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

澳大利亚库克群岛 Raivavae 和 Rapa 的氦同位素测量：对 HIMU 成分性质的新见解

• 批准号: 0509721
• 负责人: Sujoy Mukhopadhyay
• 金额: $ 5.19万
• 依托单位: Harvard University
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2005
• 资助国家: 美国
• 起止时间: 2005-06-01 至 2007-05-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0509721&HistoricalAwards=false
• 关键词: Helium; Isotopic; Measurements; Raivavae; Rapa

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

HIMU洋岛(高U/Pb值)来自于洋壳和沉积物的再循环。然而，回收的地壳物质应该基本上不含~3He，但有高浓度的~4He是由U和Th衰变产生的。因此，令人惊讶的是，HIMU海洋岛屿的平均氦同位素比率高达5-7RA。该提案要求获得资金，以测量库克-南岛拉瓦岛和拉帕岛的一套样品中的3He/4He比值，以更好地了解HIMU玄武岩中3He/4He比值的来源。之所以选择这套HIMU玄武岩是因为：1)样品具有良好的常量元素、微量元素和放射性同位素(Sr、ND、Hf、Pb和Os)特征；2)Raivavae熔岩具有强烈的HIMU特征，也是来自库克-南岛的最多样化的同位素组成，而来自Raivavae以南约500公里的Rapa岛的熔岩具有丰富的成分。以前已经注意到，地理上非常接近的HIMU和EM型火山活动频繁发生。拟议的氦同位素测量，与样品的现有地球化学数据相结合，将为我们提供关于这种联系的意义的新信息。此外，所提出的测量结果将使我们能够评估波利尼西亚之下不同地幔成分之间的混合关系，从而为HIMU玄武岩低3He/4He值的成因提供新的见解。只有通过对同一样品上测量的氦和锶、钕、铅、Os同位素的仔细研究，才能确定HIMU熔岩中低3He/4He比值的起源。氦同位素组成对原始地幔储集层的存在具有最强的制约作用。了解地幔岩石中氦的输运行为和氦同位素组成的变化，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的制约的关键。此外，拟议的研究纳入了本科生教育，并让他们参与地幔地球化学前沿的研究。及早参与激动人心的研究对招募和留住地球科学人才很重要