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S-to-P heterogeneity ratio in the lower mantle and thermo-chemical implications

机译：下地幔中S与P的异质比及热化学影响

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We evaluate the thermo-chemical state of the lower mantle by analysing the differences in the pattern of heterogeneity between shear and compressional velocity variations and the S-to-P heterogeneity ratio (RS/P=lnVS/lnVP) as mapped in our model SPani and in alternative joint models. Robust structural differences between V-P and V-S evidence the presence of compositional heterogeneity within the two Large Low Shear Velocity Provinces (LLSVPs). We find also an increasing decorrelation with depth that can be associated with compositional layering of the LLSVPs. In addition, our model shows heterogeneity in the transition zone and mid mantle by complex morphology of subducting slabs and further differences between V-P and V-S that point to an unexpected heterogeneous lower mantle. Precise estimates of compositional heterogeneities are not yet affordable because of the difficulty to provide quantitative measure of RS/P, making it difficult to use this ratio to evaluate chemical heterogeneity. For instance, RS/P global median value () drops from approximate to 2.8 to approximate to 1.9, at 2500 km depth when the V-P component of SPani is replaced by a V-P model resulting from a differently regularized inversion and obtaining an equally good data fit. An increase of 20% of the SPani V-P anomalies also drastically reduces without significantly degrading the data fit. Noise in model parameters also leads to overestimate RS/P in the two LLSVPs as we show with synthetic tests. Additional mineral physics uncertainties for compositional effects on RS/P and for the conversion of lnVS and lnVP into density further complicates a precise chemical interpretation.
机译：我们通过分析在模型SPani中绘制的剪切和压缩速度变化之间的异质性模式差异以及S-P异质比（RS / P = lnVS / lnVP），评估下地幔的热化学状态。以及其他联合模型。 V-P和V-S之间的稳健结构差异证明了两个大的低剪切速度省（LLSVP）内存在成分异质性。我们还发现，与深度的去相关性增加，这与LLSVP的成分分层有关。此外，我们的模型通过俯冲板片的复杂形态以及过渡带和V-S之间的进一步差异显示了过渡带和中地幔的非均质性，这表明了一个意想不到的非均质下地幔。由于难以提供RS / P的定量测量方法，因此难以精确估算组成的异质性，因此很难使用该比率来评估化学异质性。例如，当SPani的VP分量被不同正则化反演和获得的VP模型取代时，在2500 km深度，RS / P全局中值（）从大约2.8下降到大约1.9。同样出色的数据拟合。 SPani V-P异常的增加20％也可以大大降低而不会显着降低数据拟合度。模型参数中的噪声还会导致两个LLSVP中的RS / P过高，正如我们通过综合测试所显示的那样。对于RS / P的成分影响以及lnVS和lnVP转化为密度的其他矿物物理不确定性进一步使精确的化学解释复杂化。

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