Using Loihi Basaltic Rocks to Understand the Hawaiian Plume

热点火山被认为可以利用地球深部的地幔，并为了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地点，因为它距离任何大陆或构造板块边界(2500公里)很远，而且其热点温度(1500摄氏度)和产量(10万立方米/年)很高。这项提议的重点是Loihi火山，这是夏威夷最年轻且仍在水下的火山。Loihi熔岩有可能对岛屿下的地幔热柱提供重要的限制，因为：1)火山的年轻(前隐伏阶段)使人们能够在其广泛熔融之前确定其地幔来源的特征。2)洛伊玄武岩是夏威夷玄武岩的主要地球化学端元之一。因此，对其熔岩的分析(特别是从以前未采样的地区)将提供一个独特的视角，了解夏威夷热点--地球最热的地幔热柱--内的熔融过程和来源的不均一性。Loihi熔岩特别值得注意的是其原始的惰性气体特征。3)洛伊希的内部和早期历史被其东侧(1公里厚的地层剖面)和山顶约300m深的陨石坑中的重大滑坡所暴露。4)利用潜水器采集的大量洛伊希玄武岩(196个样品)可从其顶端坑坑、解剖的东侧和两个裂谷带的全貌中获得。这些样品中的大多数都得到了有限的地球化学表征。这些样本将提供给地球科学界进行其他地球化学研究。该项目支持对夏威夷大学研究生的培训。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缺失拼图。目前的估计范围很大。2)更好地了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趋势熔岩具有相同的地球化学特征。3)评价夏威夷地幔热柱来源是否在时间和空间上发生变化。来自Loihi深部内部和海底侧翼的熔岩将为研究Loihi组成夏威夷羽流的端元提供新的视角。4)确定夏威夷海底火山的地壳污染是如何起作用的。裂谷带岩浆是否绕过了顶端岩浆库，更忠实地保存了地幔来源的特征？以前对洛伊希峰熔岩的研究表明，它们受到地壳污染的广泛改造，而南深裂谷熔岩的稀有气体成分表明它们没有受到污染。洛伊希玄武岩的地球化学特征已经成熟，可以使用新一代地球化学仪器来确定年龄、地壳和地幔过程以及源区变化。这些分析将为夏威夷热点的运作提供基本的见解。通过潜水器精确定位和收集的新的具有地球化学特征的Loihi样品将提供给地球科学界，用于今后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).