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.

洋壳约占地球表面体积的65%，形成于大洋中脊，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特征被驱散，促进了大量潜在经济的矿藏和热液喷口的产生，这些矿藏和热液喷口拥有独特的化学合成生态系统，这些生态系统不需要阳光就能生存。在大洋中脊，来自上升地幔的熔体上升到表面，要么在海底以熔岩的形式喷发，要么在上升的过程中冻结，导致洋壳具有由辉长岩和橄榄岩制成的厚而粗的结晶底部，上面覆盖着一层玄武岩熔岩。除表层熔岩外，地壳的下部只能从构造特征或构造过程造成的海底深裂隙和裂隙中取样和研究。其中一个暴露辉长岩层的地方是东太平洋的一个叫皮托深的地方。这项研究涉及对皮托深部进行一次重大的多学科海洋考察，以采样和研究快速扩张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，还将采集样本以确定山脊轴线的古代热结构。这项工作的更广泛影响包括支持EPSCoR州(怀俄明州)的一个机构和教员；在大学、社区学院和K-12级别对学生进行良好的培训；在邮轮上包括一名K-12教师，以制作K-12教育材料并与陆地上的学生交流。该项目正在进行的科学工作有可能解决关于辉长岩下地壳如何在快速扩张的大洋中脊沉积的长期争论，并将阐明尚未解决的问题，即海底热液循环的深度、大洋中脊轴向岩浆室的宽度和时间演化以及地幔和海底之间的熔体输送和结晶过程。样品将使用远程操作工具采集，采样将沿着皮托深度约20公里长的流水线进行。皮托深部的近山脊垂直曝光跨越了多个地磁极性区间，为记录下地壳580°C化石等温线的形状，从而区分下地壳吸积模型的不同热预测提供了独特的机会。这些端元模型还预测了地球化学、岩石学和微构造参数随深度的变化。结合电子背散射衍射的岩石组构，将研究辉长岩上层相当大的垂直部分(1公里)的常量元素和微量元素地球化学，以进一步区分这些增长模式。虽然没有任何一次地壳暴露可能最终解决辉长岩下地壳的吸积过程，但Pito Deep的暴露提供了解决有关热液提取深度、轴向岩浆室的宽度和时间演化以及熔体运输和结晶过程的悬而未决的问题的可能性。