The nature and origin of the two large low-velocity provinces (LLVPs) in the lowest part of the mantle remain controversial. These structures have been interpreted as a purely thermal feature, accumulation of subducted oceanic lithosphere or a primordial zone of iron enrichment. Information regarding the density of the LLVPs would help to constrain a possible explanation. In this work, we perform a density inversion for the entire mantle, by constraining the geometry of potential density anomalies using tomographic vote maps. Vote maps describe the geometry of potential density anomalies according to their agreement with multiple seismic tomographies, hence not depending on a single representation. We use linear inversion and determine the regularization parameters using cross-validation. Two different input fields are used to study the sensitivity of the mantle density results to the treatment of the lithosphere. We find the best data fit is achieved if we assume that the lithosphere is in isostatic balance. The estimated densities obtained for the LLVPs are systematically positive density anomalies for the LLVPs in the lower 800–1000 km of the mantle, which would indicate a chemical component for the origin of the LLVPs. Both iron-enrichment and a mid-oceanic ridge basalt (MORB) contribution are in accordance with our data, but the required superadiabatic temperature anomalies for MORB would be close to 1000 K.

中文翻译：

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通过地震约束重力反演恢复的大型低速省的密度增加

位于地幔最低部分的两个大型低速省（LLVP）的性质和起源仍存在争议。这些结构已被解释为纯热特征，俯冲的海洋岩石圈或铁富集原始带的聚集。有关LLVP密度的信息将有助于限制可能的解释。在这项工作中，我们通过使用断层摄影投票图约束潜在密度异常的几何形状，对整个地幔进行了密度反演。投票图根据其与多个地震层析成像的一致性来描述潜在密度异常的几何形状，因此不依赖于单个表示。我们使用线性反演，并使用交叉验证确定正则化参数。使用两个不同的输入字段来研究地幔密度结果对岩石圈处理的敏感性。如果我们假设岩石圈处于等静平衡状态，则可以找到最佳的数据拟合。LLVPs的估计密度是地幔下800-1000 km内LLVPs的系统正密度异常，这表明LLVPs的化学成分。铁的富集和大洋中脊玄武岩（MORB）的贡献均与我们的数据一致，但MORB所需的超绝热温度异常将接近1000K。LLVPs的估计密度是地幔下800-1000 km内LLVPs的系统正密度异常，这表明LLVPs的化学成分。铁的富集和大洋中脊玄武岩（MORB）的贡献均与我们的数据一致，但是MORB所需的超绝热温度异常将接近1000K。LLVPs的估计密度是地幔下800-1000 km内LLVPs的系统正密度异常，这表明LLVPs的化学成分。铁的富集和大洋中脊玄武岩（MORB）的贡献均与我们的数据一致，但是MORB所需的超绝热温度异常将接近1000K。