Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA, 02139, USA.
Seismological Laboratory, California Institute of Technology, Pasadena, CA, 91125, USA.
Nat Commun. 2018 Mar 28;9(1):1266. doi: 10.1038/s41467-018-03654-6.
Global seismic discontinuities near 410 and 660 km depth in Earth's mantle are expressions of solid-state phase transitions. These transitions modulate thermal and material fluxes across the mantle and variations in their depth are often attributed to temperature anomalies. Here we use novel seismic array analysis of SS waves reflecting off the 410 and 660 below the Hawaiian hotspot. We find amplitude-distance trends in reflectivity that imply lateral variations in wavespeed and density contrasts across 660 for which thermodynamic modeling precludes a thermal origin. No such variations are found along the 410. The inferred 660 contrasts can be explained by mantle composition varying from average (pyrolitic) mantle beneath Hawaii to a mixture with more melt-depleted harzburgite southeast of the hotspot. Such compositional segregation was predicted, from petrological and numerical convection studies, to occur near hot deep mantle upwellings like the one often invoked to cause volcanic activity on Hawaii.
地球地幔中 410 和 660km 深处的全球地震不连续带是固态相变的表现。这些相变调节了地幔中的热流和物质通量，其深度的变化通常归因于温度异常。在这里，我们使用 SS 波在夏威夷热点下方 410 和 660 处的新型地震阵列分析。我们发现反射率的振幅-距离趋势，这意味着在 660 处横波速度和密度对比存在横向变化，热力学模型排除了热的起源。在 410 处没有发现这样的变化。推断出的 660 对比可以用地幔组成来解释，从夏威夷下方的平均（火成岩）地幔到与更多熔体亏损的方辉橄榄岩的混合物。这种成分分异是从岩石学和数值对流研究中预测的，它发生在地幔深部热上升流附近，这种上升流常常被用来引起夏威夷的火山活动。
