Ultramafic xenoliths from Lake Nyos area, Cameroon volcanic line, West-central Africa: Petrography, mineral chemistry, equilibration conditions and metasomatic features

Abstract

Spinel-bearing mantle xenoliths have been recovered in the pyroclastic breccia surrounding the Lake Nyos maar. These include spinel lherzolites, spinel harzburgites and olivine websterites. They exhibit coarse granular or protogranular to weakly porphyroclastic textures, and show variations in mineral chemistry, modal compositions and equilibrium temperature. The xenoliths consist of four mineral phases typical of upper mantle origin: olivine (Fo89–Fo91.5, NiO=0.29–0.38wt%, CaO=0.02–0.17wt%), enstatite (Mg#=90–92, Cr2O3=0.35±0.04wt%), Cr-diopside (Mg#=92–98, Cr2O3=0.7–1.65wt%, TiO2=0.26–0.6wt%) and spinel (high Mg# of 70–80, low TiO2≤0.4wt%). Spinels are aluminous (Cr#=9.7–11) in most lherzolites, and become increasingly chromiferous from websterites (Cr#Sp=15.3–19.8) to harzburgites (Cr#Sp=19–33.6). The lherzolites are composed of olivine (48–58%), orthopyroxene (22–30%), and clinopyroxene (8–15%). The harzburgites modes are olivine (60–81%), orthopyroxene (11–29%), and clinopyroxene (<5%). The websterites are mainly composed of pyroxene (∼62%) with variable amounts of olivine (23–31%). Temperatures of mineral equilibration in the xenoliths have been estimated from the two-pyroxene thermometer of Wells (1977) and range between 850 and 1050°C, corresponding to about 10–30kbar at a depth mantle of 30km at least. These P–T conditions show significant variations between different petrographical types, the maximum conditions being recorded in two spinel lherzolites (NY-05 and NY-23) that have atypical chemical compositions and textures suggesting that they were initially formed in an environment close to the garnet stability field, then re-equilibrated within the spinel stability field prior to their incorporation in the host magma. With the exception of minerals from these two lherzolite nodules, all the minerals exhibit depletion of light REE, a typical feature of abyssal peridotites implying that some xenoliths from the Cameroon volcanic line were probably sampled in a part of the sub-continental mantle that is chemically similar to sub-oceanic mantle. The variations observed in the mineral chemistry and modal compositions of xenoliths suggest that the spinel harzbugite nodules which represent residues of a significant degree of partial melting of lherzolitic mantle were affected by infiltration of alkali-enriched metasomatizing melts (or fluids) within the uppermost mantle to produce pargasitic amphiboles prior to their sampling by the host lava. The features of this metasomatism event occur in the rocks of all three petrographical facies xenoliths from Lake Nyos.