Two Contrasting H2O-rich Components in Primary Melt Inclusions from Mount Shasta

Abstract

In addition to the abundant andesite and dacite lavas of the Mt. Shasta stratocone, primitive mafic lavas have erupted during the Quaternary from the Mt. Shasta region. Two types of basic lavas are the focus of this study: nearly anhydrous high-aluminium olivine tholeiites, produced by decompression melting, and basaltic andesites, produced by melting of a metasomatized mantle source (i.e. modified by various amounts of H2O-rich components). Here we provide further insight into the origin and the coexistence of these two types of magma based on a detailed study of the dissolved volatile contents in melt inclusions trapped in magnesium-rich olivine crystals from both tholeiites and basaltic andesites. The melt inclusions fall on the primitive extension of their respective host lava compositions: (1) the tholeiite melt inclusions are nearly anhydrous melts with low and clustered volatiles compositions; (2) the basaltic andesite melt inclusions are ‘wet’ melts with higher and more variable volatile contents (up to 2·6 wt% H2O, 820 ppm CO2, 1270 ppm Cl, 1220 ppm F and 6280 ppm S) than those of the tholeiite melt inclusions. The basaltic andesite melt inclusions are enriched in fluid-mobile elements and in incompatible trace elements: they display a stronger signature of slab-derived components than their host lavas. The selective enrichment of the basaltic andesite melt inclusions in fluid-mobile elements such as K, F, Cl, Ba and B as well as their contrasting δ11B (from −10·2 ± 1·3‰ to −3·2 ± 0·9‰ for the melt inclusions from sample 95-15 and from −4·9 ± 1·1‰ to +4·4 ± 1·1‰ for the melt inclusions from samples 85-1a and 85-47) show the imprints of two distinct slab-derived components C1 and C2. Using trace element ratios, we modelled the compositions of these two H2O-rich components (C1 with Cl/F of 1·1 is rich in both in high field strength elements and incompatible trace elements, whereas C2 with Cl/F of 4·0 is poorer in trace elements) and show that they represent mixing between sediment melts and dehydration fluids from the altered oceanic crust.