Abstract
Whole rock and mineral analyses of about 30 spinel lherzolite and harzburgite xenoliths from Tertiary alkali olivine basalts, limburgites and olivine nephelinites have been performed. After testing various geothermometers, a few pyroxene solvus calibrations, the Wells geothermometer and the calibration of Ca in olivine were favoured. “Wells-temperatures” range from 1100°C to less than 900°C. Several low temperature xenoliths lack equilibrium documented in heterogeneous mineral grains and deviations from regular Al, Cr partition between coexisting minerals. P-T correlations are derived from a continental geotherm corresponding to a surface heat flow density of 65 mW/m2. Several spinel peridotites must have existed outside the field of spinel stability. All xenoliths exhibit a coarse-grained granular texture without recrystallization from shearing in diapiric mass movements. The average modal composition of 73 vol.-% olivine, 18 vol.-% orthopyroxene, 7 vol.-% clinopyroxene and 1.3 vol.-% spinel is close to xenoliths from the Eifel and from worldwide sampling. Host basalts are most probably products of partial melting from phlogopite bearing peridotites, the phlogopite being the result of interactions between peridotite and metasomatic mantle fluids.
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