Oxygen isotope geochemistry of mafic phenocrysts in primitive mafic lavas from the southernmost Cascade Range, California

Previously reported whole-rock δ¹⁸O values (5.6–7.8‰) for primitive quaternary mafic lavas from the southernmost Cascades (SMC) are often elevated (up to 1‰) relative to δ¹⁸O values expected for mafic magmas in equilibrium with mantle peridotite. Olivine, clinopyroxene, and plagioclase crystals were separated from 29 geochemically well-characterized mafic lavas for δ¹⁸O measurements by laser fluorination to assess modification of the mantle sources by ancient and modern subducted components. Oxygen isotope values of olivine phenocrysts in calc-alkaline lavas and contemporaneous high alumina olivine tholeiitic (HAOT) lavas generally exceed depleted mantle olivine values (~4.9–5.3‰). Modern addition of up to 6 wt% slab-derived fluid from Gorda serpentinized peridotite dehydration (~15‰) or chlorite dehydration (~10‰) within the serpentinized peridotite can provide the ¹⁸O enrichment detected in olivine phenocrysts (δ¹⁸O_olivine = 5.3–6.3‰) in calc-alkaline mafic lavas, and elevate ¹⁸O in overlying mantle lithosphere, as well. Specifically, although HAOT δ¹⁸O_olivine values (5.5–5.7‰) may reflect partial melting in heterogeneous ¹⁸O enriched mantle source domains that developed during multiple subduction events associated with terrane accretion (e.g.,