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

长期以来，人们一直认为地球是由一层薄薄的、富含二氧化硅的坚硬外壳和一层厚厚的、富含二氧化硅、富含镁的地幔岩石（即橄榄岩）以及一个镍铁内核组成的。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没有正常的海洋地壳，而是地幔直接暴露在北极、印度洋和大西洋大片地区的海底。究竟有多少海底是裸露的地幔目前尚不清楚，尽管据估计高达25%。这项研究由两艘船的美德合作项目的美国部分组成，该项目旨在绘制地图，收集重力和磁地球物理数据，并对西印度洋西南印度洋脊沿线的大片海底进行采样，以确定那里暴露的地幔岩石数量。在巡航后，将在岸基实验室进行主要元素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，加上法国正在进行的对西南印度海岭东部的研究，将第一次准确地估计出整个洋中海岭暴露的地幔数量。该地区80%以上的地区从未绘制过地图，采样主要局限于一个狭窄的3英里宽的裂谷的几个部分，该裂谷形成了西南印度洋脊，洋中脊扩展中心。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反应。这些反应产生氢气和甲烷，反过来，它们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，这些生物量目前还没有在我们星球上支持的生命清单中被考虑在内。海水和地幔岩石之间的反应还可能以碳酸盐矿物的形式将碳封存起来，碳酸盐矿物是通过从海水中去除二氧化碳而形成的，从而在不同的时间尺度上影响全球和大气的碳收支。这项工作的更广泛影响包括在三个机构培训研究生，支持来自科学领域性别代表性不足的早期职业科学家，向小学和高中推广，以及向公众推广。这项计划邀请一名高中教师和一名科学博主出海，他们将在海洋探险期间与学生进行现场互动，并准备适合学生年龄的教育材料和研究和海洋巡航的广播纪录片。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考察，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。本研究包括对西南印度海岭马里恩隆起顶部的海洋考察，以检验马里恩隆起是由现代洋脊下不同冈瓦南地幔省循环产生的横向地幔非均质所支持的假设，而不是由地幔柱引起的热异常。这是美国、德国和中国两个国际项目的第一站，该项目旨在研究马里恩平台及其起源。马里恩平台是海底的一个被研究过的部分。该巡航将使用多波束声纳绘制该地区的海底地图，将为随后的岸基实验室分析疏浚岩石，收集重力和磁地球物理数据，并对选定的样本进行地球化学分析。海面磁学将用于定位中心磁异常，以识别扩散中心并确定扩散速率的不对称性。岩浆海底扩张的区域将由它们的弱磁信号和缺乏容易确定的磁力线来确定。重力测量将允许计算残余地幔布格异常，从中可以根据其特征的负低点确定岩浆活跃区域。除了在轴上的工作外，这次考察还将包括广泛的离轴疏浚，以帮助划定地幔域和主要岩浆中心。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源的性质和起源。中国合作者将对疏浚的岩石进行主要元素和微量元素分析，美国实验室将分析Hf、Os的同位素和同位素比例，以及Sr、Nd和Pb的放射性同位素。利用地球化学测定的地幔密度梯度计算地幔，可以估计出暴露的地幔剖面中蛇纹岩化的程度。再加上处理过的磁化数据、收集到的岩石样本的岩性分析和多通道声纳测深技术，将用于构建该地区的地质图，以确定台地的构造演化和地壳结构。这项工作填补了大洋中脊的重要采样空白，将大大提高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。该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知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

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.