Experimental Constraints on the Relationships between Peralkaline Rhyolites of the Kenya Rift Valley

Abstract

Crystallization experiments on three comendites provide evidence for the genetic relationships between peralkaline rhyolites in the central Kenya rift valley. The crystallization of calcic clinopyroxene in slightly peralkaline rhyolites inhibits increase in peralkalinity by counteracting the effects of feldspar. Fractionation under high fO2 conditions produces residual liquids that are less, or only slightly more, peralkaline than the bulk composition. In contrast, crystallization under reduced conditions (<FMQ, where FMQ is the fayalite–magnetite–quartz buffer) and at high fF2 inhibits calcic clinopyroxene and yields residual liquids that are more peralkaline than coexisting alkali feldspar, whose subsequent crystallization increases the peralkalinity of the liquid. A marginally peralkaline rhyolite [molar (Na2O + K2O)/Al2O3 (NK/A) = 1·05] can yield a more typically comenditic rhyolite (NK/A = 1·28) after 95 wt % of crystallization. This comendite yields pantelleritic derivatives (NK/A >1·4) after 25 wt % crystallization. Upon further crystallization, extreme peralkaline compositions (NK/A ≤2·5) are obtained, with relatively low SiO2 (66 wt %) and Al2O3 (7·4 wt %), and high FeO (10·2 wt %) and Na2O (8·4 wt %) contents. In the absence of crystallization of sodic phases such as arfvedsonite or aegirine, fractionation may yield even more extreme compositions. Pantelleritic rhyolites can be produced at temperatures below 800°C, at low fO2, high fF2, by either extreme fractional crystallization or near-solidus melting of less peralkaline, but more silicic, sources.