Constraining mantle flow between Samoa and the northern Lau and N. Fiji Basins with geochemistry and geodynamics

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Intellectual MeritNumerous geochemical and geophysical anomalies within the northern portion of the Lau basin--the presence of high 3He/4He (up to 28 times atmospheric), latitudinal gradients in trace element and isotopic (Sr-Nd-Pb) enrichment, and trench parallel shear wave splitting near the Tonga arc--have been attributed to southward flow of mantle material from the nearby Samoan hotspot. However, both the total extent and the spatial distribution of any intrusion of Samoan plume mantle into the region are poorly constrained. We propose to use a combination of geochemical analysis and geodynamic modeling to test two hypotheses regarding advection of material from the Samoan plume into the northern Lau basin: 1. The observed North-South gradients in Sr, Nd, and Pb isotopic ratios reflect progressive depletion of southward flowing Samoan-plume mantle. 2. The western extent of the Samoan plume's influence in the region extends into the northwestern Lau and North Fiji basins. We will perform geochemical analyses on samples from 12 key sites within the northwest Lau and northern North Fiji basins selected to fill important geographic voids in existing geochemical data sets. This study will geochemically characterize (He, Sr, Nd, and Pb isotopes and trace elements) a suite of 28 existing lava samples along a swath covering the northern region of the Lau and North Fiji basins. These data will greatly extend the western limit of geochemical observations in the region while simultaneously improving the resolution of North-South variations in isotopic ratios at the northern extreme of the northwest Lau and North Fiji basins. He isotopic measurements, which are the most unambiguous geochemical indicator of the presence of Samoan-plume mantle, will be particularly important for identifying the (currently unconstrained) western limit of Samoan intrusion. A series of numerical geodynamic experiments will be conducted to characterize melting of isotopically enriched Samoan-plume mantle by decompression as it flows southward across the boundary between old (100 Ma), thick Pacific oceanic lithosphere and the young (5 Ma), thin oceanic lithosphere of the Lau basin in the vicinity of the Vitiaz lineament. The resulting predicted spatial patterns in mantle depletion will be compared to the observed North-South geochemical gradients in the basins and the data obtained in this study to place constraints on the morphology of mantle flow in the region.Broader ImpactsThis research will support two early career scientists (Jackson, Hall), subsequently fostering interdisciplinary collaboration between them. It will also support a BU graduate student who will have the opportunity to work across disciplines by participating directly in all aspects of the proposed work. Hall and Jackson have both incorporated undergraduate students in their research in the past and will continue to do so with this project. This proposal includes funding to allow a BU undergraduate to participate in the geochemical aspect of the proposed research. The computational geodynamic model developed for this project will be made freely available for download through the BU Geophysics website. All data generated through this proposal will be treated in compliance of EAR Data Policy and will be disseminated to the broader community both through publication and archival in on-line databases.