Compositional layering within the large low shear‐wave velocity provinces in the lower mantle
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Abstract The large low shear‐wave velocity provinces (LLSVP) are thermochemical anomalies in the deep Earth's mantle, thousands of km wide and ∼1800 km high. This study explores the hypothesis that the LLSVPs are compositionally subdivided into two domains: a primordial bottom domain near the core‐mantle boundary and a basaltic shallow domain that extends from 1100 to 2300 km depth. This hypothesis reconciles published observations in that it predicts that the two domains have different physical properties (bulk‐sound versus shear‐wave speed versus density anomalies), the transition in seismic velocities separating them is abrupt, and both domains remain seismically distinct from the ambient mantle. We here report underside reflections from the top of the LLSVP shallow domain, supporting a compositional origin. By exploring a suite of two‐dimensional geodynamic models, we constrain the conditions under which well‐separated “double‐layered” piles with realistic geometry can persist for billions of years. Results show that long‐term separation requires density differences of ∼100 kg/m 3 between LLSVP materials, providing a constraint for origin and composition. The models further predict short‐lived “secondary” plumelets to rise from LLSVP roofs and to entrain basaltic material that has evolved in the lower mantle. Long‐lived, vigorous “primary” plumes instead rise from LLSVP margins and entrain a mix of materials, including small fractions of primordial material. These predictions are consistent with the locations of hot spots relative to LLSVPs, and address the geochemical and geochronological record of (oceanic) hot spot volcanism. The study of large‐scale heterogeneity within LLSVPs has important implications for our understanding of the evolution and composition of the mantle.Keywords:
Layering
Core–mantle boundary
Hotspot (geology)
Hotspot (geology)
Mantle plume
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Hotspot (geology)
Classification of discontinuities
Core–mantle boundary
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Hotspot (geology)
Classification of discontinuities
Mantle plume
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The seismic structure of the transition-zone discontinuities was studied beneath the forty-nine hotspot locations of the catalog of Courtillot et al. (2003), using a global data set of SS precursors. Some of these hotspots are proposed to originate from plumes rising in the upper mantle or from the core-mantle boundary region. I found thin transition zones in approximately two-thirds of the twenty-six hotspot locations for which precursor observations could be made. This observation agrees with the expectation for the olivine phase transition of a systematically thin transition zone in high-temperature regions. Other hotspot locations showed a clear deepening of both the 410- and 660-km discontinuities, which is consistent with a phase transition from majorite garnet to perovskite at a depth of 660 km. Predictions from mineral physics suggest that this transition is more important than the olivine phase transition in regions with high mantle temperatures. So, a hotspot location with a deep 410-km discontinuity in combination with either a shallow or deep 660-km discontinuity might be consistent with hot upwellings rising from the lower into the upper mantle. Hotspot locations with a shallow 410-km discontinuity are not in agreement with a positive thermal anomaly from the surface down to the mantle transition zone. This new interpretation of seismic discontinuities in the transition zone has important implications for our understanding of geodynamics in potential mantle plume locations.
Hotspot (geology)
Classification of discontinuities
Core–mantle boundary
Mantle plume
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Hotspot (geology)
Core–mantle boundary
Mantle plume
Seismic Tomography
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Mantle plume
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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.
Hotspot (geology)
Classification of discontinuities
Core–mantle boundary
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