Compositional layering within the large low shear‐wave velocity provinces in the lower mantle

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/m3 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.

Geochemistry and Distribution of Recycled Domains in the Mantle Inferred From Nd and Pb Isotopes in Oceanic Hot Spots: Implications for Storage in the Large Low Shear Wave Velocity Provinces

“…However, this explanation is not consistent with the observation that many of the non-EM plumes-including Canary and Iceland-are interpreted to be thermochemical plumes due to the apparent width of their conduits in seismic studies (e.g., French & Romanowicz, 2015) and are therefore likely to have entrained chemically heterogeneous material from the base of the mantle. If oceanic hot spots without EM (all lavas have 143 Nd/ 144 Nd > 0.512630) and with EM (at least one lava has 143 Nd/ 144 Nd ≤ 0.512630) signatures are sourced by the LLSVPs, an important conclusion is that the LLSVPs are isotopically heterogeneous, a conclusion supported by recent geodynamic models (Ballmer et al, 2016;Li et al, 2014). While we cannot rule out a shallow contribution to hot spot geochemistry due to uppermost mantle heterogeneities in the melting region of the plume, a range of observations are consistent with a deep origin (e.g., Castillo, 1988;Jackson et al, 2018), and we consider that the geographic association of EM hot spots and LLSVPs is compelling and likely not coincidence.…”

“…This is because some Southern Hemisphere oceanic hot spots sample only non-EM compositions. However, we cannot exclude stratified compositional layering within the LLSVPs as a means of separating different deep mantle domains (e.g., EM vs non-EM) sourced by hot spots (Ballmer et al, 2016), but the EM material in this model would need to exhibit a latitudinal stratification to be consistent with the higher concentration of extreme EM hot spots in the Southern Hemisphere. In this way, some Southern Hemisphere plumes entrain plums of EM material, while other Southern Hemisphere plumes entrain only the non-EM LLSVP matrix, giving rise to both austral EM oceanic hot spots (i.e., hot spots that have at least one lava with 143 Nd/ 144 Nd ≤ 0.512630) and austral non-EM oceanic hot spots (i.e., hot spots that do not host any lavas with 143 Nd/ 144 Nd ≤ 0.512630).…”

“…It remains to be determined how or whether other processes-including dilution of entrained LLSVP material by greater entrainment of ambient lower mantle (i.e., non-LLSVP) material at the base of the plume during upwelling (Ballmer et al, 2016), preferential sampling of less dense deep mantle reservoirs relative to denser reservoirs , and preferential melting of less refractory domains in plumes beneath hot spots (e.g., Ito & Mahoney, 2005a, 2005bIto & Mahoney, 2006)-complicate the approach of using the geochemistry of hot spot lavas to infer deep mantle compositions. It remains to be determined how or whether other processes-including dilution of entrained LLSVP material by greater entrainment of ambient lower mantle (i.e., non-LLSVP) material at the base of the plume during upwelling (Ballmer et al, 2016), preferential sampling of less dense deep mantle reservoirs relative to denser reservoirs , and preferential melting of less refractory domains in plumes beneath hot spots (e.g., Ito & Mahoney, 2005a, 2005bIto & Mahoney, 2006)-complicate the approach of using the geochemistry of hot spot lavas to infer deep mantle compositions.…”

Lower Mantle Dynamics Perceived With 50 Years of Hindsight From Plate Tectonics

“…It may mark an increase in viscosity (Marquardt & Miyagi, ; Rudolph et al, ), which restricts, without necessarily preventing, flow between the upper and lower mantle. It might also mark a compositional boundary (Ballmer et al, , ). Many of the tomographic images in Figures can be interpreted to be consistent with isolation of the upper and lower mantle from one another, as Wen and Anderson (, ) argued more than 20 years ago.…”

“…Regarding a possible deeply held prejudice that has slowed understanding, the old‐timers—Francis Birch, Keith Bullen, Norman Haskell, and surely others—might have put the boundary between the upper and lower mantle in the right place, near 1,000 km, and not at 660 km as most have accepted, after that depth seemed to fit with simple images of plate tectonics. Recently, many have suggested that a boundary at 1,000 km is the more important for geodynamics (e.g., Ballmer et al, , ; Čížková & Bina, ; Durand et al, ; Fukao et al, , ; Jenkins et al, ; Marquardt & Miyagi, ; Morra et al, ; Rudolph et al, ; Vinnik et al, ; Wen & Anderson, , ). I argue that a boundary at 1,000 km not only fits seismological data well but also allows greater isolation between the upper and lower mantle than has, until recently, been widely assumed but not total isolation.…”