Experimental Constraints on the Origin of Icelandic Rhyolites

Abstract

Results of partial melting experiments on basalts and their metamorphic equivalents show that melts with the geochemical characteristics of Icelandic dacitic and rhyolitic lavas can form in water-undersaturated systems at below 1 kbar. The experiments also demonstrate that the residuum left after partial melting in water-poor systems will be plagioclase-rich and have pyroxene granulite mineralogy. Amphibole is stable only in water-saturated experiments with exceeding 1 kbar, but these yield melts with significantly higher and lower FeO contents compared to the Icelandic silicic lavas. Thus, petrogenetic models that require multi-stage, vapor-saturated melting of an amphibolite protolith with an amphibole-bearing residuum are untenable in light of the experimental evidence. It is concluded that dehydration melting of amphibolite, where only water of dehydration is available for the melting reactions, is the most probable mechanism for the generation of the Icelandic silicic suites.