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

GrahamOPP-0520397学术价值：Gakkel海脊是北冰洋的一个基本地质特征，因为它代表着全球大洋中脊系统重要的、超低扩散速率的终端成员。2001年夏天，在北冰洋中脊探险期间，在加克尔海脊沿线200多个地点发现了海底岩石。这次样品采集提供了一个令人兴奋的机会，以促进我们对沿世界最深和最缓慢的扩张山脊的地幔不均一性和部分熔融的理解。许多玄武岩样品挥发分丰富，气体含量相对较低，这也为我们提供了一个前所未有的机会，促进我们对稀有气体地球化学和大洋中脊碳的脱气作用的了解。初始氦同位素和其他地球化学数据揭示了加克尔山脊中段的主要地球化学地幔边界。结果还表明，与轴向熔岩相比，离轴熔岩的3He/4He可能更高，这可能为超低扩张脊下的岩浆运移提供了新的线索。通过破碎分析的两个非常新鲜的尖晶石方辉橄榄岩的氦同位素是Amore样品组中分析过的最高的3He/4He，并且相对较高的He含量。这些橄榄岩的研究结果表明，上地幔在超低扩张脊之下的熔体保留可能是地球化学预算中的一个重要考虑因素。利用对加克尔岭玄武岩和橄榄岩的惰性气体分析，这个项目解决了三个问题：1)加克尔岭玄武岩的化学和同位素变化是由下地幔中的小尺度非均质性(矿脉)引起的，还是由区域和分段尺度的差异造成的？2)加克尔岭橄榄岩中的地球化学变化是地幔熔融的准确指示，还是与上地幔熔体保留有关？3)轴向深度(喷发压力)或岩浆成分(如结晶程度)是否控制着沿加克尔岭挥发分的脱气？更广泛的影响：北极地区因其地处偏远、独特的海洋和陆地栖息地、对环境的敏感性以及探索和发现的历史，成为公众关注的焦点。对该项目的支持将有助于发展与其他科学团体的协同联系，如喷口生物地理学家和物理海洋学家。由于北冰洋深处相对孤立，加克尔海脊很可能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预算，这些元素可能是由海水在高温下与洋壳相互作用提供的。玄武岩中C/3He比值的测量将为评估热液输入对北极深处的作用提供有用的约束。

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.