Investigating Mantle Mixing and Chemical Layering through a Comprehensive Understanding of Transition Zone Seismic Discontinuities

过渡带地震不连续性(TZSD)是上地幔和下地幔之间矿物相变或/和成分变化的表现，是解开地幔质量和热量传输之谜以及地球内部长期演化的关键。然而，由于与TZSD相关的数据频带和灵敏度的限制，TZSD的地震特征通常是不完整的。我们创新了一种简单、有效和高分辨率的地幔不连续探测方法，在远震接收函数方法的背景下，通过检查宽带前向和后向散射波来探测地幔不连续。这种方法将使我们能够全面地描述大陆以下的TZSD，包括不连续地形、锐度和坡度、剪切速度跳跃和密度跳跃等属性。到目前为止，还没有一项研究能够同时确定TZSD的这些基本地震属性。这些对TZSD的更新描述将用于探索悬而未决的问题，包括过渡带的矿物学模型和挥发/熔体的存在。特别是，我们的目标是研究现今和过去的俯冲如何决定短期和长期的地幔混合，以及这种混合过程是否会反过来塑造板块下沉动力学。我们的新地震观测可以更好地解决一系列悬而未决的问题：数十亿年的长期混合是否像地球动力学模型所指示的那样导致明显的化学分层？如果存在这种化学分层，横向非均质性的程度和长度尺度是什么？原始结构是否有可能长期混合并被困在过渡带？过渡带是否可能是一个相对较浅的储集层，用于长期储存玄武岩的地球化学演化？是化学分层还是大规模原始结构决定了板块下沉的动力学？现代和古代俯冲是否将水循环到深部地幔和过渡带？水化过渡带是否会导致对流不稳定并导致板内火山活动？在拟议的工作中，我们将使用一种创新和有效的宽带前向和后向散射波观测来提供TZSD的全面表征，包括不连续地形、锐度、跨越边界的速度和密度跳跃以及不连续面上方/下方的坡度。这些空前丰富的观测将对与地球内部和演化相关的基本过程提供新的制约。

Transition zone seismic discontinuities (TZSDs), manifestations of mineral phase transitions or/and compositional changes between the upper mantle and the lower mantle, hold the key to resolve the mystery of mass and heat transport in the Earth's mantle and the long-term evolution of the Earth's interior.However, seismic characterizations of TZSDs are typically incomplete because of the limit in the data frequency bandwidth and sensitivity relevant to TZSDs. We innovate a simple, effective and high resolution probing of mantle discontinuity through examination of broadband forward and backward scattering waves in the context of the teleseismic receiver function method. This approach will allow us to comprehensively characterize TZSDs beneath the continents, including properties such as discontinuity topography, sharpness and gradient, shear velocity jump and density jump.To date, there has been no single study that is capable of simultaneously determining these essential seismic properties in the TZSDs. These renewed descriptions of TZSDs will be used to explore outstanding questions including mineralogical models of the transition zone and the presence of volatile/melt. In particular, we aim to address how current and past subduction determine short-term and long-term mantle mixing and whether such a mixing process may in turn shape slab sinking dynamics.A series of outstanding questions can be much better addressed with our new seismic observations:Did long-term mixing of billions of years result in apparent chemical layering as indicated in geodynamic models?What are the degree and the length scale of lateral heterogeneity if such a chemical layering exists?Is it possible that primordial structure may survive long term mixing and become trapped in the transition zone?Is the transition zone potentially a relatively shallow reservoir for long-term storage and geochemical evolution of basalt?Does chemical layering or large-scale primordial structure dictate the slab sinking dynamics?Does modern and ancient subduction recycle water into the deep mantle and transition zone?Does hydrated transition zone induce convective instability and contribute to intraplate volcanism?In the proposed work, we will use an innovative and effective observation with broadband forward and backward scattering waves to provide a comprehensive characterization of TZSDs, including properties such as discontinuity topography, sharpness, velocity and density jumps across the boundaries, and the gradient above/below the discontinuities. These unprecedentedly rich observations will provide renewed constraints on fundamental processes relevant to the Earth's interior and evolution.