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Olivine in Hawaiian tholeiitic lavas have high NiO at given forsterite (Fo) contents (e.g., 0.25–0.60 wt% at Fo 88 ) compared to MORB (e.g., 0.10–0.28 wt% at Fo 88 ). This difference is commonly related to source variables such as depth and temperature of melting and/or lithology. Hawaiian olivine NiO contents are also highly variable and can range from 0.25–0.60 wt% at a given Fo. Here we examine the effects of crustal processes (fractional crystallization, magma mixing, diffusive re-equilibration) on the Ni content in olivine from Hawaiian basalts. Olivine compositions for five major Hawaiian volcanoes can be subdivided at ≥Fo 88  into high-Ni (0.25–0.60 wt% NiO; Ko‘olau, Mauna Loa, and Mauna Kea) and low-Ni (0.25–0.45 wt% NiO; Kllauea and Lō‘ihi), groups that are unrelated to major isotopic trends (e.g., Loa and Kea). Within each group, individual volcanoes show up to 2.5× variation in olivine NiO contents at a given Fo. Whole-rock Ni contents from Ko‘olau, Mauna Loa, Mauna Kea, and Kīlauea lavas overlap significantly and do not correlate with differences in olivine NiO contents. However, inter-volcano variations in parental melt polymerization (NBO/T) and nickel partition coefficients(   D   Ni    O  1   /   melt    )  ,    $(D^{\text O1/\text{melt}}_{\text{Ni}}),$caused by variable melt SiO 2 , correlate with observed differences in olivine NiO at Fo 90 , indicating that an olivine-free source lithology does not produce the inter-volcano groups. Additionally, large intra-volcano variations in olivine NiO can occur with minimal variation in lava SiO 2  and NBO/T. Minor variations in parental melt NiO contents (0.09–0.11 wt%) account for the observed range of NiO in ≥Fo 88  olivine. High-precision electron microprobe analyses of olivine from Kīlauea eruptions (1500–2010 C.E.) show that the primary controls on &lt;Fo 88  olivine NiO contents are fractional crystallization, magma mixing, and diffusive re-equilibration. Core-rim transects of normally zoned olivine crystals reveal marked differences in Fo and NiO zoning patterns that cannot be related solely to fractional crystallization. These Fo-NiO profiles usually occur in olivine with &lt;Fo 88  and are common in mixed magmas, although they are not restricted to lavas with obvious petrographic signs of mixing. Three-dimensional numerical diffusion models show that diffusive re-equilibration decouples the growth zoning signatures of faster diffusing Fe-Mg (Fo) from the somewhat slower Ni. This diffusive “decoupling” overprints the chemical relationships of Fe-Mg, Ni, and Mn inherited from crystal growth and influences the calculated fraction of pyroxenite-derived melt (Xpx). Sections of numerical olivine that have been affected by diffusive re-equilibration indicate that larger phenocrysts (800 μm along  c -axis) are &gt;50% more likely to preserve original Xpx compared to smaller phenocrysts (400 μm along  c -axis) which rarely (6%) recover original Xpx. Sections that are parallel or sub-parallel to the  c -axis and/or pass near the core of the crystal best preserve growth signatures. Thus, diffusive reequilibration, crystal size, and sectioning effects can strongly influence the characterization of mantle source lithologies for Hawaiian volcanoes.

Nickel variability in Hawaiian olivine: Evaluating the relative contributions from mantle and crustal processes