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Les dépôts volcaniques du lac Nyos (Cameroun) contiennent des enclaves ultrabasiques de natures variées : lherzolites, harzburgites et webstérites, dont certaines contiennent de l'amphibole. Les compositions en éléments majeurs et en éléments traces des amphiboles et des clinopyroxènes permettent de mettre en évidence un épisode ancien de fusion partielle. Deux épisodes d'enrichissement en éléments incompatibles sont identifiés, l'un riche en terres rares légères et affectant des échantillons préalablement appauvris, l'autre très riche en U et Th et plus pauvre en terres rares. La relation entre ces deux derniers épisodes n'est pas établie.

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Thermobarometric calculations show equilibrium temperatures ranging between 850 and 950 °C and pressures of 8 to 17 kbar consistent with the spinel lherzolite stability field. These data suggest that the xenoliths come from a depth of 28-31km in the uppermost mantle situated just below a thinned crust; they are in agreement with the geophysical data previously determined in the Adamawa Massif. On the basis of these features, and considering the evidence for textural, mineralogical, and chemical equilibrium in the studied xenoliths prior to their entrainment in the host magma, we conclude that the source of these xenoliths was a chemically and petrographically homogeneous, spinel lherzolite lithospheric mantle. But the occurrence of mantle-derived xenoliths of various types (dunite, lherzolite, wehrlite, harzburgite, websterite, clinopyroxenite, and orthopyroxenite) in alkali basalts from many other localities of Cameroon (Oku Massif, Lake Nyos area and Mount Cameroon) is consistent with upper-mantle heterogeneities on a regional scale and implies that the nature of the upper mantle beneath the continental sector of the Cameroon Volcanic Line varies under its different volcanic centres.

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