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Abstract

The Jurassic Polaris Alaskan-type ultramafic-mafic intrusion is a 14 km by 3 km sill-like intrusion hosted by sedimentary and volcanic rocks of the Lay Range Assemblage in the Quesnel terrane of north-central British Columbia. Olivine, the dominant mineral in the intrusion, displays a wide range of textures and compositions that reflect different environments of crystallization. Olivine compositions (n = 326) determined by electronprobe microanalysis (EPMA) span a range of forsterite contents (Fo94-79), with the most Mg-rich olivine present in chromitite, due to subsolidus exchange with chromite, and most Mg-poor olivine present in olivine clinopyroxenite. Nickel concentrations in olivine vary significantly from 3108 ppm Ni at Fo89 to 275 ppm Ni at Fo81. The lack of systematic Fo-Ni-Mn zonation from core-to-rim in Polaris olivine is indicative of pervasive diffusion and re-equilibration at high temperatures. The Mg-rich compositions of olivine in dunite, olivine wehrlite, and werhlite (Fo87.5-92) are evidence for the involvement of primitive, mantle wedge-derived parent magmas that experienced little fractionation prior to emplacement in the crust. The Fo-Ni relationships of olivine are consistent with a fractional crystallization control from dunite-olivine werhlite-wehrlite (Fo87.5-92, 1605- 3108 ppm Ni) through magnetite-rich olivine clinopyroxenite (Fo79-84, 275-550 ppm Ni), whereas Ni-rich olivine in magnetite-poor olivine clinopyroxene (Fo81.5-84, 1305-2090 ppm Ni) is accounted for by mixing between fractionated and primitive magmas. Outcrop-scale field relationships (i.e., chaotic mixing, ultramafic dikes, chromitite schlieren) in the Polaris intrusion support the operation of periodic recharge and mixing processes. Polaris olivine is distinguished by extremely low Ca concentrations (