Experimental Studies of the System Na2CO3-CaCO3-MgF2 at 0{middle dot}1 GPa: Implications for the Differentiation and Low-temperature Crystallization of Natrocarbonatite

Oldoinyo Lengai (Tanzania) is the world’s only active carbonatite volcano and until recently has been characterized by the eruption of natrocarbonatite lavas consisting principally of nyerereite [Na2Ca(CO3)2] and gregoryite [(Na,Ca)2CO3] phenocrysts, in a mesostasis of nyerereite, gregoryite, halite–sylvite, fluorite, potassium neighborite [(Na,K)MgF3], and khanneshite. The pseudoternary system Na2CO3–CaCO3–MgF2 (NC–CC–MF) includes synthetic analogs of nyerereite (NY), gregoryite (NC) and neighborite (PV). Phase relationships along five pseudobinary joins in the subsystem NC–NY–MF have been determined at 0·1 GPa at diverse temperatures to establish liquidus phase relationships and elucidate liquid lines of descent. Additional experiments at subliquidus and subsolidus temperatures were undertaken to assess the crystallization sequence of natrocarbonatite magmas and predict the mineralogy of rocks formed under hypabyssal conditions within the volcano. Primary liquidus phases encountered in NC–NY–MF include gregoryite, nyerereite, calcite and neighborite. Along the join NC–MF there is a pseudoeutectic (NC + PV + L) at 20 wt % MgF2, and the subsolidus assemblage is NC + PV+ eitelite [Na2Mg(CO3)2]. This pseudoeutectic extends into the ternary system as a pseudocotectic leading to a pseudoternary eutectic, involving NC, NY and PV, with the approximate composition [NC49CC28(MgF2)23] at ∼525°C. In the more CaCO3-rich join NY–MF, the phase assemblage is characterized by the crystallization of calcite (CC) for compositions with >5 wt % MgF2, and PV at ∼45 wt % MgF2. In the ternary system NC–NF–MF pseudocotectics involving CC + NY and CC + PV terminate at a reaction point [∼575°C; ∼NY46CC30(MgF2)24], where CC reacts with liquid to form NY. Phase relations on other joins in the ternary system indicate that the NC–NY pseudocotectic terminates at the NC–NY–PV pseudoternary eutectic. For compositions with <5 wt % MgF2, this pseudocotectic is an odd reaction curve; for those with >5 wt % MgF2, this pseudocotectic is an even reaction curve. Subsolidus assemblages consist of NC, NY, PV, dolomite and eitelite. For natrocarbonatite magmas, our phase equilibrium data explain the typical crystallization of gregoryite before nyerereite, the crystallization of nyerereite before gregoryite, and why their differentiation leads towards magnesium enrichment and formation of the fluoroperovskite neighborite. Crystallization under hypabyssal conditions could result in gregoryite–nyerereite cumulates, and dolomite- and eitelite-bearing assemblages.

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