Mantle Xenolith Evidence for Lithospheric Thinning and Metasomatic Modification Beneath the Eastern Fiji Plateau: Implications for Samoan Plume Supply, Intraplate Volcanism, and Orocline Formation

We provide petrographic and geochemical characterisation of mantle xenoliths from Koro Island, eastern Fiji. Several lherzolite xenoliths display spinel–pyroxene symplectite textures, including preserved former grain boundaries, consistent with garnet breakdown that we attribute to thinning of a preexisting garnet‐facies lithospheric mantle beneath Fiji. Visible melt infiltration, particularly the growth of secondary clinopyroxene in Koro wehrlites, indicates intense interaction with metasomatic melts. Several geochemical characteristics (e.g., variable spinel Cr#–Ti trends and REE systematics inconsistent with a single process) cannot be explained by a single melt or fluid event, indicating overprinting by multiple metasomatic processes, dominated by silicate metasomatism. Arc‐related fluid geochemical signatures are less pronounced than in forearc peridotite xenoliths but resemble those in peridotite xenoliths from back‐arc mantle settings, despite Koro xenoliths being erupted by intraplate volcanism. We suggest that metasomatism was linked to the Samoan plume, whereas surface volcanism was most likely triggered by melt focusing through thinned mantle lithosphere. We build on previous findings by suggesting that the timing of Fiji's switch from arc to ocean island basalt (OIB)‐like volcanism at ∼3 Ma, coincident with the cessation of the Fiji Plateau's ∼135° counter‐clockwise rotation (e.g., Taylor et al. 2020), implies that rotation impeded ascent of OIB melts. Silicate melts at the base of the lithosphere likely facilitated the rotation, leading to the development of the Fiji–Lau Ridge Bend as a full‐lithosphere‐scale orocline.