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：由于其扩散率沿其长度逐渐变化，并且没有主要的偏移或倾斜度，因此扩散的Gakkel Ridge是测试将扩散速率与地幔融化和地壳形成相关的假设的理想实验室。 北极中部山脊探险队（Amore 2001）是首次采样Gakkel Ridge的综合巡游，超出了预期的范围，并产生了第一个高分辨率的，精心策划的Gakkel Ridge图表。 首席研究人员从200多个地点（计划只有60个）中恢复了地下样品，并发现了丰富的热液活动。 新的测深地图和样品显示了一个惊人的边界，山脊从3级西部的火山稳健变为东部几乎是岩浆（橄榄岩）。 火山活动甚至恢复到东方，这表明扩散率的变化对火山主义的变化无关。 玄武岩地球化学揭示了一个显着的地球化学边界，与山脊的形态边界紧密相吻合。 与其他山脊相比，在3级西部，玄武岩具有高H2O / Cesium（CE）和钡（BA） / Niobium（NB），以及印度洋同位素特征。 在东部，H2O /CE和BA /NB是其他山脊的典型特征，同位素比类似于太平洋玄武岩。为了检验有关这个地球化学省的起源的假设，主要研究者将根据可以将其与地球化学储层相比的关键要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，并寻找证据表明地球化学信号是通过检查玄武岩和橄榄岩中钡的熔体夹杂物和晶体分区而产生的。将Gakkel Ridge的地球化学数据与其他海洋中部山脊，尤其是西南印度山脊（SWIR）进行了比较，以确定可以将高Ba和H2O归因于超级隆的扩散率，而不是北极构造历史。 研究的第二部分介绍了最高岩石圈和地壳的结构。 他将通过确定玄武岩玻璃，囊泡和熔体夹杂物中的二氧化碳丰度来研究岩浆最终流动以建立北极海的外壳的途径。这些数据将用于描述气体实体，以测试有关岩浆上升深度的假设以及到达与火山中心相距甚远的山脊轴的部分路径。 在研究的第三方面，他将研究H2O的富集在多大程度上影响了Gakkel Ridge融化的深度和程度。 为玄武岩生成的H2O数据将与主要元素数据集成。将构建模型，以考虑H2O的影响。 特别是，首席研究者将检验假设H2O在沿Gakkel Ridge沿火山强度的巨大变化中发挥了重要作用，并且在超震差散布速率下，脊对这种变化的反应更为明显。 在整个研究的所有方面，他都将整合地球物理的约束，并与其他山脊，尤其是SWIR进行比较。 拟议研究的更广泛的影响：本科生将在分析数据中发挥重要作用。 至少有两个学生将参与该项目，并将从中得出本科论文项目。 该项目的很大一部分将构成研究生的硕士论文项目。首席调查员将继续向小学和中学课程，童军团体和公民团体进行有关Gakkel Ridge的插图演讲。