Geodynamics and Melting at Ultra-slow and Oblique Spreading Centers

孟德西(0649103)在众多使我们的星球在太阳系中独一无二的突出特征中，很少有像全球板块构造的存在那样令人费解的。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机制。大洋中脊(MORS)作为大多数陆地火山活动和板块分离的地点特别重要。丰富的火山活动突显了莫尔斯岩浆产生和运输的重要性。一些全球地幔对流模型甚至表明，MORS的熔融是板块构造的关键要求。然而，火山活动在最慢的山脉，如最近探索的北极山脊和西南印度山脊(SWIR)并不是持续的。这些本质上是山顶洞人的MOR是如何运作的？我们建议建立地幔流动的数值模型，以及超低脊火山活动的分布、强度和地球化学，以阐明最慢的MORS的地球动力学。最近的研究揭示了最慢的MORS的许多特征。火山作用强烈减弱，局限于离散的和广泛分布的火山中心，以及指示有限熔融和深层原地结晶的地球化学示踪。显然，熔融提取，如果不是生产，在这种环境下是严重受限的。缓慢的扩散肯定会导致相对较冷的地幔，从而将熔融限制在很深的地方。然而，仍有两个问题需要解决：为什么火山活动是局部性的？是否有足够的熔体存在于深度，以促进板块发散？为了回答这些问题，我们将建立超低MORS地幔流动的数值模型，将地幔隆起与山脊几何形状和固有的地幔流动不稳定性联系起来，计算超低山脊的地幔热结构，并估计岩浆的轨迹及其化学演化。这些模型产生的火山流、玄武岩和橄榄岩化学成分的轴向变化将直接与最慢的MORS(SWIR，Gakkel Ridge)的数据进行比较，这些数据可以进行精确的水深测量和地球化学分析。具体地说，我们将比较模拟和观测到的局部火山中心的强度和间距，将这些中心的位置与山脊分段联系起来，将预期的熔融程度和结晶深度与橄榄岩化学进行比较，并评估以玄武岩化学为重点的富饶榴辉岩成分和轴线熔融的重要性。这项研究将产生MORS先进的3D地球动力学和化学模型。通过弄清楚最慢的MORS的地球动力学，这个项目着重于板块构造的起源，以及当山脊减速和地幔充分冷却时板块构造的可能终止。我们将讨论如何在玄武岩和橄榄岩地球化学中记录熔融、岩浆迁移和山脊分段。我们将评估熔融和岩浆迁移在山脊力学中的作用，使用熔化有限的超低山脊环境，作为充分研究的无熔融构造环境和熔岩丰富的大部分山脊之间的垫脚石。除了培训早期研究生和促进两名初级教员的职业生涯，该项目将通过提供一个新的、灵活的工具包来改进计算基础设施，该工具包分布在网上，用于从三维地幔流动模型计算地球化学特征和合成重力场。这个工具包将通过使用COMSOL多物理的循序渐进的教程，通过在MOR的拐角流动的基准问题来详细记录和说明。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.