Collaborative Research: Thin Crust Over the Marion Rise: Remelting the Gondwanan Mantle

合作研究：马里恩海隆上的薄地壳：冈瓦纳地幔的重熔

• 批准号: 1657983
• 负责人: Masako Tominaga
• 金额: $ 63.98万
• 依托单位: Woods Hole Oceanographic Institution
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2018
• 资助国家: 美国
• 起止时间: 2018-10-01 至 2024-09-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1657983&HistoricalAwards=false
• 关键词: Collaborative; Research; Thin; Crust; Over

It has long been assumed that the Earth consists of a thin, outer, silica-rich, hardened crust overlying a thick layer of silica-poor, magnesium-rich, mantle rock known as peridotite and an inner, nickel-iron core. Compared to Earth's ~40 kilometer thick continental crust, ocean crust is generally considered to be relatively thin (i.e., 6-7 km thick). One of the most exciting discoveries in ocean sciences over the last 15 years has been the discovery that parts of the seafloor do not have normal ocean crust, but rather Earth's mantle is exposed directly on the seafloor over large regions of the Arctic, Indian, and Atlantic Oceans. Just how much of the seafloor is exposed mantle not presently known, although estimates have been made that predict up to 25%. This research comprises the US portion of a two-ship, US-German, collaborative project designed to map, collect gravity and magnetic geophysical data, and sample a large areas of the seafloor along the Southwest Indian Ridge in the western Indian Ocean to determine how much mantle rock is exposed there. Geophysical data and geochemical analysis of major and trace elements and various isotopes will be carried out post-cruise at shore-based laboratories. The results of this work, combined with ongoing French studies on the eastern portion of the southwest Indian Ridge will allow, for the first time, an accurate estimate of how much mantle is exposed along an entire mid-ocean ridge. More than 80% of this region has never been mapped and sampling has been largely restricted to a few sections of a narrow 3-mile-wide rift valley that forms the southwest Indian Ridge mid-ocean ridge spreading center. This work is important because mantle rock is very unstable at the Earth's surface, particularly on the seafloor where it is exposed to and extensively reacts with seawater. These reactions produce hydrogen and methane, which, in turn, provide energy for bacterial life in the deep sea and support what could be an extensive biomass below the seafloor that is presently not accounted for in the inventory of life supported by our planet. The reactions between seawater and mantle rock also potentially sequester carbon in the form of carbonate minerals that form by the removal of CO2 from seawater, thereby affecting global and atmospheric carbon budgets at different time scales. Broader impacts of the work include training of graduate students at three institutions, support of an early career scientist from a gender under-represented in the sciences, outreach to elementary and high schools, and public outreach. This project takes to sea a high school teacher and a scientific blogger, who will conduct live interaction sessions with students during the oceanographic expedition and who will prepare age-appropriate educational materials and radio documentaries of the research and oceanographic cruise. There is also a significant component of international collaboration with German, Italian, and Chinese scientists some of whom will participate in the expedition and interact closely with the students and US faculty, further building international relations with these scientific communities.This research consists of an oceanographic expedition to the crest of the Marion Rise on the Southwest Indian Ridge to test the hypothesis that that the Marion Rise is supported by lateral mantle heterogeneity produced by the recycling different Gondwanan mantle provinces beneath the modern ocean ridge, as opposed to a thermal anomaly due to a mantle plume. This is the first leg of a two-leg US-German-Chinese international program to study the Marion platform, a little studied part of the seafloor, and its origin. The cruise will use multibeam sonar to map the seafloor in the area, will dredge rocks for later shore-based laboratory analysis, collect gravity and magnetic geophysical data, and carry out geochemical analyses of select samples. Sea surface magnetics will be used to locate central magnetic anomalies to identify spreading centers and determine spreading rate asymmetries. Regions of a amagmatic seafloor spreading will be determined by their weak magnetic signal and lack of easily definable magnetic lineations. Gravity surveys will allow calculation of residual mantle Bouguer anomalies from which magmatically robust regions can be identified on the basis of their characteristic negative lows. In addition to on-axis work, the expedition will include extensive off-axis dredging to help delineate mantle domains and major magmatic centers. Post-cruise research includes petrographic and geochemical analysis of collected seafloor basalts and peridotites to determine the nature and origin of the mantle source in the Marion Rise area. Chinese collaborators will conduct major and trace element analyses of dredged rocks and US laboratories will analyze the isotopes and isotopic ratios of Hf, and Os as well as radiogenic isotopes of Sr, Nd, and Pb. Calculation of the mantle using geochemically determined mantle density gradients will estimate the extent of serpentinization in the exposed mantle sections. This, together with processed magnetization data, lithologic analysis of collected rock samples, and multi-channel sonar bathymetry will be used to construct a geologic map of the area to determine the tectonic evolution and crustal architecture of the Platform. This work fills a significant sampling gap on mid-ocean ridges and will enhance significantly our understanding of global mantle variability as well as provide new insights into the nature of shallow mantle convection and on the origin of mantle hotspots.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.

长期以来，人们一直认为，地球是由一层薄薄的、外层富含二氧化硅的坚硬地壳和一层厚厚的贫硅、富镁的地幔岩石组成的，这层地幔岩石被称为橄榄岩，而内部则是镍铁核。与地球约40公里厚的大陆地壳相比，海洋地壳通常被认为是相对较薄的(即6-7公里厚)。在过去的15年里，海洋科学中最令人兴奋的发现之一是，发现部分海底并不具有正常的洋壳，而是地球的地幔直接暴露在北冰洋、印度洋和大西洋大片区域的海底。到底有多少海底暴露在地幔之外，目前尚不清楚，尽管已有估计高达25%。这项研究包括美国和德国两艘船合作项目的美国部分，该项目旨在绘制地图，收集重磁地球物理数据，并对西印度洋西南印度洋海脊沿线的大片海底进行采样，以确定那里暴露了多少地幔岩石。巡航后将在岸基实验室进行主要和微量元素以及各种同位素的地球物理数据和地球化学分析。这项工作的结果，结合法国正在进行的对西南印度洋海脊东部的研究，将首次能够准确估计沿着整个大洋中脊暴露出多少地幔。该地区80%以上的地区从未被绘制过地图，采样主要局限于一个狭窄的3英里宽的裂谷的少数部分，该裂谷形成了西南印度洋海脊中脊的扩张中心。这项工作很重要，因为地幔岩石在地球表面非常不稳定，特别是在海底，它暴露在海水中并与海水发生广泛的反应。这些反应产生氢气和甲烷，进而为深海中的细菌生命提供能量，并支持海底下可能存在的大量生物量，目前我们的地球支持的生命清单中没有考虑到这些生物量。海水和地幔岩石之间的反应还可能以碳酸盐矿物的形式隔离碳，碳酸盐矿物是通过从海水中去除二氧化碳而形成的，从而影响不同时间尺度的全球和大气碳收支。这项工作的更广泛影响包括在三个机构培训研究生，支持一名来自科学界代表性不足的性别的早期职业科学家，扩大到小学和高中，以及公共宣传。该项目邀请了一名高中教师和一名科学博客作者出海，他们将在海洋考察期间与学生进行现场互动，并将准备适合年龄的研究和海洋巡航的教育材料和广播纪录片。与德国、意大利和中国科学家的国际合作也是一个重要组成部分，他们中的一些人将参加这次探险，并与学生和美国教师密切互动，进一步建立与这些科学界的国际关系。这项研究包括一支前往西南印度洋海脊马里恩隆起顶端的海洋探险，以检验马里恩隆起是由现代洋脊下不同冈德瓦南地幔区域的再循环产生的侧向地幔不均一性支持的假设，而不是地幔热柱造成的热异常。这是美国-德国-中国两个国际项目的第一阶段，目的是研究马里恩平台及其起源，马里恩平台是海底的一部分，研究得很少。邮轮将使用多波束声纳绘制该地区的海底地图，将为稍后的岸基实验室分析疏浚岩石，收集重磁地球物理数据，并对选定的样品进行地球化学分析。海面磁学将被用来定位中央磁异常，以识别扩散中心并确定扩散速率不对称性。岩浆海底扩张的区域将由其微弱的磁信号和缺乏容易定义的磁性线条所决定。重力测量将允许计算剩余的地幔布格异常，从这些异常中，可以根据其特征的负低来确定具有魔力的强健区域。除了轴上工作，探险队还将包括广泛的轴外疏浚，以帮助圈定地幔区域和主要的岩浆中心。巡航后的研究包括对收集的海底玄武岩和橄榄岩进行岩石学和地球化学分析，以确定马里恩隆起地区地幔来源的性质和来源。中国合作者将对疏浚岩石进行主要和微量元素分析，美国实验室将分析Hf、Os以及锶、钕和铅的放射性同位素和同位素比率。使用地球化学确定的地幔密度梯度计算地幔将估计暴露的地幔剖面中蛇纹岩化的程度。这将与处理后的磁化数据、采集的岩石样品的岩性分析和多通道声纳测深一起用于构建该地区的地质图，以确定该平台的构造演化和地壳结构。这项工作填补了大洋中脊的重大采样空白，将显著增强我们对全球地幔变异性的理解，并为浅地幔对流的性质和地幔热点的起源提供新的见解。这一奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

项目成果

Plate-Driven Micro-Hotspots and the Evolution of the Dragon Flag Melting Anomaly, Southwest Indian Ridge

板块驱动的微热点和西南印度洋中脊龙旗融化异常的演化

• DOI: 10.1016/j.epsl.2019.116002
• 发表时间: 2020
• 期刊: Earth and Planetary Science Letters
• 影响因子: 5.3
• 作者: Yu Xing;Dick H.J.B.
• 通讯作者: Dick H.J.B.