Mantle Heterogeneity, Partial Melting and Magma Degassing Along the Ultraslow-Spreading Gakkel Ridge: Constraints from Helium and Argon Isotopes In Basalts and Peridotites

沿超慢速扩散的加克尔海脊的地幔异质性、部分熔融和岩浆脱气：来自玄武岩和橄榄岩中氦和氩同位素的限制

• 批准号: 0520397
• 负责人: David Graham
• 金额: $ 15.46万
• 依托单位: Oregon State University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2005
• 资助国家: 美国
• 起止时间: 2005-09-01 至 2008-08-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0520397&HistoricalAwards=false
• 关键词: Mantle; Heterogeneity; Partial; Melting; Magma

ABSTRACTGrahamOPP-0520397 Intellectual Merit: The Gakkel Ridge is a fundamental geological feature of the Arctic Ocean because it represents the important, ultraslow-spreading rate end-member for the global mid-ocean ridge system. Seafloor rocks were recovered from over 200 localities along the Gakkel Ridge during the Arctic Mid-Ocean Ridge Expedition (AMORE) in the summer of 2001. This sample collection provides an exciting opportunity to advance our understanding of mantle heterogeneity and partial melting along the world's deepest and slowing spreading ridge. Many of the basalt samples are volatile-rich and relatively undergassed, which also provides an unprecedented opportunity to advance our understanding of noble gas geochemistry, and the degassing of carbon along mid-ocean ridges. Initial helium isotope and other geochemical data reveal a major geochemical mantle boundary in the mid-section of the Gakkel Ridge. Results also indicate that 3He/4He may be higher in off-axis lavas compared to axial lavas, potentially carrying new clues about magma transport beneath ultraslow-spreading ridges. Two extremely fresh spinel harzburgites analyzed for helium isotopes by crushing have the highest 3He/4He of the AMORE sample suite yet analyzed, and relatively high He contents. These peridotite results suggest that melt retention in the upper mantle beneath ultraslow-spreading ridges may be an important consideration in geochemical budgets. Using noble gas analyses of basalts and peridotites from the Gakkel Ridge, this projectaddresses three questions: 1) Is the chemical and isotopic variability in Gakkel Ridge basalts caused by small scale heterogeneity (veins) in the underlying mantle, or by regional and segment-scale differences? 2) Is geochemical variability in Gakkel Ridge peridotites an accurate indicator of mantle melting or is it related to upper mantle melt retention? 3) Does axial depth (eruption pressure) or magma composition (e.g., extent of crystallization) control the degassing of volatiles along the Gakkel Ridge? Broader Impacts: The Arctic region is a focus of greater than usual public awareness, through its remoteness, its unique oceanic and terrestrial habitat, its environmental sensitivity, and its history of exploration and discovery. Support for this project will help to develop synergistic connections with other scientific groups, such as vent biogeographers and physical oceanographers. Because the deep Arctic Ocean is relatively isolated, it is likely that unique vent communities exist along the Gakkel Ridge. An adequate understanding of these biological communities will require understanding the budgets of key elements, such as carbon and sulfur, that are potentially supplied by seawater interaction with the ocean crust at high temperatures. Measurements of C/3He ratios in basalts will provide useful constraints for assessing the role of hydrothermal inputs to the deep Arctic.

知识点：加克尔海脊是北冰洋的一个基本地质特征，因为它代表了全球大洋中脊系统重要的、超低速扩张的端元。2001年夏天，在北极大洋中脊考察期间，在加克尔脊沿着200多个地点发现了海底岩石。这个样品收集提供了一个令人兴奋的机会，以促进我们的地幔异质性和部分熔融沿着世界上最深和缓慢扩张的山脊的理解。许多玄武岩样品富含挥发物，相对而言，气体不足，这也提供了一个前所未有的机会，以促进我们对稀有气体地球化学的理解，以及沿着大洋中脊的碳脱气。 初始氦同位素和其他地球化学数据揭示了一个主要的地球化学地幔边界在中段的加克尔海岭。结果还表明，3 He/4 He可能更高的轴向熔岩相比，轴向熔岩，可能携带新的线索下的岩浆运输超低扩张脊。两个非常新鲜的尖晶石方辉橄榄岩的氦同位素分析粉碎有最高的3 He/4 He的AMORE样品套件尚未分析，相对较高的He含量。这些橄榄岩的结果表明，熔体保留在上地幔超低速扩张脊下可能是一个重要的考虑在地球化学预算。利用加克尔海岭玄武岩和橄榄岩的稀有气体分析，本项目解决了三个问题：1）加克尔海岭玄武岩的化学和同位素变异是由下伏地幔中的小尺度不均匀性（脉）引起的，还是由区域和段尺度差异引起的？2)加克尔海岭橄榄岩的地球化学变异性是地幔熔融的准确指示还是与上地幔熔体滞留有关？3)轴向深度（喷发压力）或岩浆成分（例如，结晶的程度）控制挥发物的脱气沿沿着加克尔岭？更广泛的影响：北极地区是一个比平常更大的公众意识的焦点，因为它的偏远，其独特的海洋和陆地栖息地，其环境敏感性，以及其探索和发现的历史。对这一项目的支持将有助于发展与其他科学团体的协同联系，如喷口生物学家和物理海洋学家。由于北冰洋深处相对孤立，因此可能沿着加克尔海脊存在独特的喷口群落。要充分了解这些生物群落，就需要了解碳和硫等关键元素的收支，这些元素可能是海水在高温下与洋壳相互作用而提供的。玄武岩中C/3 He比率的测量将为评估热液输入到北极深处的作用提供有用的限制。