Peralkaline Nephelinite/Carbonatite Liquid Immiscibility: Comparison of Phase Compositions in Experiments and Natural Lavas from Oldoinyo Lengai

Abstract

New low-pressure, low-temperature experiments (50–375 MPa, 700–850°C) utilizing starting compositions of carbonated peralkaline nephelinite have generated immiscible carbonatite liquids that are alkali-rich. The results suggest the P-T conditions operating to produce the Oldoinyo Lengai natrocarbonatite lavas are quite low i.e. ≤100 MPa and ≤750°C. These low temperatures are consistent with both calculated (silicate) and measured (carbonate) eruption temperatures for lavas from Oldoinyo Lengai. The conjugate silicate liquids are of peralkaline wollastonite nephelinite composition. The differentiation of peralkaline wollastonite nephelinite strongly depends on both pressure and to what degree the melt is saturated in CO₂. High pressure coupled with CO₂ saturation favours formation of an immiscible alkalic carbonate liquid, which becomes increasingly K2O-rich with lower pressures of exsolution (P ≤ 200 MPa). Exsolution of an alkali-rich carbonate liquid from a silicate melt during further cooling and differentiation buffers the peralkalinity of the residual silicate liquid at near constant peralkalinity ([Na+K]/Al≈2.15). At very low pressure (P ≤ 35 MPa) and CO₂ undersaturated conditions, carbonate liquid is not exsolved, and wollastonite nephelinite fractionate to exceptionally peralkaline [Na+K]/Al=4.0–7.0) combeite-bearing nephelinite. Liquid natrocarbonatite and combeite nephelinite are not conjugate, but may be expected to be closely associated. Alkalic carbonate liquids exsolved at low temperatures (≤800 °C) are saturated with ferromagnesian solid phase(s) and are not superheated. Differentiation of alkalic carbonate melts (after these liquids have separated from their silicate host), consisting of fractionation of previously formed ferromagnesians, coupled with the precipitation of nyerereite+gregoryite produces a halogen-rich and SiO₂, TiO₂, A1₂O₃, MgO and FeO depleted natrocarbonatite magma.