Helium Isotopic Measurements From Raivavae and Rapa, Cook-Austral Islands: New Insights into the Nature of the HIMU Component

HIMU洋岛（高U/Pb比）是由于再循环的洋壳和沉积物。然而，再循环的地壳物质应该基本上没有3 He，但具有由U和Th衰变产生的高浓度4 He。因此，令人惊讶的是，在HIMU海洋岛屿的平均氦同位素比值高达5-7 RA。该提案要求提供资金，以测量库克-南方群岛Raivavae和Rapa的一套样品中的3 He/4 He比率，以更好地了解HIMU玄武岩中3 He/4 He比率的起源。选择这套HIMU玄武岩是因为：1）样品在主元素、微量元素和放射性同位素方面具有良好的特征（Sr、Nd、Hf、Pb和Os）; 2）Raivavae熔岩具有强烈的HIMU特征，也具有库克-南方群岛最多样化的同位素组成，而来自Raivavae以南约500公里的拉帕岛的熔岩，具有丰富的成分。HIMU和EM型火山活动在地理位置上的频繁发生以前已经注意到。建议的氦同位素测量，结合现有的地球化学数据的样品，将给我们提供新的信息的意义，这样的协会。此外，建议的测量将使我们能够评估波利尼西亚下存在的不同地幔成分之间的混合关系，因此，将提供新的见解低3 He/4 He比的HIMU玄武岩的起源。只有通过氦和Sr、Nd、Pb、Os同位素的研究，才能确定HIMU熔岩中低3 He/4 He比值的成因，氦同位素组成对原始地幔储层的存在提供了最强的制约。了解幔源岩石中氦同位素组成的迁移行为和变化是对地球起源和化学演化以及地幔对流的性质和动力学提供强有力约束的关键。此外，拟议的研究纳入本科生教育，并让他们参与地幔地球化学前沿的研究。早期参与激动人心的研究对于招募和留住地球科学人才非常重要

HIMU ocean islands (high U/Pb ratio) are attributed to recycled oceanic crust and sediments. However, recycled crustal material should be essentially devoid of 3He but have high concentrations of 4He produced from the decay of U and Th. It is therefore surprising that the average helium isotopic ratios at HIMU ocean islands are as high as 5-7 RA. This proposal requests funding to measure 3He/4He ratios in a suite of samples from Raivavae and Rapa from the Cook-Austral Islands to better understand the origin of the 3He/4He ratios in HIMU basalts. This suite of HIMU basalts were selected because: 1) the samples are well characterized with respect to major elements, trace elements, and radiogenic isotopes (Sr, Nd, Hf, Pb, and Os); 2) Raivavae lavas have a strong HIMU signature and also have the most diverse isotopic composition from the Cook-Austral Islands while lavas from the island of Rapa, approximately 500 km south of Raivavae, have an enriched composition. The frequent occurrence of HIMU and EM-type volcanism in close geographical proximity has been previously noted. The proposed helium isotopic measurements, when combined with the existing geochemical data on the samples, will give us new information on the meaning of such an association. In addition, the proposed measurements will enable us to evaluate mixing relations between the different mantle components present under Polynesia and will, therefore, provide new insights into the origin of the low 3He/4He ratios of HIMU basalts. Only through careful studies involving helium and the isotopes of Sr, Nd, Pb, and Os measured on the same samples can the origin of low 3He/4He ratios in HIMU lavas be ascertained.Helium isotopic composition provides the strongest constraint on the existence of a primitive mantle reservoir. Understanding the transport behavior and the variations in helium isotopic composition in mantle-derived rocks is key to providing strong constraints on the origin and chemical evolution of the Earth as well as the nature and dynamics of mantle convection. In addition, the proposed research incorporates undergraduate student education and involves them in research at the forefront of mantle geochemistry. Early involvement in exciting research is important for recruiting and retaining talented individuals to the Earth Sciences