Extreme Ni Concentrations in High Forsterite Olivine from a Cascade Forearc Basalt

We have evaluated olivine phenocrysts and host glass from a basalt lava flow and associated scoria cone in the Quartzville mining district located on the western forearc margin of the Oregon Cascades. Both lavas and scoria are contemporaneous determined through detailed field mapping, and they share similar petrological and geochemical characteristics. Ar-Ar dating indicates a lava eruption age of 82.3±3.1 ka. Minor clinopyroxene is also present as a phenocryst phase while the groundmass contains apatite, magnetite and plagioclase. Abundant Cr-spinel and lesser magnetite occur as mineral inclusions within olivine phenocrysts and olivine-hosted melt inclusions. Olivine compositions vary from Fo88 to Fo94, and are accompanied by Ni concentrations that range from 0.06 to 0.63 wt%, but average ~0.4 wt%. Ni abundances are positively correlated with olivine composition from Fo88-92. In this compositional range, olivines from the lavas tend to have higher Fo contents and consequently higher Ni concentrations, which is consistent with the idea that the scoria is slightly more evolved than the basaltic melt. At forsterite values above Fo92, however, Ni concentrations decrease to concentrations comparable to Fo88-92 olivines. Fo92-94 olivine compositions are predominantly located at the rims of the olivine phenocrysts. Olivine-hosted melt inclusion compositions were also determined to provide important constraints on volatiles and oxygen fugacity of the magma at time of entrapment. Oxygen fugacity was determined in melt inclusions through sulfur speciation measurements and ranged from ΔFMQ + 1.9 log units. This high fO2 is corroborated by oxygen fugacity calculations from spinel-olivine pairs. Additionally, oxygen isotope measurement on an olivine separate records a typical mantle value of δ18O = 5.6‰. Prior studies have identified high Ni olivine in calc-alkaline and highly potassic lavas and have proposed that these magmas are not derived from a peridotite source but rather a pyroxenite source. Additionally, the high Fo and Ni concentrations may be a function of the high oxygen fugacities as recorded by olivine-hosted melt inclusions and spinel-olivine pairs. In the present study we apply detailed petrography and geochemistry of mineral phases and glass to better constrain the source composition and magma evolution for this magmatic system. Our plan is to test our observations against the current hypothesis that high-Ni olivines can be produced from a progressively hybridized peridotite or pyroxenite source (Straub et al., 2008).