Résumé
Hanang is the southern volcano of the East branch of the East African Rift and represents volcanic activity at early stage rifting (0.9 Ma). Lavas are highly alkaline Mg-poor nephelinites (Mg#=24.4-35.2) with cpx, garnet, nepheline, titanite, and apatite and result from fractional crystallisation of primary melilitite magmas (Parat et al. AGU2016). In this study, we investigate glassy melt inclusions at the rim of nepheline phenocrysts to constrain the late stage of nephelinite evolution and the behaviour of volatiles (CO (sub 2) , H (sub 2) O, S, F, Cl) during magma storage and ascent. The melt inclusions have a green silicate glass, a microcrystalline carbonate phase and a shrinkage bubble free of gas phase (Raman analyses) suggesting that carbonatite-silicate liquid immiscibility (85:15) occurred during nephelinite differentiation. The silicate glasses have trachytic composition (Na+K/Al=1.6-7.2, SiO (sub 2) =54-65.5 wt%) with high CO (sub 2) (0.43 wt% CO (sub 2) , SIMS analyses), sulfur (0.21-0.92 wt% S) and halogens (0.28-0.84 wt% Cl; 0.35-2.54 wt% F) content and very low H (sub 2) O content (<0.1 wt%, Raman analyses). The carbonate phase is an anhydrous Ca-Na+ or -S,K- carbonate with 33 wt% CaO, 20 wt% Na (sub 2) O, 3 wt% K (sub 2) O, and 3 wt% S. The pre-immiscible silicate liquid (e.g. silicate melt + carbonatite) in equilibrium with nepheline and cpx phenocrysts has CO (sub 2) -rich phonolitic composition (Na+K/Al=6.2-6.9) with 6 + or - 1.5 wt% CO (sub 2) at pressure of 700-1100 MPa. The entrapped melt in nepheline corresponds to evolved interstitial silicate melt after crystallisation of cpx (16.7%), nepheline (40%) garnet (6.5%) and apatite (1.7%) from Mg-nephelinite magma. The immiscibility process leading to glassy silicate melt and microcrystalline carbonatitic melt occurred in closed system during rapid ascent at crustal level at 200-230 MPa. The absence of gas phase in shrinkage bubble in melt inclusions suggests CO (sub 2) -undersaturated conditions during quenching. The absence of carbonatite lavas at Hanang volcano is then explained by open system differentiation with primary melilititic magma replenishment preventing alkaline enrichment and carbonatite-silicate liquid immiscibility at depth as observed for the nephelinite-natrocarbonatite Oldoinyo Lengai volcano in the Natron Basin (150 km north).