Mantle Petrology and Tectonics at the Gakkel Ridge

加克尔海岭的地幔岩石学和构造

• 批准号: 0327591
• 负责人: Henry Dick
• 金额: $ 38.5万
• 依托单位: Woods Hole Oceanographic Institution
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2003
• 资助国家: 美国
• 起止时间: 2003-08-01 至 2007-01-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0327591&HistoricalAwards=false
• 关键词: Mantle; Petrology; Tectonics; Gakkel; Ridge

ABSTRACTDickOPP-0327591The Principal Investigator will study the petrology and tectonic setting of Arctic Ocean abyssal peridotites from the Gakkel Ridge, a previously inaccessible geologic end-member for sea floor spreading. It provides a window into the Earth's interior which critical for geochemical mapping. Due to ultraslow spreading rates, the Gakkel is tectonically distinct from other ridges with no transform faults along its 1800 km length, with thin highly variable crust, and abundant mantle exposures. The Arctic Mid-Ocean Ridge Expedition (AMORE) was the first attempt to sample this ridge and the expedition succeeded beyond all expectations. There were 200 successful dredges. From 3degreesE to 87degreesE, the Ridge proved to consist of local volcanic centers linked by sparsely magmatic segments and peridotites were dredged at 36 rift valley walls and floor locations. The Principal Investigator will analyze peridotite modal mineralogy and trace and major elements of about ~150-200 samples. He will also study the tectonic setting and emplacement using dredge results and the new high-resolution Seabeam data. This research will constrain the relationship between peridotites and basalts, the nature of mantle melting at its lower limit, and the composition of the Arctic mantle. This will be a unique opportunity to assess models for mantle melting and the controls on mid-ocean ridge basalt composition. Because peridotites representing the lowest melting temperatures ocean ridges, they can also be used to direct constrain models for the primary composition and variability of the MORB source.Broader Impacts: The Principal Investigator will continue use various outreach efforts to publicize the expedition and the importance of Arctic research to the general public and scientific community. He has already made a number of presentations at scientific meetings and held press group interviews. He made presentations to a number of prominent congressional staff and senators. There have been three comprehensive studies of abyssal peridotites that show that these samples have much more complicated histories that this proposal implies. Refertilization, melt impregnation, high-pressure melt-rock interactions, "excess olivine" etc. have all been identified in abyssal peridotites. These processes are also widely recognized in xenoliths and massif-type peridotites, but the PI of this proposal wants to interpret these GR data in a vacuum. This field is rapidly advancing and the PI needs to become current.In this proposal the PI states that approximately 2/3 of abyssal peridotites 'are like simple residues of melting'. This statement is perhaps contradicted by studies that show that most abyssal peridotites are enriched in Na and Ti above what is expected based on fractional melting (Elthon, JGR, 1992; Asimow, EPSL 1999; Baker and Beckett, EPSL, 1999). It could be that most abyssal peridotites are not particularly like simple residues of melting, and this proposal doesn't suggest that the PI plans to consider this issue. Moreover, in the PIs compendium of mineral composition and modal data from abyssal peridotites (Dick, 1989; in Magmatism in the Ocean Basins), the Mg and Fe contents of minerals are noted to be potentially inaccurate in a footnote from the author. Finally, it seems that Ni in olivine data published by the PI is perhaps systematically low, relative to studies by other workers (in Dick, 1989, olivine in most samples are listed as having NiO between 0.22 and 0.28 wt% and this range is definitely low relative to results on abyssal peridotites published by most other workers). I am somewhat concerned that the PI sends samples to be analyzed by someone else at MIT, rather than personally overseeing the work.Objective 3 makes the case that the Gakkel ridge allows one to look at AP away from fracture zones-so we should be able to test the idea that there are more dunites within magmatic segments. Surely the PI has an idea about the answer to this right now simply by looking at the rocks collected in hand sample. Indeed, an annoying statement is that there is 'a growing body of data, LARGELY UNPUBLISHED, that dunite is common in rift valley walls.' I have to suspect that much of these data are the PIs-why not published-why aren't the data at least put in some assessable form rather than this statement.

摘要DickOPP-0327591首席研究员将研究来自 Gakkel 海脊的北冰洋深渊橄榄岩的岩石学和构造环境，Gakkel 海脊是以前无法到达的海底扩张的地质端元。它提供了一个了解地球内部的窗口，这对于地球化学测绘至关重要。 由于扩张速度极慢，Gakkel 在构造上与其他山脊不同，沿其 1800 公里的长度没有转换断层，地壳薄且高度变化，地幔暴露丰富。 北极大洋中脊探险队 (AMORE) 是对该山脊进行采样的首次尝试，这次探险的成功超出了所有人的预期。 共有200艘成功的挖泥船。从东经 3 度到东经 87 度，山脊被证明由由稀疏的岩浆段连接的当地火山中心组成，并且在 36 个裂谷壁和谷底位置挖掘了橄榄岩。 首席研究员将分析约 150-200 个样品的橄榄岩形态矿物学以及微量和主要元素。 他还将利用挖泥机结果和新的高分辨率 Seabeam 数据研究构造环境和就位。这项研究将约束橄榄岩和玄武岩之间的关系、地幔熔化下限的性质以及北极地幔的成分。这将是评估地幔融化模型和洋中脊玄武岩成分控制的独特机会。由于橄榄岩代表了最低融化温度的洋脊，因此它们还可用于直接约束 MORB 源的主要成分和变异性模型。 更广泛的影响：首席研究员将继续利用各种外展工作向公众和科学界宣传这次探险以及北极研究的重要性。 他已经在科学会议上做了多次演讲并接受了新闻组采访。 他向一些著名的国会工作人员和参议员做了演讲。对深海橄榄岩进行的三项综合研究表明，这些样本具有比该提议所暗示的更为复杂的历史。 再肥化、熔体浸渍、高压熔体-岩石相互作用、“过量橄榄石”等都已在深海橄榄岩中被发现。 这些过程在捕虏体和山体型橄榄岩中也得到了广泛认可，但该提案的 PI 希望在真空中解释这些 GR 数据。 这一领域正在迅速发展，PI 需要与时俱进。在这项提案中，PI 指出，大约 2/3 的深渊橄榄岩“就像简单的熔化残留物”。 这一说法可能与研究相矛盾，研究表明大多数深海橄榄岩中 Na 和 Ti 的富集量高于基于分级熔融的预期（Elthon，JGR，1992；Asimow，EPSL 1999；Baker 和 Beckett，EPSL，1999）。 大多数深海橄榄岩可能并不特别像简单的熔化残留物，并且该提案并不表明 PI 计划考虑这个问题。 此外，在深海橄榄岩矿物成分和模态数据的 PI 简编中（Dick，1989；《大洋盆地的岩浆作用》），作者在脚注中指出矿物中的 Mg 和 Fe 含量可能不准确。 最后，相对于其他工作者的研究，PI 发表的橄榄石数据中的 Ni 似乎可能系统性较低（Dick，1989 年，大多数样品中的橄榄石被列为 NiO 含量在 0.22 至 0.28 wt% 之间，并且相对于大多数其他工作者发表的深海橄榄岩结果而言，这个范围绝对较低）。 我有点担心 PI 将样本发送给麻省理工学院的其他人进行分析，而不是亲自监督工作。目标 3 表明，Gakkel 山脊允许人们在远离断裂带的地方观察 AP，因此我们应该能够检验岩浆段内存在更多沙丘的想法。当然，PI 现在只需查看手中收集的岩石样本就知道这个问题的答案。事实上，一个令人恼火的说法是，“越来越多的数据表明，纯橄榄岩在裂谷壁中很常见，但大部分尚未发表。”我不得不怀疑这些数据中的大部分都是 PI——为什么不发布——为什么这些数据至少不以某种可评估的形式而不是这个声明。