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

超慢速扩张的加克尔海脊下方的地幔异质性、熔融和岩浆输送：来自挥发物和微量元素的证据

• 批准号: 0425892
• 负责人: Peter Michael
• 金额: --
• 依托单位: University of Tulsa
• 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 2010-07-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0425892&HistoricalAwards=false
• 关键词: Mantle; Heterogeneity; Melting; Magma; Transport

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.

智力优势：由于它的扩张速度沿着它的长度逐渐变化，而且它没有主要的偏移或倾斜，超低速扩张的加克尔海岭是一个理想的实验室，用于测试扩张速度与地幔熔融和地壳形成有关的假设。 北极大洋中脊考察（AMORE 2001）是对加克尔脊进行取样的第一次全面考察，取得了超出预期的成功，绘制了第一张高分辨率、导航良好的加克尔脊测深图。 主要调查人员从200多个地点（仅计划60个）回收了基底样品，并发现了丰富的热液活动。 新的测深图和样本显示了一个明显的边界，在那里，海脊从东经3度以西的火山岩变成了东部的岩浆岩（橄榄岩）。 火山活动在更远的东部恢复，这表明扩张速度的变化不是火山活动变化的原因。 玄武岩的地球化学特征揭示了一个显著的地球化学边界，它与海岭的形态边界非常吻合。 3degreesE以西的玄武岩与其他海脊相比具有较高的H2O /铯（Ce）和钡（Ba）/铌（Nb）比值，并具有印度洋同位素特征。 在东部，H2O /Ce和Ba/Nb是其他海脊的典型比值，同位素比值类似于太平洋玄武岩。为了测试关于这个地球化学省起源的假设，首席研究员将根据可以与地球化学储层进行比较的关键元素和挥发物来定义其地球化学。 他还将根据该地区的构造历史来定义其地理范围，并通过检查玄武岩和橄榄岩中的熔融包裹体和钡的晶体分带来寻找地球化学信号来自地幔中离散成分的证据。Gakkel海岭的地球化学数据与其他洋中脊，特别是西南印度洋海岭（SWIR），以确定在何种程度上高Ba和H2O可以归因于超低扩张率，而不是北极构造历史。 研究的第二部分涉及最上层岩石圈和地壳的结构。 他将通过确定玄武岩玻璃、其囊泡和熔融包裹体中的CO2丰度，研究岩浆流动最终形成北冰洋地壳的途径。这些数据将被用来描述气体出溶，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山脊轴部分的路径。 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加克尔海岭融化的深度和范围。 为玄武岩生成的H2O数据将与主要元素数据相结合。将构建考虑H2O影响的模型。 特别是，首席研究员将测试假设，即H2O发挥了重要作用，创造了巨大的变化，火山强度沿着加克尔岭，和山脊更敏锐地响应这种变化在超低的蔓延率。 在整个研究的各个方面，他将整合来自地球物理学的限制，并与其他山脊，特别是SWIR进行比较。 拟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。 至少两名学生将参与这个项目，并将从中衍生出本科论文项目。该项目的大部分将构成研究生的硕士论文项目。首席研究员将继续向小学和中学班级、童子军团体和民间团体提供关于加克尔岭的插图介绍。