Liquid separation in the presence of vapor in synthetic fluid inclusions obtained from Na2CO3 solutions

Abstract

Phase relations in the Na2CO3–H2O system are of special interest for geochemistry in the context of the problems of carbonatite genesis and particularly Old� onyo Lengai sodium carbonate lavas. The solubility of anhydrous Na2CO3 at 25°С is 8.13 wt % and decreases practically to zero with temperature growth up to the critical point of water. Consequently, the constitution diagram for this system belongs to the second (P–Q) type for which critical phenomena proceed both in undersaturated and saturated (in two points P and Q) solutions. The temperature of salt melting at normal pressure is 851°С and increases 17°С per 1 kbar. The peculiarities of sodium carbonate solutions at high temperatures and pressures were studied by M.I. Rav� ich (up to 550°С and 1900 bar) by the fixation of the trend of the curves of pressure dependence on temper� ature and volume during the phase transition [1] and Koster van Groos by differential thermal analysis up to 925°С and 400–3700 bar [2]. The coordinates of the lower critical point P are slightly above the critical point of water. The position of upper critical point Q was determined by M.I. Ravich [1] as 480°С, 1600 bar, and 30–40 wt % Na2CO3. Koster van Groos gives the following coordinates of Q point: 500 ± 10°С, 1505 ± 0.01 kbar, and 12 ± 2 wt % Na2CO3 [2]. Both publica� tions contain no data on liquid separation at tempera� tures below the Q point. However, according to [1], two liquids in equilibrium, but not liquid and vapor as was reported in [2], were registered in the upper two� phase area. This should provide evidence for the fact that the upper section of the critical curve connecting the critical Q point with the critical point of salt is not a continuation of the lower section passing from the critical point of water to point P. Consequently, both