Interactions between multiple-scale thermochemical heterogeneities in the lowermost mantle, specifically ultralow velocity zones (ULVZs) and large low shear velocity provinces (LLSVPs), are critical in lower mantle dynamics. However, the evolution of ULVZs formed outside LLSVPs has not been thoroughly explored. Here we perform two-dimensional numerical experiments to examine the evolution of highly dense ULVZs originating beneath cold downwellings and their interactions with the LLSVP. We find that ULVZs with an intrinsic density anomaly more than 500 kg/m³ compared with the ambient lowermost mantle cannot fully enter the LLSVP and would dwell at LLSVP margins for an indefinitely long time. This suggests that dense ULVZ within LLSVPs might have different sources from those outside LLSVPs. The buoyancy number and compositional viscosity of ULVZs are controlling factors on their dynamics and imprints on the core-mantle boundary (CMB), such as how much the ULVZ protrudes into the LLSVP and the CMB topography beneath the ULVZ. The excess density of ULVZs dictates their width but not their thickness. The oscillations of ULVZ morphology suggest that various types of plumes occur at the LLSVP margin. The mobility of ULVZ implies that the bottom margin of the LLSVP moves much faster than its center. Hot zones exist within the LLSVP near its margins, which may affect the evolution of ULVZs and subducted material nearby. The CMB topography under dense ULVZs are positive unless the buoyancy number of ULVZs exceeds 6.0. These results have intriguing implications for the distribution of ULVZs as well as thermochemical evolution in the lowermost mantle.

中文翻译：

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源自 LLSVP 之外的密集 ULVZ 的演变及其对 LLSVP 边缘动态的影响


最下层地幔中多尺度热化学异质性之间的相互作用，特别是超低速区 （ULVZ） 和大型低剪切速度省 （LLSVP），在下地幔动力学中至关重要。然而，在 LLSVP 之外形成的 ULVZ 的演变尚未得到彻底探索。在这里，我们进行了二维数值实验，以检查源自冷下的高密度 ULVZ 的演变以及它们与 LLSVP 的相互作用。我们发现，与环境最低层地幔相比，本征密度异常超过 500 kg/m³ 的 ULVZ 不能完全进入 LLSVP，并且会无限期地停留在 LLSVP 边缘。这表明 LLSVP 内的密集 ULVZ 可能与 LLSVP 外部的 ULVZ 具有不同的来源。ULVZ 的浮力数和成分粘度是其动力学和在核-地幔边界 （CMB） 上的印记的控制因素，例如 ULVZ 突出到 LLSVP 中的程度以及 ULVZ 下方的 CMB 地形。ULVZ 的过量密度决定了它们的宽度，而不是它们的厚度。ULVZ 形态的振荡表明，各种类型的羽流出现在 LLSVP 边缘。ULVZ 的移动性意味着 LLSVP 的下边缘移动速度比其中心快得多。LLSVP 内靠近其边缘的地方存在热区，这可能会影响附近 ULVZ 和俯冲物质的演变。除非 ULVZ 的浮力数超过 6.0，否则密集 ULVZ 下的 CMB 地形为正。这些结果对 ULVZ 的分布以及最下层地幔中的热化学演变具有有趣的意义。