Elasticity of davemaoite as a primary contributor to lower-mantle heterogeneities.

Geophysical detection of subducted mid–ocean ridge basalt (MORB) in the lower mantle is hindered by uncertainties in the elasticity of Fe,Al,Mg,Ti–bearing davemaoite, a key MORB component. Using Brillouin spectroscopy and x-ray diffraction, we determined the elasticity of a Ca0.906(1)Fe2+0.027(1)Fe3+0.042(1)Mg0.033(1)Al0.072(1)Ti0.020(1)Si0.912(1)O3 davemaoite up to 113 gigapascals and 2294 K. We found that it exhibited a shear wave velocity 10 to 20% slower than end-member davemaoite, making it the slowest phase among major lower-mantle minerals. Our models show that MORB, containing 20 to 25 volume percent davemaoite, potentially contributes to large low-shear-velocity provinces (LLSVPs), whereas a cumulate layer enriched in davemaoite crystallized from basal magma ocean may comprise ultralow-velocity zones (ULVZs). Davemaoite's ability to host incompatible and heat-producing elements possibly links LLSVPs and ULVZs to mantle plume initiation and geochemical signatures of ocean island basalts. Editor's summary: Seismic images of Earth's lower mantle revealed two continental-scale regions with anomalously slow shear-wave velocities rising from the core-mantle boundary. Understanding their origin depends on knowing their mineralogy, a challenge to measure under conditions at ~2500 kilometers depth. Zhou et al. used Brillouin spectroscopy and x-ray diffraction, with carbon dioxide laser heating, to synthesize and measure at lower-mantle conditions the elasticity of Fe-, Al-, Mg-, and Ti-bearing davemaoite, a common component of subducted oceanic crust. Their results showed that davemaoite is seismically slow, making it a likely contributor to low-velocity regions of the lower mantle, possibly through accumulation of subducted oceanic crust. —Angela Hessler [ABSTRACT FROM AUTHOR]