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

知识产权优点Lau盆地北方部分内的许多地球化学和地球物理异常-存在高的3 He/4 He（高达大气的28倍），微量元素和同位素（Sr-Nd-Pb）富集的纬度梯度，以及汤加弧附近的海沟平行剪切波分裂-都归因于附近萨摩亚热点的地幔物质向南流动。然而，无论是总的程度和空间分布的任何侵入萨摩亚地幔柱到该地区的约束不足。我们建议使用地球化学分析和地球动力学模型相结合，以测试两个假设平流的物质从萨摩亚羽流到北方劳盆地：1。观察到的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地幔柱地幔的逐渐耗尽。2.萨摩亚羽流在该区域的影响向西延伸到劳西北部和北斐济盆地。我们将对来自劳西北部和北斐济盆地北方的12个关键地点的样品进行地球化学分析，以填补现有地球化学数据集中的重要地理空白。这项研究将对一套现有的28个熔岩样品进行地球化学表征（He、Sr、Nd和Pb同位素和微量元素），这些样品沿着覆盖Lau和北斐济盆地北方地区的一条狭长地带。这些数据将大大扩展该区域地球化学观测的西部界限，同时提高劳西北部和北斐济盆地北方极端同位素比率南北变化的分辨率。他的同位素测量，这是最明确的地球化学指标的存在萨摩亚地幔柱，将是特别重要的确定（目前不受约束）萨摩亚入侵的西部限制。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地幔柱在向南穿过老的（100 Ma）厚的太平洋岩石圈和年轻的（5 Ma）之间的边界时减压融化的特征，在Vitiaz线性构造附近的Lau盆地的薄的海洋岩石圈。由此产生的预测空间模式地幔耗尽将进行比较，观察到的南北地球化学梯度的盆地和在这项研究中获得的数据，以限制地幔流的形态在region.Broader ImpactsThis研究将支持两个早期的职业科学家（杰克逊，霍尔），随后促进他们之间的跨学科合作。它还将支持BU研究生谁将有机会通过直接参与拟议工作的各个方面跨学科的工作。霍尔和杰克逊过去都将本科生纳入他们的研究，并将继续这样做。该提案包括资助BU本科生参与拟议研究的地球化学方面。为该项目开发的计算地球动力学模型将通过BU地球物理学网站免费提供下载。通过这一建议产生的所有数据将按照《联合国数据政策》处理，并将通过出版和在线数据库存档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.