Fluid inclusion studies in apatite from some East African carbonatites and ijolites.

Primary inclusions are present in apatites from carbonatites and ijolites of the Wasaki, Homa and Tororo complexes of E. Africa. These unique portions of crystallising fluids give valuable information about the nature of carbonatite and ijolite magmas. CO2 - rich, aqueous inclusions, which contain substantial amounts of alkalis, predominate in apatites from these carbonatites. The Na+:K+ weight ratio was found to be in the range 2.4-16.0. Significant amounts of ore metals are also present, as shown by the occurrence of minute, magnetic, crystalline specks within the inclusion fluid. It is concluded that the carbonatitic fluid from which the apatites crystallised, consisted of a highly mobile, CO2 - rich, aqueous saline fluid. Homogenisation temperatures, which represent the minimum formation temperature of the apatites, ranged from about 200°C to 485°C. The mode of homogenisation of one sample, the Uyi carbonatite, indicated that this fluid was supercritical. A higher estimate of the formation temperature (500-590°C) of the apatites from this sample was obtained from the solution temperature of a transient phase (probably Na2CO3) precipitated from the inclusion fluid at high temperatures during heating-runs. Melt inclusions predominate in apatites from the Wasaki ijolites. Carbonate-rich and silicate-rich inclusions are co-genetic in the apatites from some samples. Heating studies have shown that liquid immiscibility between carbonate-rich and silicate melts can occur in ijolitic magmas. Aqueous, CO2-rich, saline fluids may also occur in these magmas. Carbonatitic fluids can be derived from ijolitic magmas. The minimum formation temperature of the Wasaki ijolite apatites was in the range 680°C-950°C. Inclusion studies in apatites from the Homa ijolites showed that the apatites crystallised from a highly saline, aqueous fluid. The minimum formation temperature was in the range 458°C-232°C. Nahcolite (NaHCO3) is described in these aqueous inclusions. This is the first reported occurrence of this mineral in igneous rocks.