Phase relations of peralkaline silicic magmas and petrogenetic implications

The phase relationships of three peralkaline rhyolites from the Kenya Rift have been established at 150 and 50 MPa, at oxygen fugacities of NNO - 1.6 and NNO + 3.6 (log fO(2) relative to the Ni-NiO solid buffer), between 800 and 660 degreesC and for melt H2O contents ranging between saturation and n ominally anhydrous. The stability fields of fayalite, sodic amphiboles, che vkinite and fluorite in natural hydroux silicic magmas are established. Add itional phases include quartz, alkali feldspar, ferrohedenbergite, biotite, aegirine, titanite, montdorite and oxides. Ferrohedenbergite crystallizati on is restricted to the least peralkaline rock, together with fayalite; it is replaced at low melt water contents by ferrorichterite. Riebeckite-arfve dsonite appears only in the more peralkaline rocks, at temperatures below 7 50 degreesC (dry) and below 670 degreesC at H2O saturation. Under oxidizing conditions, it breaks down to aegirine. In the more peralkaline rocks, bio tite is restricted to temperatures below 700 degreesC and conditions close to H2O saturation. At 50 MPa, the tectosilicate liquidus temperatures are r aised by 50-60 degreesC, and that of amphibole by 30 degreesC. Riebeckite-a rfvedsonite stability extends down nearly to atmospheric pressure, as a res ult of its F-rich character. The solidi of all three rocks are depressed by 40-100 degreesC compared with the solidus of the metaluminous granite syst em, as a result of the abundance of F and Cl. Low fO(2) lowers solidus temp eratures by at least 30 degreesC. Comparison with studies of metaluminous a nd peraluminous felsic magmas shows that plagioclase crystallization is sup pressed as soon as the melt becomes peralkaline, whatever its CaO or volati le contents. In contrast, at 100 MPa and H2O saturation, the liquidus tempe ratures of quartz and alkali feldspar are not significantly affected by cha nges in rock peralkalinity, showing that the incorporation of water in pera lkaline melts diminished the depression of liquidus temperatures in dry per alkaline silicic melts compared with dry metaluminous or peraluminous varie ties. At 150 MPa, pre-eruptive melt H2O contents range from 4 wt % in the l east peralkaline rock to nearly 6 wt % in the two more peralkaline composit ions, in broad agreement with previous melt inclusion data. The experimenta l results imply magmatic fO(2) at or below the fayalite-quartz-magnetite so lid buffer, temperatures between 740 and 660 degreesC, and melt evolution u nder near H2O saturation conditions.