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Evolution of the sub-oceanic mantle lithosphere beneath the SW Pacific region

  • Author / Creator
    Barrett, Natasha
  • This thesis investigates the processes and evolution of the sub-oceanic lithospheric mantle beneath the Southwest (SW) Pacific region via two direct sampling methods: 1) basalt-hosted peridotite xenoliths erupted by volcanic eruptions from Koro Island in eastern Fiji, and 2) tectonically exposed sections of mantle lithosphere as ophiolite peridotites from the Marum ophiolite and Papuan Ultramafic Belt (PUB) in Papua New Guinea (PNG). Both studies are located in the SW Pacific region that formed via complex tectonic interactions between the Pacific and Indo-Australian plate, and collectively, provide information on the melt depletion and subsequent melt enrichment history of the underlying mantle lithosphere of this vast region. Chapter 2 reports new petrological, geochemical, highly siderophile element (HSE: Os, Ir, Ru, Pt, Pd, and Re), and Os isotope data from a suite of spinel peridotite xenoliths hosted in <3 Ma alkali basalts from Koro Island in eastern Fiji. These xenoliths are dominated by lherzolites, with a smaller population of harzburgites, dunites, and wehrlites. Residual signatures from orthopyroxene, olivine and the whole-rock chemistry correspond to partial melting of ~5% to <25%. Residual clinopyroxene is scarce but displays lower HREE than orthopyroxene when present, a disequilibrium feature providing evidence of former equilibration with garnet and pre-existing thickened lithosphere. The dominant metasomatic process recorded in these samples occurs as silicate melt-rock interaction that resulted in the production of wehrlites comprised of metasomatic clinopyroxene and olivines with lower forsterite (<Fo88) contents than typical mantle peridotite, likely formed at the expense of residual orthopyroxene. Whole-rock platinum group element (PGE) signatures reflect the addition of metasomatic sulfides (<30 μm) that have precipitated along grain boundaries and within crystalized late-stage clinopyroxene melts. The mean 187Os/188Os ratio for Koro peridotites is 0.1269 ± 0.0027 and overlaps with the most recent peak (~0.1260) observed in OIB-hosted peridotite xenoliths from Samoa, to the east of Fiji. The widespread silicate melt metasomatism and perturbed geothermal gradient beneath Koro support indications from seismology studies that the removal of back-arc lithosphere could have been linked to the ingress of Samoan plume material beneath Fiji as a result of slab rollback of the subducting Pacific Plate. Chapter 3 focuses on the formation of tectonite peridotites from the PUB and Marum ophiolites of PNG that represent some of the most melt-depleted mantle peridotites on Earth. The origin of these ultra-depleted mantle residues is re-examined through new major element, trace element, HSE and Os isotope data to better understand the timing and processes involved in the formation of such ultra-depleted mantle compositions. Comparison with residues produced from experimental studies and modelling indicate that the geochemistry of these mantle tectonites reflect ~25 to > 40 % partial melting of fertile mantle peridotite. The most melt-depleted examples of these peridotites are observed in the PUB harzburgite tectonites, and exceed levels of melting predicted from single-stage anhydrous melt experiments. While initial melting could have occurred under anhydrous conditions, the ultra-depleted compositions indicate that a water-fluxed melt environment is essential, at some stage in their evolution, to reach such extensive levels of melt depletion. The Marum harzburgite tectonite compositions are not as melt depleted as the PUB harzburgite tectonites, and do not necessitate hydrous conditions if produced at low pressures (≤2 GPa) and high temperatures (~1250‒1350 ⁰C). This evidence collectively points towards a two-stage melt evolution. The first melt stage most likely occurred during subduction initiation and production of FAB-like lavas, while the second stage, for some peridotites, involved a water-fluxed environment resulting in the production the extremely depleted peridotite residues of the PUB and eruption of the overlying boninite lavas. Unusual whole-rock HSE signatures in the Marum and PUB tectonites are characterized by very low Os, Ir and Pt abundances and do not reflect typical Ir-group (Ir, Os, Ru) PGE to Pt-group (Pd, Pt) PGE depletion patterns observed in large degree melt residues such as cratonic peridotites or the most depleted oceanic peridotites. Such HSE patterns may reflect dissolution of PGE-rich phases by melt-rock interaction, possibly by interaction with an aggressive subduction-related fluid. 187Os/188Os ratios reflect values within the range of modern convecting mantle, indicating that these very depleted compositions are not linked to ancient melt depletion events as commonly perceived.

  • Subjects / Keywords
  • Graduation date
    Spring 2021
  • Type of Item
    Thesis
  • Degree
    Doctor of Philosophy
  • DOI
    https://doi.org/10.7939/r3-rrzk-zm26
  • License
    This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.