The Origin of the Lehmann Discontinuity Beneath the Ancient Craton: Insight From the High Pressure‐Temperature Elasticity Measurements of Topaz

In this study, we concentrate on the seismic signature of subducted sediments and suggest the formation of the L‐discontinuity beneath the ancient craton related to migrated sediment dehydration. We first determined the single‐crystal elasticity of topaz, the product of sediment dehydration, at high pressures and temperatures by Brillouin scattering. Using the derived elastic parameters, we establish the velocity and density profiles of subducted sediments in the upper mantle. According to our modeling results, 8.5–17.5 vol.% sediments intruding into the upper mantle will induce a 2%–4% low‐VSanomaly at 210–260 km. Meanwhile, continuous heating will lead to the dehydration of phengite in sediments. The dehydration of this amount sediments can generate a 3%–6% ISSwith negative Clapeyron slopes, satisfying the observed L‐discontinuity in northern Finland and northern America without the anisotropy changes but accompanied by low‐velocity anomalies. Our study thus provides new insights into the origin of the L‐discontinuity. The origin of the global seismic discontinuities in the Earth's mantle has been well understood, but the formation mechanism of some local seismic discontinuities is still vague. Lehmann discontinuity (L‐discontinuity) is one of the regional discontinuities in the upper mantle with a 2%–6% impendence contrast (ISS) and negative Clapeyron slope. However, the deformation mechanism changes of olivine cannot explain the formation of the L‐discontinuity without the anisotropy changes. Here, we emphasize the significance of the migration and dehydration of the subducted sediments in the origin of the L‐discontinuity beneath the ancient craton. We determined the single‐crystal elasticity of topaz, the product of the sediment dehydration, and then investigated the seismic signature of subducted sediments. Our modeling results indicated that ∼17.5 vol.% sediments intruding into the upper mantle would lead to ∼4% low‐VSanomalies at 210–260 km. After the continuous heating, these intruding sediments would dehydrate by the reaction with the negative Clapeyron slopes, thus producing significant ISS. Therefore, the migration and dehydration of the subducted sediments can reasonably interpret the seismic characteristics of the L‐discontinuity beneath the cratonic regions such as northern Finland and northern America. Our results deepen the understanding of the origin of the L‐discontinuity. Single‐crystal elasticity of topaz under high pressure and temperature has been investigated by Brillouin scatteringThe obtained elasticity parameters are used to model the density and velocity profiles of the subducted sedimentsWe suggest the formation of the L‐discontinuity beneath the ancient craton related to migrated sediment dehydration Single‐crystal elasticity of topaz under high pressure and temperature has been investigated by Brillouin scattering The obtained elasticity parameters are used to model the density and velocity profiles of the subducted sediments We suggest the formation of the L‐discontinuity beneath the ancient craton related to migrated sediment dehydration