The 1999 and 2000 eruptions of Mount Cameroon: eruption behaviour and petrochemistry of lava

Abstract

Mount Cameroon (4,095 m high and with a volume of ∼1,200 km3) is one of the most active volcanoes in Africa, having erupted seven times in the last 100 years. This stratovolcano of basanite and hawaiite lavas has an elliptical shape, with over a hundred cones around its flanks and summit region aligned parallel to its NE-SW-trending long axis. The 1999 (28 March–22 April) eruption was restricted to two sites: ∼2,650 m (site 1) and ∼1,500 m (site 2). Similarly, in the eruption in 2000 (28 May–19 June), activity occurred at two sites: ∼4,095 m (site 1) and ∼3,300 m (site 2). During both eruptions, the higher vents were more explosive, with strombolian activity, while the lower vents were more effusive. Accordingly, most of the lava (∼8×107 m3 in 1999 and ∼6×106 m3 in 2000) was emitted from the lower sites. The 1999–2000 lavas are predominantly basanites with low Ni (5–79 ppm), Cr (40–161 ppm) and mg numbers (34–40). Olivine (Fo77–85, phenocrysts and Fo68–72, microlites), clinopyroxene (Wo47En41Fs10 to Wo5lEn34FS15), plagioclase (An49–67) and titanomagnetite are the principal phenocryst and groundmass phases. The lavas contain xenocrysts of olivine and clinopyroxene, which are interpreted as fragments of intrusive rocks disrupted by magma ascent. Major and trace element characteristics point to early fractionation of olivine. The clinopyroxenes (Al2O3 1.36–7.83 wt%) have high Aliv/Alvi ratios (1.3–1.8) and are rich in TiO2, characteristics typical of low pressure clinopyroxenes. Geochemical differences between the 1999–2000 lavas and those from previous eruptions, such as higher Nb/Zr of the former, suggest that different eruptions discharged magmas that evolved differently in space and time. Geophysical and petrological data indicate that these fractionated magmas originated just below the geophysical Moho (at 20–22 km) in the lithospheric mantle. During ascent, the magmas disrupted intrusions and earlier magma pockets. The main ascent path is below the summit, where newly arrived magma degasses. Degassed magma simultaneously intrudes the flank rift zones where most lava is extruded.