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.

智力优点：由于其扩张速度沿其长度逐渐变化，并且没有大的偏移或倾斜，超慢扩张的加克尔海脊是检验将扩张速度与地幔熔化和地壳形成联系起来的假设的理想实验室。 北极大洋中脊探险（AMORE 2001）是第一次对加克尔海脊进行采样的综合巡航，其成功远远超出了预期，并绘制了第一张高分辨率、导航良好的加克尔海脊测深图。 首席研究员从 200 多个地点（仅计划了 60 个）回收了地下室样本，并发现了丰富的热液活动。 新的测深图和样本显示了一个引人注目的边界，其中的山脊从东经 3 度以西的火山活动变为东部的几乎岩浆岩（橄榄岩）。 火山活动在更远的东部地区重新开始，这表明火山活动的变化并不是由扩散速率的变化造成的。 玄武岩地球化学揭示了一个显着的地球化学边界，与山脊的形态边界非常吻合。 与其他海脊和印度洋同位素特征相比，东经 3 度以西的玄武岩具有较高的 H2O/铯 (Ce) 和钡 (Ba)/铌 (Nb) 含量。 在东部，H2O/Ce 和 Ba/Nb 是其他海脊的典型特征，同位素比类似于太平洋玄武岩。为了检验有关该地球化学省起源的假设，首席研究员将根据可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。将 Gakkel 海脊的地球化学数据与其他大洋中脊（尤其是西南印度洋中脊 (SWIR)）进行比较，以确定高 Ba 和 H2O 在多大程度上可归因于超慢扩散速率，而不是北极构造历史。 该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、其囊泡和熔体包裹体中的二氧化碳丰度来研究岩浆最终流动形成北冰洋地壳的路径。这些数据将用于描述气体逸出，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 的富集对加克尔海脊融化的深度和程度的影响程度。 为玄武岩生成的 H2O 数据将与主要元素数据集成。将构建考虑 H2O 影响的模型。 特别是，首席研究员将测试以下假设：H2O 在造成 Gakkel 山脊沿线火山强度的巨大变化方面发挥了重要作用，并且山脊以超慢的扩散速度对这种变化做出了更敏锐的反应。 在研究的各个方面，他将整合地球物理学的约束，并与其他海脊（尤其是短波红外）进行比较。 拟议研究的更广泛影响：本科生将在分析数据中发挥重要作用。 至少两名学生将参与该项目，并从中衍生出本科论文项目。 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、童子军团体和公民团体提供有关 Gakkel Ridge 的图文并茂的演示。