Tracing the Origin and Evolution of the Lithospheric Mantle Beneath the Fijian Archipelago — Insights from Koro Island Peridotite Xenoliths

The southwest Pacific volcanic Fijian islands formed via complex tectonic interactions between the Pacific and Indo-Australian plates. The mantle underpinning the Fijian archipelago, however, remains an uncharacterized portion of the oceanic lithosphere that could provide important information for deciphering the melt and enrichment history of the sub-oceanic mantle, and its tectonic regime.

We report new petrological, geochemical, platinum group element (PGE), and Os isotope data from a suite of spinel peridotite xenoliths hosted in < 3 Ma alkali basalts from Koro Island in eastern Fiji. The lack of hydrous phases, along with spinel fO2 values ranging from moderately reduced to slightly oxidized (FMQ -1.1 to 0.74) rule out typical fore-arc mantle signatures, but have similarities to both back-arc and abyssal peridotites, suggesting a multi-stage melt and re-enrichment history of interactions involving a MORB(BABB) like source. Melt-rock interaction also resulted in the production of wehrlites comprised of metasomatic clinopyroxene and low Fo ol (<Fo88) formed at the expense of residual orthopyroxene. In the lherzolites and harzburgites, residual clinopyroxene is scarce. When evident, it has low HREE_N, suggesting former equilibration with garnet.

Whole-rock PGE signatures reflect the addition of metasomatic sulfides (< 30 μm) that have precipitated along grain boundaries and within crystalized late-stage clinopyroxene melts. ¹⁸⁷Os/¹⁸⁸Os ratios range from 0.1201 to 0.1306, and show no relationship with typical melt depletion indices (i.e. Al₂O₃ wt. %, Cr# in spinel and Fo content in olivine), reflecting convecting mantle heterogeneity. The collective evidence suggests this mantle lithosphere accreted during back-arc development and played a role in enhancing melt production, however, the absence of arc signatures in the surface volcanics and shallow lithosphere indicate that the pre-existing lithosphere has since been removed and/or undergone significant metasomatic modification.