Abstract
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.
Highlights
Global seismic discontinuities near 410 and 660 km depth in Earth’s mantle are expressions of solid-state phase transitions
We present direct and clear evidence for lateral variation in composition near the base of the Mantle transition zone (MTZ), from joint seismological and mineral physics analysis of the amplitudes of so-called SS precursors (S waves that bounce off MTZ discontinuities)
Shearer and Flanagan[23] used such amplitude versus offset (AVO) analysis to estimate a global average for Δρ and Δβ across the 410 and 660 from data beyond 110° (Fig. 2a)
Summary
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. We present direct and clear evidence for lateral variation in composition near the base of the MTZ, from joint seismological and mineral physics analysis of the amplitudes of so-called SS precursors (S waves that bounce off MTZ discontinuities). This shows that this is a promising technique to get constraints on the far elusive distribution of compositional heterogeneity within Earth’s mantle
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