Collaborative Research: Tracking the thermal and petrologic evolution of magmatically robust fast spread lower ocean crust

合作研究：追踪岩浆强劲、快速扩张的下洋地壳的热学和岩石学演化

• 批准号: 1459387
• 负责人: Jeffrey Gee
• 金额: $ 27.51万
• 依托单位: University of California-San Diego Scripps Inst of Oceanography
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2015
• 资助国家: 美国
• 起止时间: 2015-06-15 至 2021-05-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1459387&HistoricalAwards=false
• 关键词: Collaborative; Research; Tracking; thermal; petrologic

Ocean crust, which makes up about 65% of the volume of Earth's surface, is created at mid-ocean ridges, one of the most dynamic geological environments on the planet. Vast quantities of heat are dissipated along these features, facilitating the generation of large, potentially economic, mineral deposits and hydrothermal vents that host unique chemosynthetic ecosystems that live without the need for sunlight. At mid-ocean ridges, melts from the upwelling mantle rise toward the surface and either erupt as lavas on the seafloor or freeze on their way up, resulting in ocean crust that has a thick coarsely crystalline base made of gabbroic and peridotitic rock covered by a veneer of basaltic lava. With the exception of the surface lavas, the lower portions of crust can only be sampled and studied from deep rips and tears in the seafloor caused by structural features or tectonic processes. One of the places where the gabbroic layer is exposed is a place called the Pito Deep in the eastern Pacific Ocean. This research involves a major multidisciplinary oceanographic expedition to the Pito Deep to both sample and study the lower crustal gabbros of fast spread ocean crust and their relation to overlying basalts and underlying crustal components. On the expedition, samples will also be taken to determine the ancient thermal structure at the ridge axis. Broader impacts of the work include support of an institution and faculty in an EPSCoR state (Wyoming); good student training at the college, community college, and K-12 level; and the inclusion of a K-12 teacher on the cruise to produce educational materials for K-12 and communicate with students back on land. The science being done in this project has the potential to resolve long standing debates about how gabbroic lower crust is accreted at fast spread mid-ocean ridges and will shed light on unresolved questions regarding the depth of hydrothermal circulation in the seafloor, the width and temporal evolution of the axial magma chamber of mid-ocean ridges, and the processes of melt transport and crystallization between the mantle and the seafloor. Samples will be collected using a remotely operated vehicle, with sampling taking place along a ~20-km long flow line at Pito Deep. The nearly ridge-perpendicular exposures at Pito Deep span multiple geomagnetic polarity intervals, providing a unique opportunity to document the shape of the fossil 580°C isotherm in the lower crust and thus to differentiate between the different thermal predictions of lower crustal accretion models. These end-member models also make predictions of the variation of geochemical, petrologic and microstructural parameters with depth. Major and trace element geochemistry combined with petrofabrics from electron backscattered diffraction will be examined for a significant vertical portion (1km) of the upper gabbroic layer to further discriminate between these accretion models. While no single crustal exposure is likely to definitively resolve the processes by which the gabbroic lower crust is accreted, the exposures at Pito Deep offer the possibility of addressing outstanding questions regarding the depth of hydrothermal heat extraction, the width and temporal evolution of the axial magma chamber, and the processes of melt transport and crystallization.

海洋地壳占地球表面体积的65%，形成于海洋中脊，这是地球上最具活力的地质环境之一。 大量的热量沿着这些地貌消散，促进了具有潜在经济价值的大型矿床和热液喷口的形成，这些矿床和热液喷口拥有独特的化学合成生态系统，不需要阳光就能生存。 在洋中脊，来自上涌地幔的熔融物上升到表面，要么在海底喷发成熔岩，要么在上升的过程中冻结，形成了由辉长岩和橄榄岩组成的厚而粗糙的结晶基底，上面覆盖着玄武岩熔岩。 除表层熔岩外，地壳下部只能从海底结构特征或构造过程造成的深裂和撕裂中取样和研究。 辉长岩层暴露的地方之一是东太平洋的皮托深海。这项研究涉及对皮托深海进行一次重大的多学科海洋学考察，以取样和研究快速扩展的洋壳的下地壳辉长岩及其与上覆玄武岩和下伏地壳成分的关系。在这次考察中，还将采集样品以确定脊轴处的古代热结构。 这项工作的更广泛的影响包括支持EPSCoR州（怀俄明州）的一个机构和教师;在大学，社区大学和K-12水平的良好学生培训;以及在游轮上包括一名K-12教师，为K-12制作教育材料，并与陆地上的学生交流。该项目正在进行的科学研究有可能解决长期存在的关于辉长岩下地壳如何在快速扩展的洋中脊增生的争论，并将揭示有关海底热液循环深度、洋中脊轴向岩浆房的宽度和时间演变以及地幔与海底之间的熔体输送和结晶过程等未决问题。将使用遥控潜水器采集样本，采样将在Pito Deep沿沿着约20公里长的流线进行。皮托深海的近脊垂直暴露跨越多个地磁极性间隔，提供了一个独特的机会来记录下地壳中化石580°C等温线的形状，从而区分下地壳增生模型的不同热预测。这些端元模型还预测了地球化学、岩石学和显微构造参数随深度的变化。主要和微量元素地球化学结合电子背散射衍射的岩石组构将检查一个显着的垂直部分（1公里）的上部辉长岩层，以进一步区分这些吸积模型。虽然没有一个单一的地壳暴露是可能明确解决的辉长岩下地壳增生的过程中，暴露在皮托深提供了解决悬而未决的问题，热液热提取的深度，轴向岩浆房的宽度和时间演变的可能性，熔体运输和结晶的过程。