Mantle Heterogeneity and Melting and Magma Transport Beneath the Ultraslow-Spreading Gakkel Ridge: Evidence from Volatiles and Trace Elements

学术价值：由于其扩展速度沿其长度逐渐变化，并且没有主要的偏移或倾斜，所以超低扩展的Gakkel Ridge是一个理想的实验室，用于测试扩展速度与地幔融化和地壳形成有关的假设。北极洋中脊探险（AMORE 2001）是对Gakkel Ridge进行的第一次全面考察，取得了超出预期的成功，并制作了第一张高分辨率，导航良好的Gakkel Ridge水深图。主要研究人员从200多个地点（只有60个计划）回收了地下室样本，并发现了丰富的热液活动。新的测深图和样品显示了一个引人注目的边界，在那里，山脊从3度以西的火山活跃到东部的几乎是岩浆（橄榄岩）。火山活动甚至在更远的东方恢复，这表明扩散速度的变化不是火山活动变化的原因。玄武岩地球化学揭示了一个显著的地球化学边界，该边界与山脊的形态边界密切吻合。在东经3度以西，玄武岩的水/铯（Ce）和钡(Ba)/铌（Nb）比其他脊高，具有印度洋同位素特征。在东部，H2O /Ce和Ba/Nb是其他山脊的典型特征，同位素比值与太平洋玄武岩相似。为了验证关于这个地球化学省起源的假设，首席研究员将根据关键元素和挥发物来定义其地球化学，这些元素和挥发物可以与地球化学储层进行比较。他还将根据该地区的构造历史来确定其地理范围，并通过检查玄武岩和橄榄岩中钡的熔融包裹体和晶体带来寻找地球化学信号来自地幔中离散成分的证据。Gakkel Ridge的地球化学数据与其他洋中脊，特别是西南印度洋脊（SWIR）进行了比较，以确定在何种程度上高Ba和H2O可归因于超低扩张速率，而不是北极构造历史。研究的第二部分讨论了最上层岩石圈和地壳的结构。他将通过测定玄武岩玻璃、它们的囊泡和熔体包裹体中的二氧化碳含量，来研究岩浆最终形成北冰洋地壳的途径。这些数据将用于描述气体的溶解，以测试有关岩浆上升深度的假设，以及它们到达远离火山中心的脊轴部分的路径。在研究的第三个方面，他将研究H2O的富集在多大程度上影响了Gakkel Ridge上融化的深度和程度。玄武岩生成的H2O数据将与主元素数据进行整合。我们将构建考虑H2O影响的模型。特别地，首席研究员将验证这样的假设，即水在造成Gakkel山脊火山强度的巨大变化中发挥了主要作用，并且山脊对这种变化的反应更强烈，以超低的扩散速度。在研究的各个方面，他将整合地球物理学的约束条件，并与其他脊，特别是SWIR进行比较。建议研究的更广泛影响：本科生将在分析数据方面发挥重要作用。至少有两名学生将参与该项目，并将从中获得本科论文项目。该项目的很大一部分将构成研究生的硕士论文项目。首席研究员将继续向中小学班级、童子军团体和民间团体进行有关Gakkel Ridge的插图演讲。

Intellectual Merit: Because its spreading rate varies progressively along its length and it has no major offsets or obliquity, the ultraslow-spreading Gakkel Ridge is an ideal laboratory for testing hypotheses that relate spreading rate to mantle melting and the formation of crust. The Arctic Mid-Ocean Ridge Expedition (AMORE 2001), the first comprehensive cruise to sample Gakkel Ridge, succeeded well beyond expectations, and produced the first high-resolution, well-navigated bathymetric chart of Gakkel Ridge. The Principal Investigators recovered basement samples from more than 200 sites (only 60 were planned) and discovered abundant hydrothermal activity. The new bathymetric map and samples show a striking boundary where the ridge changes from being volcanically robust west of 3degreesE to virtually magmatic (peridotitic) to the east. Volcanic activity resumes even farther to the east, suggesting that spreading rate changes are not responsible for the variations in volcanism. Basalt geochemistry has revealed a remarkable geochemical boundary that coincides closely with the ridge's morphologic boundary. West of 3degreesE, basalts have high H2O /Cesium (Ce) and Barium (Ba)/ Niobium (Nb) compared to other ridges, and Indian Ocean isotopic signatures. To the east, H2O /Ce and Ba/Nb are typical of other ridges, and isotopic ratios resemble Pacific basalts. To test hypotheses about the origin of this geochemical province, the Principal Investigator will define its geochemistry in terms of key elements and volatiles that can be compared to geochemical reservoirs. He will also define its geographic extent in relation to the tectonic history of this region and look for evidence that the geochemical signal arises from a discrete component in the mantle by examining melt inclusions and crystal zoning of barium in basalts and peridotites. Geochemical data for Gakkel Ridge is compared with other mid-ocean ridges, especially Southwest Indian Ridge (SWIR), to determine the extent to which high Ba and H2O can be attributed to ultraslow spreading rate as opposed to Arctic tectonic history. The second part of the study addresses the structure of the uppermost lithosphere and crust. He will examine the pathways by which magmas flow eventually to build the crust of the Arctic Ocean by determining CO2 abundances in basalt glasses, their vesicles and in melt inclusions. These data will be used to describe the gas exsolution to test hypotheses about the depths from which magmas ascend, and the paths that they have taken to reach parts of the ridge axis that are distant from volcanic centers. In the third aspect of the study, he will examine the extent to which the enrichment of H2O has influenced the depth and extent of melting on Gakkel Ridge. The H2O data generated for basalts will be integrated with major element data. Models will be constructed that take into account the influence of H2O. In particular, the Principal Investigator will test hypotheses that H2O has played a major role in creating the large variations in volcanic intensity along Gakkel Ridge, and that ridges respond more acutely to such changes at ultraslow spreading rates. Throughout all aspects of the study, he will integrate constraints from geophysics and make comparisons with other ridges, especially SWIR. Broader Impacts of the Proposed study: Undergraduate students will play an important role in analyzing the data. At least two students will be involved in this project, and will derive undergraduate thesis projects from it. A large part of the project will constitute the Masters thesis project for a graduate student. The Principal Investigator will continue giving illustrated presentations about Gakkel Ridge to primary and secondary school classes, scout groups, and civic groups.