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

摘要GrahamOPP-0520397 智力价值：Gakkel 海脊是北冰洋的一个基本地质特征，因为它代表了全球洋中脊系统重要的、超慢扩散速率的终端成员。 2001 年夏天，在北极大洋中脊探险 (AMORE) 期间，从加克尔海脊沿线的 200 多个地点回收了海底岩石。这个样本采集提供了一个令人兴奋的机会，可以加深我们对世界上最深且扩张缓慢的洋脊沿线的地幔异质性和部分熔融的了解。许多玄武岩样本富含挥发性物质且气体含量相对较低，这也为增进我们对稀有气体地球化学以及大洋中脊碳脱气的理解提供了前所未有的机会。 初始氦同位素和其他地球化学数据揭示了加克尔海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，离轴熔岩中的 3He/4He 可能更高，这可能为超慢速扩张山脊下的岩浆输送提供新线索。通过粉碎分析两种极其新鲜的尖晶石方辉橄榄石的氦同位素，其具有迄今为止分析的 AMORE 样品套件中最高的 3He/4He，并且 He 含量相对较高。这些橄榄岩结果表明，超慢速扩张脊下方上地幔的熔体滞留可能是地球化学预算中的一个重要考虑因素。该项目利用对 Gakkel 海脊的玄武岩和橄榄岩进行稀有气体分析，解决了三个问题：1) Gakkel 海脊玄武岩的化学和同位素变异性是由底层地幔中的小尺度异质性（脉）引起的，还是由区域和分段尺度的差异引起的？ 2）Gakkel海岭橄榄岩的地球化学变异性是地幔熔融的准确指标还是与上地幔熔融滞留有关？ 3) 轴向深度（喷发压力）或岩浆成分（例如结晶程度）是否控制沿加克尔山脊挥发物的脱气？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、环境敏感性以及探索和发现的历史而成为公众关注的焦点。对该项目的支持将有助于与其他科学团体（例如喷口生物地理学家和物理海洋学家）建立协同联系。由于北冰洋深处相对孤立，加克尔海脊沿线很可能存在独特的喷口群落。要充分了解这些生物群落，需要了解碳和硫等关键元素的预算，这些元素可能由高温下海水与洋壳的相互作用提供。玄武岩中 C/3He 比率的测量将为评估北极深处热液输入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.