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，人们一直认为地球由一层薄薄的、富含二氧化硅的硬化地壳组成，上面覆盖着一层厚厚的贫硅、富镁的地幔岩石（称为橄榄岩）和内部的镍铁核。 与地球约 40 公里厚的大陆地壳相比，海洋地壳通常被认为相对较薄（即 6-7 公里厚）。过去 15 年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洋壳，而是地幔直接暴露在北极、印度洋和印度洋大片地区的海底。大西洋。目前尚不清楚海底有多少暴露的地幔，但据估计，这一比例高达 25%。这项研究包括美德两艘船合作项目的美国部分，该项目旨在绘制地图、收集重力和磁力地球物理数据，并对西印度洋西南印度洋脊沿线的大片海底区域进行采样，以确定如何许多地幔岩石暴露在那里。主量元素、微量元素和各种同位素的地球物理数据和地球化学分析将在巡航结束后在岸上实验室进行。这项工作的结果与法国正在进行的西南印度洋中脊东部研究相结合，将首次准确估计整个大洋中脊暴露的地幔数量。该地区超过 80% 的区域从未绘制过地图，采样主要局限于 3 英里宽的狭窄裂谷的几个部分，该裂谷形成了西南印度洋中脊和大洋中脊的扩张中心。这项工作很重要，因为地幔岩石在地球表面非常不稳定，尤其是在暴露于海水并与海水发生广泛反应的海底。这些反应产生氢气和甲烷，进而为深海细菌生命提供能量，并支持海底以下可能存在的大量生物量，而这些生物量目前尚未计入地球支持的生命清单中。 海水和地幔岩石之间的反应还可能以碳酸盐矿物的形式封存碳，这些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预算。这项工作的更广泛影响包括培训三个机构的研究生、支持科学界性别代表性不足的早期职业科学家、向中小学和公共宣传。该项目将派遣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，并准备适合年龄的教育材料和有关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很重要，其中一些科学家将参加这次探险，并与学生和美国教师密切互动，进一步与这些科学界建立国际关系。这项研究包括海洋学研究西南印度洋中脊马里恩隆起顶峰的探险验证了这一假设：马里恩隆起是由现代洋脊下方不同冈瓦纳地幔省循环产生的侧向地幔异质性所支持的，而不是地幔柱引起的热异常。这是美-德-中两阶段国际项目的第一阶段，该项目旨在研究马里恩平台（对海底的一部分及其起源进行了一些研究）。该游轮将使用多波束声纳绘制该地区的海底地图，疏浚岩石以供随后的岸基实验室分析，收集重力和磁性地球物理数据，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不对称性。非岩浆海底扩张的区域将由其微弱的磁信号和缺乏易于定义的磁线来确定。重力调查将允许计算残余地幔布格异常，从中可以根据其特征性的负低点来识别岩浆活跃的区域。除了轴上工作外，这次探险还将包括广泛的离轴疏浚，以帮助描绘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相学和地球化学分析，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岩石进行主量元素和微量元素分析，美国实验室将分析铪、锇的同位素和同位素比以及锶、钕和铅的放射性同位素。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部分中蛇纹石化的程度。结合处理后的磁化数据、收集的岩石样本的岩性分析和多通道声纳测深，将用于绘制该地区的地质图，以确定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，将显着增强我们对全球地幔变化的理解，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 NSF 的法定使命和通过使用基金会的智力价值和更广泛的影响审查标准进行评估，该项目被认为值得支持。

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脊龙旗融化异常的演化

• DOI: 10.1016/j.epsl.2019.116002
• 发表时间: 2020-02-01
• 期刊: Earth and Planetary Science Letters
• 影响因子: 5.3
• 作者: Xinghe Yu;H. Dick
• 通讯作者: H. Dick