Using Loihi Basaltic Rocks to Understand the Hawaiian Plume

热点火山被认为可以挖掘地球的深层地幔，并为理解其组成和过程提供基本的见解。夏威夷是研究热点熔岩的绝佳场所，因为它远离任何大陆或构造板块边界（2500公里），其热点的高温（1500摄氏度）和体积输出（100，000立方米/年）。该计划的重点是Loihi，这是夏威夷最年轻的水下火山。Loihi熔岩有可能提供重要的约束下的地幔柱的岛屿，因为：1）火山的青年（preshield阶段）允许其地幔源的表征之前，它已被广泛熔化。2)Loihi玄武岩是夏威夷玄武岩的关键地球化学端员之一。因此，分析其熔岩（特别是来自以前未采样的地区）将提供一个独特的视角融化过程和来源的异质性在夏威夷热点，地球上最热的地幔柱。Loihi熔岩因其原始惰性气体特征而特别值得注意。3)Loihi的内部和早期历史暴露在其东侧的主要滑坡（1公里厚的地层剖面）和山顶约300米深的陨石坑中。4)潜水器收集的大量洛伊希玄武岩（196个样本）可从其山顶坑陨石坑、其切开的东侧和其两个裂谷带的全部范围获得。这些样品中的大多数都得到了有限的地球化学表征。这些样品将提供给地球科学界用于其他地球化学研究。 该项目支持在夏威夷大学培训一名研究生。该项目的目标是：1）使用Loihi熔岩确定夏威夷盾状火山的寿命，以记录早期阶段的持续时间，这是解决这一问题的主要难题。目前的估计相差很大。2)更好地了解夏威夷烟羽的结构（例如，它是否如目前所认为的那样是双边不对称的）。Loihi位于LOA趋势的年轻端，但矛盾的是，它与KEA趋势熔岩具有相同的地球化学特征。3)评估夏威夷地幔柱源是否随时间和空间变化。来自Loihi深层内部和海底侧翼的熔岩将为夏威夷羽流的Loihi组成端成员提供新的视角。4)确定地壳污染如何在夏威夷海底火山工作。裂谷带岩浆是否绕过山顶岩浆库，更忠实地保存了地幔源的特征？先前对洛伊希山顶熔岩的研究表明，它们被地壳污染广泛改变，而南部裂谷深处熔岩的稀有气体成分表明它们没有受到污染。利用新一代的地球化学仪器确定年龄、地壳和地幔过程以及来源变化，对洛伊希玄武岩进行地球化学表征的时机已经成熟。这些分析将为夏威夷热点的运作提供基本的见解。新的具有地球化学特征的Loihi样本，全部由潜水器精确定位和收集，将提供给地球科学界用于未来的其他补充地球化学研究（例如，Os同位素、铂族元素、稀有气体）。

Hotspot volcanoes are thought to tap the Earth's deep mantle, and to provide fundamental insights into understanding its composition and processes. Hawaii is an excellent venue for studying hotspot lavas because of its distant location from any continent or tectonic plate boundary (2500 km), and the high temperature (1500 degrees C) and volume output (100,000 cubic meters/year) of its hotspot. This proposal focuses on Loihi, the youngest and still underwater Hawaiian volcano. Loihi lavas have the potential to provide important constraints on the mantle plume beneath the islands because: 1) The volcano's youth (preshield stage) allows for the characterization of its mantle source before it has been extensively melted. 2) Loihi basalts are one of the key geochemical end members for Hawaiian basalts. Thus, analyses of its lavas (especially from previously unsampled areas) will provide a unique perspective into melting processes and source heterogeneity within the Hawaiian hotspot, the Earth's hottest mantle plume. Loihi lavas are especially noteworthy for their primordial noble gas signatures. 3) Loihi's interior and early history are exposed by major landslides on its eastern flank (1 km thick stratigraphic section) and in ~300 m deep craters at its summit. 4) An extensive suite (196 samples) of Loihi basalts collected by submersible are available from its summit pit craters, its dissected east flank and the full extent of its two rift zones. Most of these samples have received limited geochemical characterization. These samples will be made available to the Earth science community for other geochemical studies. This project supports the training of a graduate student at the University of Hawaii.The goals of this project are: 1) Determine the lifespan of Hawaiian shield volcanoes using Loihi lavas to document the duration of the early stage, which is the major missing puzzle piece for resolving this issue. Current estimates range widely. 2) Better understand the structure of the Hawaiian plume (e.g., whether it is bilaterally asymmetrical as currently thought). Loihi is at the young end of the LOA trend but paradoxically, it shares geochemical features with KEA trend lavas. 3) Evaluate whether the Hawaiian mantle plume source varies in time and space. Lavas from Loihi deep interior and submarine flanks will provide a new perspective on the Loihi compositional end member of the Hawaiian plume. 4) Determine how crustal contamination works at a submarine Hawaiian volcano. Do rift zone magmas bypass the summit magma reservoir and more faithfully preserve mantle source signatures? Previous studies of Loihi summit lavas showed they were extensively modified by crustal contamination, whereas rare gas compositions of deep south rift lavas suggest they were uncontaminated. Loihi basalts are ripe for geochemical characterization using a new generation of geochemical instruments for determining ages, crustal and mantle processes, and source variations. These analyses will provide fundamental insights into the workings of the Hawaiian hotspot. The newly geochemically characterized Loihi samples, all precisely located and collected by submersible, will be made available to the Earth science community for other complementary future geochemical studies (e.g., Os isotopes, platinum group elements, rare gases).