Collaborative Research: Petrology and Geochemistry of Gakkel Ridge Basalts

合作研究：Gakkel 岭玄武岩的岩石学和地球化学

• 批准号: 0425785
• 负责人: Charles Langmuir
• 金额: $ 19.85万
• 依托单位: Harvard University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2004
• 资助国家: 美国
• 起止时间: 2004-08-15 至 2007-07-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0425785&HistoricalAwards=false
• 关键词: Collaborative; Research; Petrology; Geochemistry; Gakkel

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.

知识价值：Gakkel Ridge作为全球洋脊系统中扩展最慢的末端成员，其扩展速度向东逐渐减小，提供了丰富的研究机会。随着扩张速率的减小，上覆岩石圈的厚度增加。这一现象使我们能够研究并可能解决与海洋地壳形成有关的两个主要问题。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，它们将提供当前和过去地幔温度变化的宝贵记录。其次，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”成分的优先采样控制？这个问题的解决对于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，北极洋中脊考察队（AMORE）从200多个站点绘制了第一张洋中脊和基底岩石的高分辨率地图。对主要元素、微量元素、锶、钕和铅同位素的初步分析表明了一个意想不到的发现的重要性——地幔域边界的存在，它出现在沿山脊部分的“稀疏岩浆带”中，岩浆活动降至零，橄榄岩位于扩张轴上。西边的样本与印度洋相似，东边的样本与太平洋相似。尽管存在这些复杂性，但东部地区低融化程度的预测信号在主要元素和微量元素上都是明确的。大量可用样本的相干数据集将允许分离变量——地幔成分、扩散速率和地幔温度的相对作用。首席研究人员将承担一项主要的分析和建模项目：完成地球化学项目，以精确定义地幔域边界的位置和清晰度，利用现有和新的主要元素和微量元素数据对融化过程进行建模，对两个域的微量元素和同位素演化进行建模，并将北极盆地信号与其他海洋盆地进行详细比较。新的数据和模型将允许在岩石圈厚度增加的影响与其他地方观察到的全球变化之间进行清晰的比较。它们是可区分的吗？这些差异是否与模型预测一致？重要的新方法将包括使用铪同位素来评估石榴石在熔化过程中的重要性，以及对熔体内含物进行激光烧蚀ICP-MS分析，以深入了解熔化过程和熔体偏析以及它们如何沿着山脊变化。新的地图和定位良好的样品也提供了测试岩浆分割模型的机会，以及明确定义和孤立的火山中心是地幔非均质性、地幔流动聚焦还是均匀流动的熔融聚焦的结果。这项工作将继续与塔尔萨大学的彼得·迈克尔博士合作，他也将进行挥发性元素的分析工作，与俄勒冈州立大学的大卫·格雷厄姆博士合作，他正在测量稀有气体，并与同事一起研究从该地区回收的橄榄岩样本。更广泛的影响：这项工作解决了与海洋地壳和地幔有关的两个最重要的问题——地幔温度的分布和地幔非均质性的本质，这在许多学科中都具有广泛的重要性。它已经并将继续是教育和外联的一个重要组成部分。学生们参与了这次海上探险，这项工作将由两所大学的研究生和本科生参与，并将举行一系列公开讲座，让更多的人了解美国新的破冰能力所带来的历史性发现。