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

大量地球化学和地球物理异常——高3He/4He（高达大气的28倍）的存在，微量元素和同位素（Sr-Nd-Pb）的纬度梯度富集，以及汤加弧附近的海沟平行横波分裂——都归因于附近萨摩亚热点的地幔物质向南流动。然而，萨摩亚地幔柱侵入该地区的总体程度和空间分布都没有得到很好的限制。我们建议使用地球化学分析和地球动力学建模相结合的方法来验证关于萨摩亚羽流物质平流进入北劳盆地的两个假设：观测到的Sr、Nd和Pb同位素比值的南北梯度反映了向南流动的萨摩亚柱地幔的逐渐枯竭。2. 萨摩亚羽流在该地区的影响向西延伸到西北的劳盆地和北斐济盆地。我们将对劳盆地西北部和北斐济盆地北部的12个关键地点的样本进行地球化学分析，以填补现有地球化学数据集中的重要地理空白。本研究将对覆盖劳盆地北部和北斐济盆地的28个现有熔岩样品（He, Sr, Nd， Pb同位素和微量元素）进行地球化学表征。这些数据将大大扩展该地区地球化学观测的西界，同时提高西北劳盆地和北斐济盆地北端同位素比率南北变化的分辨率。同位素测量是萨摩亚地幔柱存在的最明确的地球化学指标，对于确定（目前不受限制的）萨摩亚侵入的西部界限将特别重要。将进行一系列数值地球动力学实验，以描述同位素富集的萨摩亚柱地幔在向南流过维提亚兹（Vitiaz）沉积层附近的老（100 Ma）、厚的太平洋岩石圈和年轻（5 Ma）、薄的海洋岩石圈之间的边界时通过减压而熔化的特征。由此预测的地幔衰竭空间格局将与观测到的盆地南北地球化学梯度和本研究获得的数据进行比较，以限制该地区地幔流动的形态。更广泛的影响这项研究将支持两位早期职业科学家（杰克逊，霍尔），随后促进他们之间的跨学科合作。它还将支持一名波士顿大学研究生，该研究生将有机会通过直接参与拟议工作的各个方面来开展跨学科工作。霍尔和杰克逊在过去的研究中都有本科生参与，并将在这个项目中继续这样做。该提案包括资助波士顿大学本科生参与拟议研究的地球化学方面。为这个项目开发的计算地球动力学模型将通过BU地球物理网站免费提供下载。通过本提案产生的所有数据将按照EAR数据政策进行处理，并将通过出版物和在线数据库存档的方式向更广泛的社区传播。

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.