Collaborative Research: Petrology and Geochemistry of Gakkel Ridge Basalts

学术价值：Gakkel海脊作为全球洋脊系统中扩展最慢的末端成员，提供了丰富的研究机会，其扩展速度向东递减。随着扩张速度的减小，上覆岩石圈的厚度增加。这一现象使研究和可能解决与洋壳形成有关的两个主要问题成为可能。首先，岩石圈厚度和地幔温度对洋脊玄武岩成因的相对重要性是什么？如果洋脊玄武岩记录了地幔温度，它们将为现在和过去的地幔温度变化提供有价值的记录。第二，洋脊玄武岩的同位素和微量元素组成是否受小范围熔融的“脉状地幔”成分优先取样的控制？这一问题的解决对解释整个海洋火山的微量元素和同位素记录具有重要意义。2001年，北极大洋中脊考察队(AMORE)从200多个站点制作了第一张高分辨率的海脊和基底岩石地图。对主要元素、微量元素以及锶、钕和铅同位素的初步分析表明，一项意想不到的发现的重要性--存在地幔区域边界，该边界出现在沿山脊的一段“稀疏岩浆带”中，在那里岩浆作用降至零，橄榄岩侵位在扩张轴上。西面的样本与印度洋相似，而东面的样本与太平洋相似。尽管有这些复杂性，但预测的东部地区熔融程度较低的信号在常量元素和微量元素中都是明确的。大量可用样本上的相关数据集将允许分离变量--地幔成分、扩散速度和地幔温度的相对作用。首席调查员将进行一项主要的分析和模拟方案：完成地球化学方案，以精确确定地幔区域边界的位置和锐度，利用现有和新的主要和微量元素数据模拟融化过程，模拟两个区域的微量元素和同位素演化，并详细比较北极盆地与其他大洋盆地的信号。新的数据和模型将使岩石圈厚度增加的影响与在其他地方观察到的全球变化之间有明确的比较。它们是可区分的吗？这些差异是否与模型预测相符？重要的新方法将包括使用Hf同位素评估石榴石在熔化过程中的重要性，以及激光烧蚀对熔体包裹体的电感耦合等离子体质谱分析，以深入了解熔化和熔体分离的过程及其沿山脊的变化。新的地图和位置良好的样品也提供了测试岩浆分段模型的机会，以及明确定义和孤立的火山中心是地幔不均匀、地幔流动的集中还是均匀流动的熔体集中的结果。这项工作将继续与塔尔萨大学的彼得·迈克尔博士合作，他还将对挥发性元素进行分析工作，与俄勒冈州立大学的大卫·格雷厄姆博士合作，他正在测量贵重气体，以及与从该地区回收的橄榄岩样本的同事合作。更广泛的影响：这项工作解决了与洋壳和地幔有关的两个最重要的问题--地幔温度分布和地幔不均匀的性质，这两个问题在许多学科中具有广泛的重要性。它曾经是，也将继续是教育和外联的重要组成部分。学生们参与了这次远洋探险，这项工作将有两个机构的研究生和本科生参与，并将有一系列的公开讲座，让更多的社区成员了解美国新的破冰能力带来的历史性发现。

Intellectual Merits: The Gakkel Ridge provides rich research opportunities as the slowest spreading end member of the global system of ocean ridges, with the spreading rate decreasing progressively towards the east. As the spreading rate decreases, the thickness of the overlying lithosphere increases. This phenomenon permits examination and possibly resolution of two major questions pertinent to the formation of the ocean crust. First, is what the relative importance of lithospheric thickness and mantle temperature on the genesis of ocean ridge basalts? If ocean ridge basalts record mantle temperature, they will provide a valuable record of current and past mantle temperature variations. Second, are ocean ridge basalt isotope and trace element compositions controlled by preferential sampling of a "veined mantle" component at small extents of melting? Resolution of this question has implications for interpretations of the entire trace element and isotope record of oceanic volcanics. The Arctic Mid-Ocean Ridge Expedition (AMORE) in 2001 produced the first high resolution map of the ridge and basement rocks from over 200 stations. Preliminary analyses for major elements, trace elements, and strontium, neodymium, and lead isotopes demonstrate the importance of an unexpected discovery-the existence of a mantle domain boundary that occurs in a "sparsely magmatic zone" part way along the ridge, where magmatism drops to zero and peridotites are emplaced at the spreading axis. Samples to the west are similar to the Indian Ocean, while those to the east are similar to the Pacific. Despite these complexities, the predicted signal of low extents of melting in the eastern region is clear in both major and trace elements. Coherent data sets on the large suite of samples available will permit a separation of variables-the relative roles of mantle composition, spreading rate, and mantle temperature. The Principal Investigators will undertake a major analytical and modeling program: to complete the geochemical program to precisely define the location and sharpness of the mantle domain boundary, to model the melting process using existing and new major and trace element data, to model the trace element and isotope evolution of both domains, and to make detailed comparisons of the Arctic Basin signal to other ocean basins. The new data and modeling will permit a clear comparison between the effects of increasing lithospheric thickness and the global variations observed elsewhere. Are they distinguishable, and do these differences accord with model predictions? Important new approaches will include using hafnium isotopes to evaluate the importance of garnet in the melting process, and laser ablation ICP-MS analysis of melt inclusions to gain insights into processes of melting and melt segregation and how they vary along the ridge. The new maps and well located samples also give the opportunity to test models of magmatic segmentation, and whether clearly defined and isolated volcanic centers are the result of mantle heterogeneity, focusing of mantle flow, or melt focusing of uniform flow. This work will continue the collaboration with Dr. Peter Michael, University of Tulsa, who will also do analytical work on volatile elements, with Dr. David Graham, Oregon State University, who is measuring noble gasses, and with colleagues working on the peridotite samples recovered from this region. Broader Implications: This work addresses two of the most significant issues pertinent to the ocean crust and mantle-the distribution of mantle temperature and the nature of mantle heterogeneity, which have broad importance across many disciplines. It has had, and will continue, to have an important component of education and outreach. Students were involved in the sea-going expedition, the work will involve participation by both graduate and undergraduate students at two institutions, and there will be a series of public lectures to expose more of the community to the historic discoveries made possible by the new U. S. ice-breaking capability.