All the samples of Bioko Island belong to the alkaline series. Only basaltic lavas (basalts and hawaiites) have been found (Figure 2) whilst felsic lavas (trachytes and phonolites) are present in the other islands of CHL

(Pagalu, Principe and Sao Tome). A phonolite has been described by [6] , but it has been sampled as a loosed block on the sea shore and could originate from Sao-Tome or Principe islands from which it could have been carried as ship ballast: phonolite has never been found anymore on Bioko Island. Basaltic lavas only have also been sampled on the neighbouring continental Mt Cameroon volcano (Figure 2). The nomenclature used in this paper is based upon the differentiation index (D.I., [7] ) and on peculiar mineralogical distribution. Basalts have D.I. < 35 (42 samples) and hawaiites have 35 < D.I. < 50 (15 samples). Sample FP1K which has hornblende phenocrysts, high SiO₂ content (>53.0 wt%) and normative quartz (>4.0 wt%) has a D.I. of 49.2 (value obtained on a recalculated anhydrous analysis): it is classified as a mugearite.

The lavas have a microlitic texture. Basalts contain euhedral to subhedral phenocrysts (>1 mm) of olivine (0.83 < Mg# < 0.87) and clinopyroxene. Plagioclase (An_62-67Ab_35-32Or_3-1) is also present in hawaiites. The matrix contains microlites of clinopyroxene, plagioclase and magnetite and microphenocrysts of olivine. Diopside and Ti-augite occur respectively as euhedral to subhedral microphenocrysts (<2.5 mm) and as phenocrysts of variable size (2.5 to 6.0 mm). One lava sample (B17) of basaltic affinity contains skeletal hypersthene phenocrysts, embayed olivine phenocrysts and euhedral cordierite microphenocryts with opacified core. Cordierite has already been described in hypersthene-bearing lava from St. Helena Island [8] .

Crystal size distribution (CSD) measurements have been performed on clinopyroxene crystals from 12 basaltic lavas using CSD Corrections software of [9] . The data were plotted following general convention [10] as linear crystal size vs population density―ln(n) (mm⁻⁴) vs l (mm) (Figure 3). Most of the CSDs appear curved with nearly straight distribution for small (<2 mm) grains and curved patterns for larger (>2 mm) crystals (Fig- ure 4). The small grains are distributed along a steep slope with intercepts between 8.5 and 12.0 mm⁻⁴ while the larger grains plot along a more gentle slope with lower intercepts (4 to 6 mm⁻⁴). Following [11] , [9] and [12] , these curved CSDs can be interpreted as mixing of two populations of crystals with contrasted crystallization histories. The population of large crystals could represent clinopyroxene that has crystallized under different physical condition. To strengthen this hypothesis, the crystallization pressure of the clinopyroxenes was estimated using the [10] structural barometer of [13] . This barometer is suitable for anhydrous alkaline melts as the Bioko basaltic melt. The small phenocrysts of Ti-augite composition were equilibrated at low P (0 - 2 kbar) while the few larger ones (of diopside composition) crystallized at significantly higher pressure (~6 kbar). In few samples, the clinopyroxene phenocrysts are zoned: the diopside cores crystallized at high pressure and the Ti-augite rims at low pressure.

CSD pattern and P estimate suggest a polybaric crystallization of Bioko basaltic magma. Similar multi- chamber magmatic evolution is common in alkaline [14] and peralkaline [15] volcanic rocks.

Some basaltic flows and tephra deposits contain abundant ultramafic and mafic xenoliths. Xenoliths of gabbro are composed of clinopyroxene and cloudy plagioclase (due to the presence of numerous scattered small green

spinel crystals) sometimes with olivine. The variety of ultramafic xenoliths is quite large. In a basalt lava flow at the NE flank of Santa Isabel volcano, harzburgites, lherzolites and dunites occur together scattered in a restricted area of few m². In fact, xenoliths are mainly harzburgites with a protogranular texture and lherzolites; dunites are rare. Olivine is Mg-rich (Fo ≈ 90.3 ± 0.5) as well as clinopyroxene and orthopyroxene (Mg#: 0.92 - 0.95 and 0.91 - 0.92, respectively). Clinopyroxene is also Cr-rich (0.13 < Cr a.p.f.u. < 0.31) when compared to other crystals in xenoliths from the CHL (Cr a.p.f.u. < 0.18) ([16] and references therein, [17] ). Harzburgites contain holly-leaf and vermicular spinel. This spinel has a composition similar to that of crystals in xenoliths from the Kapsiki plateau but is quite disctinct from xenoliths in other basalts all along the CHL. Rare phlogopite and very scarce plagioclase crystals are present in veinlets between olivine and orthopyroxene grains. Amphibole occurs in veinlets in the dunites. High fO₂ (above the FMQ buffer) characterize dunites. Similar xenoliths have also been found at Mt Cameroon, Mt Bambouto, Mt Oku and Nyos (see [16] ) in the continental sector of the CHL. Equilibrium temperatures for pyroxene pairs calculated after [9] are between 889˚C and 1033˚C (mean 943; 2 sigmas: 38˚C).

The three volcanoes of Bioko Island only emitted basaltic lavas (basalts and hawaiites); this is similarly to what is observed on the nearby Mt Cameroon. These volcanoes are the only ones of the CHL that have only basaltic lavas. Moreover, apart the phreatic eruptions of Monoun and Nyos lakes, Bioko and Mt Cameroon are the only volcanoes that had eruptions during the last century. Mt Etinde which is located at the very shore of the continent between Bioko and Mt Cameroon only consists of nephelinitic lavas which are completely unknown on the other volcanoes.

Evolution by crystal fractionation has been modeled for the short differentiation series of Bioko. Despite the restricted compositional range observed for the data set, the mineralogical (progressive evolution of mineral compositions with increasing differentiation) and geochemical (regular and progressive major and trace element evolution) data suggest that fractional crystallization had a major role in the differentiation of the lava series. Mass balance modelling for major elements gave results that are consistent with the distribution of mineralogical phases present as phenocrysts in the lavas. For example, the evolution from basalt (D) to hawaiite (W) can be modeled by fractional crystallization of ol + cpx + pl + ox in the proportions 6.9 + 0.7 + 0.3 + 0.1 (Σr² < 0.6) and to hawaiite FP1F by fractionation of ol + cpx + ox in the proportions 5.3 + 14.2 + 2.0 (Σr² < 0.5). The limited number of mineral phases involved in the process lead to quite large Σr².

Basaltic lavas from Bioko lavas have low Hf contents (4.2 - 9.2 ppm) and consequently higher Zr/Hf ratios (50 - 90) than those (<50) of similar lavas from other volcanoes of the CHL. Such high ratios are commonly observed in alkaline basaltic lavas associated with carbonatitic and/or nephelinitic magmatism. It is interesting to remind that Bioko is situated quite near the nephelinitic Mt Etinde volcano.

The occurrence of amphibole (pargasite) in ultramafic xenoliths of Bioko and of many other volcanoes of the CHL (see review in [17] ) suggests that the lithospheric mantle beneath the CHL has been metasomatized. Metasomatized peridotites are likely to be representative of enriched lithosphere, the commonly accepted metasomatic agent being partial melts that form a network of thin veins lacing mantle peridotite beneath zones of recent volcanism and that induce the crystallization of K-rich amphibole [21] . The low Al₂O₃ (<4 wt%) and high CaO (21 - 24 wt%) contents of clinopyroxenes and also the relatively high fO₂ (above the FMQ buffer) in dunites are suggestive of a metasomatic influence by carbonatitic liquids.