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）是由海洋地壳和沉积物的再循环形成的。然而，再循环的地壳物质应该基本上不含3He，但有高浓度的由U和Th衰变产生的4He。因此，令人惊讶的是，HIMU海洋岛屿的平均氦同位素比高达5-7 RA。该提案要求提供资金，以测量来自Cook-Austral群岛的Raivavae和Rapa的一套样品中的3He/4He比率，以更好地了解HIMU玄武岩中3He/4He比率的来源。选择这组玄武岩是因为：1)样品在主元素、微量元素和放射性成因同位素（Sr、Nd、Hf、Pb和Os）方面具有良好的特征；2) ravavae熔岩具有强烈的HIMU特征，其同位素组成也最多样化，而位于ravavae以南约500 km的Rapa岛熔岩的同位素组成更丰富。HIMU和em型火山活动的频繁发生在地理上接近的地区。提出的氦同位素测量，当与样品上现有的地球化学数据相结合时，将为我们提供有关这种关联意义的新信息。此外，所提出的测量将使我们能够评估波利尼西亚不同地幔组分之间的混合关系，从而为HIMU玄武岩低3He/4He比率的起源提供新的见解。只有通过对氦和在同一样品上测量的Sr、Nd、Pb和Os同位素的仔细研究，才能确定HIMU熔岩中低3He/4He比率的来源。氦同位素组成对原始地幔储层的存在提供了最强的约束。了解地幔源岩石的输运行为和氦同位素组成的变化，是研究地球起源和化学演化以及地幔对流性质和动力学的重要依据。此外，建议的研究纳入本科生教育，并使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为地球科学招募和留住人才非常重要

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