1Cr 14). This band is almost identical with the bands for podiform deposits in alpine-type ultramafics and for inclusions in basalts, and is distinct from bands for Archaean rocks and for layered basic intrusions. The Cr-rich spinels from the kimberlites tend to derive from lherzolites rather than harzburgites, and the principal correlation with high Cr-content of the spinel is with absence of Ca-rich pyroxene rather than with presence of garnet. There is a good positive correlation between Cr (Cr + Al) for coexisting spinel and pyroxene. When rutile coexists, Ti is high in the spinel. The Jackson chromite-olivine thermometer yields temperatures from 1000 to 2280°C with an unreasonably high mean value of 1550°C. Picroilmenites from a dunite and a glimmerite from Bultfontein contain, respectively, MgO 7.3–9.0 Cr 2O 3 1.8–2.0 and 10.1–12.0, 1.7–1.9 wt.%. The dunite also contains disseminated magnetite crystals with TiO 2 9.0 Al 2O 3 0.8 Cr 2O 3 17.2 MgO 5.8. The ilmenite-pyroxene thermometer of A. T. Anderson yields temperatures in the range 900–1200°C for ilmenites currently available in the literature, mostly from intergrowths. Rutiles from eclogites have low Cr 2O 3 (0. 0n wt.%) while those from lherzolites and a glimmerite have 1.6 to 7.2 wt.%. The Fe 2O 3 content ranges from 0.2 to 2.6 wt.%, the higher values suggesting temperatures of at least 1000°C.