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.

Mangaia hosts the most radiogenic Pb‐isotopic compositions observed in ocean island basalts and represents the HIMU (high µ = ²³⁸U/²⁰⁴Pb) mantle end‐member, thought to result from recycled oceanic crust. Complete geochemical characterization of the HIMU mantle end‐member has been inhibited due to a lack of deep submarine glass samples from HIMU localities. We homogenized olivine‐hosted melt inclusions separated from Mangaia lavas and the resulting glassy inclusions made possible the first volatile abundances to be obtained from the HIMU mantle end‐member. We also report major and trace element abundances and Pb‐isotopic ratios on the inclusions, which have HIMU isotopic fingerprints. We evaluate the samples for processes that could modify the volatile and trace element abundances postmantle melting, including diffusive Fe and H₂O loss, degassing, and assimilation. H₂O/Ce ratios vary from 119 to 245 in the most pristine Mangaia inclusions; excluding an inclusion that shows evidence for assimilation, the primary magmatic H₂O/Ce ratios vary up to ∼200, and are consistent with significant dehydration of oceanic crust during subduction and long‐term storage in the mantle. CO₂ concentrations range up to 2346 ppm CO₂ in the inclusions. Relatively high CO₂ in the inclusions, combined with previous observations of carbonate blebs in other Mangaia melt inclusions, highlight the importance of CO₂ for the generation of the HIMU mantle. F/Nd ratios in the inclusions (30 ± 9; 2σ standard deviation) are higher than the canonical ratio observed in oceanic lavas, and Cl/K ratios (0.079 ± 0.028) fall in the range of pristine mantle (0.02–0.08).