Phase Relationships of Hydrous Alkalic Magmas at High Pressures: Production of Nepheline Hawaiitic to Mugearitic liquids by Amhibole-Dominated Fractional Crystallization within the Lithospheric Mantle

Abstract

Experimental melting studies were conducted on a nepheline mugearite composition to pressures of 31 kbar in the presence of 0-30% added water. A temperature maximum in the near-liquidus stability of amphibole (with olivine) was found for a water content of 3·5 wt % at a pressure of 14 kbar. This is interpreted to have petrogenetic significance for the derivation of nepheline mugearite magmas from nepheline hawaiite by amphibole-dominated fractional crystallization at depth within the lithospheric mantle. Synthetic liquids at progressively lower temperatures range to nepheline benmoreite compositions very similar to those of natural xenolith-bearing high-pressure lavas elsewhere, and support the hypothesis that continued fractional crystallization could lead to high-pressure phonolite liquids. Independent experimental data for a basanite composition modeled on a lava from the same igneous province (the Newer Basalts of Victoria) permit the inference that primary asthenospheric basanite magmas undergo polybaric fractional crystallization during ascent, and may evolve to liquids ranging from nepheline hawaiite to phonolite upon encountering cooler lithospheric mantle at depths of 42-50 km. Such a model is consistent with the presence in some evolved alkalic lavas of both lithospheric peridotite xenoliths indicative of similar depths and of megacryst suites that probably represent disrupted pegmatitic segregations precipitated from precursor alkalic magmas in conduit systems within lithospheric mantle.