Geodynamics and Melting at Ultra-slow and Oblique Spreading Centers

摘要蒙特西（0649103）在许多突出的特点，使我们的星球在太阳系中独一无二，很少有像全球板块构造的存在令人困惑。为了解释板块构造的起源以及相关的地震和火山灾害，必须首先阐明板块边界的力学。大洋中脊作为大多数陆地火山活动和板块分离的场所特别重要。丰富的火山活动突出了岩浆生成和迁移的重要性。一些全球地幔对流模型甚至认为，MORs的熔融是板块构造的一个关键要求。然而，火山活动在最慢的MOR上并不连续，例如最近勘探的北极海脊和西南印度洋海脊（SWIR）。这些基本上无火山的莫尔是如何工作的？我们建议建立地幔流的数值模型，并在超低脊火山活动的分布，强度和地球化学来阐明最慢MORs的地球动力学。火山作用强烈减少，局限于离散和广泛分布的火山中心，和地球化学示踪剂，表明有限的熔融和深就地结晶。显然，熔体萃取，如果不是生产，在这种环境中受到严重限制。缓慢的扩散当然会导致相对较冷的地幔，这限制了大深度的融化。然而，还有两个问题有待解决：为什么火山活动是局部的？是否有足够的熔体存在于深处以促进板块的分离？为了回答这些问题，我们将建立在超低MORs地幔流动的数值模型，连接地幔隆起的山脊的几何形状和内在的地幔流动的不稳定性，计算地幔的热结构在超低的山脊，并估计岩浆的轨迹和它们的化学演化。由这些模型产生的火山流量和玄武岩和橄榄岩化学成分的沿轴变化将直接与最慢的MORs（SWIR，Gakkel Ridge）的数据进行比较，其中精确的水深测量和地球化学分析正在进行中。具体来说，我们将比较模拟和观察到的强度和局部火山中心的间距，这些中心的位置与山脊分割，比较预期的熔融程度和结晶深度与橄榄岩化学，并评估熔融的肥沃的榴辉岩成分的重要性，并沿轴向专注于玄武岩chemical.This研究将产生先进的三维地球动力学和化学模型MORs。通过对最慢的MOR的地球动力学的研究，这个项目强调了板块构造的起源，以及当山脊减缓和地幔充分冷却时板块构造的终止。我们将讨论如何熔融，岩浆迁移，和脊分段记录在玄武岩和橄榄岩地球化学。我们将评估熔融和岩浆迁移对洋脊力学的作用，使用熔融有限的超低速洋脊环境，作为充分研究的无熔融构造环境和熔融丰富的大多数洋脊之间的垫脚石。除了培养早期研究生和促进两名初级教师的职业生涯外，该项目将通过提供一种新颖的，在线分发的灵活工具包，用于根据三维地幔流模型计算地球化学特征和合成重力场。通过使用COMSOL MULTIPHYSICS的分步教程，将通过莫尔的拐角流基准问题对该工具包进行全面记录和说明。由于一些模型将使用澳大利亚开发的Underworld软件包，因此该项目涉及国际合作。

Abstract Montesi(0649103)Among the many outstanding characteristics that make our planet unique in the solar system, few are as puzzling as the presence of global plate tectonics. To explain the origins of plate tectonics and related seismic and volcanic hazard, the mechanics of plate boundary must first be elucidated. Mid-Ocean Ridges (MORs) are particularly important as the locus of the majority of terrestrial volcanism and plate separation. The abundant volcanism highlights the importance of magma generation and transport at MORs. Some global mantle convection models even suggest that melting at MORs is a key requirement of plate tectonics. However, volcanism is not continuous at the slowest MORs such as the recently explored Arctic ridges and the Southwest Indian Ridge (SWIR). How do these essentially avolcanic MOR work? We propose to build numerical models of mantle flow and the attending distribution, intensity, and geochemistry of volcanism at ultraslow ridges to elucidate the geodynamics of the slowest MORs.Recent studies have exposed the many peculiarities of the slowest MORs. Volcanism is strongly reduced, localized onto discrete and widely spaced volcanic centers, and geochemical tracers that indicate limited melting and deep in-situ crystallization. Clearly, melt extraction, if not production, is severely limited in that environment. Slow spreading certainly results in a relatively cold mantle, which limits melting to great depth. However, two questions remain to be addressed: Why is volcanism localized? Is enough melt present at depth to facilitate plate divergence? To answer these questions, we will construct numerical models of mantle flow at ultraslow MORs, linking mantle upwellings to ridge geometry and intrinsic mantle flow instabilities, compute the thermal structure of the mantle at ultraslow ridges, and estimate the trajectory of magma and their chemical evolution. The along-axis variations in volcanic flux and basalt and peridotite chemistry produced by these models will be directly compared with data from the slowest MORs (SWIR, Gakkel Ridge) for which precise bathymetry is available and geochemical analysis is underway. Specifically, we will compare the modeled and observed intensity and spacing of localized volcanic centers, relate the location of these centers to ridge segmentation, compare the expected extent of melting and depth of crystallization with peridotite chemistry, and evaluate the importance of melting of a fertile eclogite component and along-axis focusing on basalt chemistry.This research will produce advanced 3D geodynamical and chemical models of MORs. By puzzling out the geodynamics of the slowest MORs, this project weighs on the origins of plate tectonics and its proposed cessation when ridges slow down and the mantle cools sufficiently. We will address how melting, magma migration, and ridge segmentation are recorded in basalt and peridotite geochemistry. We will evaluate the role of melting and magma migration on ridge mechanics, using the ultraslow ridge environment, where melting is limited, as a stepping stone between well-studied melt-free tectonic settings and the majority of ridges in which melt is abundant.Beyond training an early graduate student and advancing the careers of two junior faculty members, this project will improve the computation infrastructure by providing a novel, flexible toolkit, distributed online, to compute the geochemical signature and synthetic gravity fields from three-dimensional mantle flow models. This toolkit will be thoroughly documented and illustrated by a benchmark problem of corner flow at a MOR through a step-by-step tutorial using COMSOL MULTIPHYSICS. As some models will use the Underworld software package developed in Australia, this project involves an international collaboration.