Using Loihi Basaltic Rocks to Understand the Hawaiian Plume

热点火山被认为挖掘了地球的深层地幔，并为了解其组成和过程提供了基本的见解。夏威夷是研究热点熔岩的绝佳地点，因为它距离任何大陆或构造板块边界都很远（2500公里），而且它的热点温度高（1500摄氏度），体积大（10万立方米/年）。这个提议的重点是Loihi，这是夏威夷最年轻的水下火山。Loihi熔岩有可能对岛屿下面的地幔柱提供重要的限制，因为：1)火山的年轻期（护盾前阶段）允许在它被广泛融化之前表征它的地幔源。2) Loihi玄武岩是夏威夷玄武岩的关键地球化学端元之一。因此，对其熔岩（特别是来自以前未采样地区的熔岩）的分析将为夏威夷热点（地球上最热的地幔柱）的融化过程和来源异质性提供一个独特的视角。Loihi熔岩特别值得注意的是它们的原始惰性气体特征。3) Loihi的内部和早期历史是由其东侧翼（1 km厚的地层剖面）的大型滑坡和山顶约300 m深的火山口暴露出来的。4)通过潜水器收集的大量Loihi玄武岩（196个样本）可从其顶坑火山口、其解剖的东侧翼和两个裂谷带的全部范围内获得。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。该项目支持夏威夷大学的一名研究生的培训。这个项目的目标是：1)确定夏威夷盾状火山的寿命，使用Loihi熔岩来记录早期阶段的持续时间，这是解决这个问题的主要缺失的拼图。目前的估计范围很广。2)更好地理解夏威夷羽流的结构（例如，它是否像目前认为的那样是双边不对称的）。Loihi处于LOA趋势的年轻端，但矛盾的是，它与KEA趋势的熔岩具有相同的地球化学特征。3)评价夏威夷地幔柱源是否存在时空变化。来自Loihi深层内部和海底侧翼的熔岩将为夏威夷羽流的Loihi成分末端成员提供新的视角。4)确定地壳污染如何影响夏威夷海底火山。裂谷带岩浆是否绕过山顶岩浆储层，更忠实地保存了地幔源特征？先前对Loihi山顶熔岩的研究表明，它们受到地壳污染的广泛影响，而深南裂谷熔岩的稀有气体成分表明它们没有受到污染。Loihi玄武岩的地球化学特征已经成熟，可以使用新一代的地球化学仪器来确定年龄、地壳和地幔过程以及来源变化。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样品全部由潜水器精确定位和收集，将提供给地球科学界用于其他补充的未来地球化学研究（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).