Stable isotope (oxygen-18/oxygen-16 and deuterium/hydrogen) studies of Cascade volcanic arc magmatism

Stable isotope ratios (18O/16O and D/H) potentially constrain origins of magma and volatile sources in igneous rocks. Modification of magmas by closed system processes (fractional crystallization and closed system devolatilization) or open-system processes (assimilation and fractional crystallization, magma mixing, degassing) affect 18 O/16O and D/H ratios in known ways. Magma degassing during volcanic eruptions and subsequent rehydration or alteration of groundmass glass reduces accuracy of whole rock stable isotope measurements, the traditional method of measuring glassy volcanic rocks. However, phenocrysts separated from fresh volcanic rocks may retain magmatic 18O/16 O and D/H values. Accordingly, oxygen isotopes in olivine, clinopyroxene, and plagioclase crystals were measured to determine whether Quaternary little-evolved mafic lavas of the Southernmost Cascades (SMC) represent only melts of heterogeneously contaminated mantle sources. In variably degassed silicic volcanic rocks from the 1980-1986 eruptions at Mount St. Helens (MSH) and the 1915 eruptions at Lassen Volcanic Center (LVC), hydrogen isotopes in amphibole and biotite phenocrysts were measured to evaluate shallow subvolcano magmatic processes. Magmas in each study are strongly influenced by crustal stress fields associated with each tectonic setting. The mantle source of the SMC little-evolved mafic lavas is heterogeneous, but these continental arc magmas acquire crustal contamination that reflects vent location across a region of extended heterogeneous crust. Hydrous phenocrysts in comparatively shallow, sill-like LVC silicic magma bodies recorded heating and devolatilization associated with the periodic injections of mafic magma. The broadly distributed crustal extension enables crystallizing silicic magmas to devolatilize as they are variably remobilized by heat and volatiles of recharging mafic magmas. At MSH, numerous small dacite magma pulses crowded into the narrow extensional volume below the vent. This focusing of all magmas and exsolved volatiles from a deeper main magma body produced the explosive May 18, 1980, eruption that also extensively damaged at least the upper 5 km of subvolcano plumbing. Through 1986, pulses of volatile-rich dacite magma degassed in a structurally and thermally evolving plumbing system. [Mineral data are located in separate Supplemental Data Files.