Numerous 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 Impacts This 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.
Hotspot volcanism is thought to be caused by buoyantly upwelling plumes that rise from the deep mantle and melt beneath the lithosphere. The lavas erupted at hotspot volcanoes have unique chemical signatures which suggests that lavas derived from plumes can be distinguished from lavas derived from the shallow upper mantle. The Samoan hotspot is located ~100 km north of the northern terminus of the Tonga trench, where the Pacific plate subducts into the mantle. The juxtaposition of the Samoan plume with the northern terminus of the Tonga subduction zone offers a unique opportunity to explore how mantle flows around the edge of a subducting plate. In particular, the unique chemical signatures associated with the Samoan plume can be used as tracers to "map out" mantle flow around the subducting Tonga slab. In this project, we characterized the distribution the Samoan plume beneath the Lau Basin, which overlies the subducting Pacific plate. While a Samoan plume chemical signature has previously been identified at some volcanoes in the northern Lau Basin, the westward extent of the Samoan plume’s incursion into the region has not been explored. The goal of this proposal was to map the extent of the Samoan plume’s incursion into the region to the west of the subduction zone and to perform numerical geodynamic models to understand the evolution of the invading plume material as it transits from beneath the older, thicker Pacific lithosphere to the younger, thinner lithosphere of the Lau Basin. We identified evidence for a geochemically-enriched mantle component that we interpret to be associated with the Samoan plume in both seamount and back-arc basin lavas in the region of the northern North Fiji Basin, more than 1000 km to the west of the Samoan hotspot and the Tonga trench. We argue that the Samoan plume entered the region 4 million years ago, when the northern terminus of the Tonga trench was located more than 1000 km west of its current location. The new data are consistent with the broad distribution of a Samoan plume component throughout the mantle beneath the Lau Basin and the northern North Fiji Basin. The geodynamic modeling effort demonstrates that material derived from the Samoan plume will experience significant decompression melting as it passes from beneath the older, thicker Pacific lithosphere to the younger, thinner lithosphere of the Lau basin. We argue that this novel mechanism for melt generation could explain the high density of volcanic seamounts to the south of the boundary between the old Pacific plate and the young Niuafo’ou plate, as defined by the Vitiaz lineament. Additional geochemical sampling of these seamounts would allow this hypothesis to be fully tested. The project provided support to two early-career scientists, a geochemist and a geodynamicist, and fostered interdisciplinary collaboration between them. The project also supported a promising female graduate student and provided her with the resources to publish her first scientific paper as lead author, which details the geochemical results of the proposal. It also provided a funded summer research opportunity for an undergraduate from Colby College, who worked on the geodynamic modeling. The undergraduate presented the results of his research at two scientific conferences and has submitted a scientific paper detailing their work for publication (student is lead author). Finally, the project provided support for a Boston University undergraduate to perform geochemical research, resulting in a published scientific paper with the undergraduate as lead author.
