Insight into differentiation in alkalic systems: Nephelinite-carbonate-water experiments aimed at Ol Doinyo Lengai carbonatite genesis

Ol Doinyo Lengai (ODL, Tanzania, East African Rift) is the only known volcano currently erupting carbonatite on Earth with 30 yr. cycles alternating between quiescent effusion and explosive, compositionally-zoned silicate eruptions. We performed isothermal crystallization thermal gradient experiments involving ODL nephelinite, Na 2 CO 3 H O to understand magmatic differentiation in this system using SEM-EDS x-ray analysis, tomography, SIMS LA-ICPMS characterize samples. Isothermal document that hydrous liquids coexist nepheline+feldspar; as peralkalinity increases, temperatures decrease. Presence of increases solubility water liquid. Experiments placing nephelinite O+ a 1,000–350°C show rapid reaction occurs, resulting virtually melt-free mineral aggregates having layering reflecting systematic throughout capsule. Both types argue continuous interconnected melt exists over large temperature range alkalic systems allowing for reactive mush zone process. Liquid compositions change from carbonate-water bearing nephelinites at high down carbonate <400°C. propose model eruption behavior: 1) magmas pond build sill complex downward time; 2) melts form buoyantly rise, ultimately natrocarbonatites observed; 3) contents up bottom complex, eventually leading vapor saturation explosive The accounts cycling unusual compositional zoning tephras.