Melting at the Edge of a Slab in the Deepest Mantle

• Published in: Geophysical research letters 2019-07, Vol.46 (14), p.8000-8008
• Main Authors: Thorne, Michael S., Takeuchi, Nozomu, Shiomi, Katsuhiko
• Format: Article
• Language: English
• Subjects: Basalt; Broadband; Core-mantle boundary; Earth; Earthquakes; Lower mantle; Magma; Melting; Melting point; Melting points; mid‐ocean ridge basalt; Oceans; P waves; Seismic activity; Seismic velocities; Seismic wave velocities; Seismic waves; Seismograms; Slabs; Subduction; Subduction (geology); Tiltmeters; ultralow‐velocity zone; Velocity; Wave velocity; Waveforms
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2019GL082493

We analyzed new recordings of SPdKS seismic waveforms from a global set of broadband seismograms and horizontal tiltmeters from the Hi‐net array in Japan from 26 earthquakes in the Central American region. The anomalous waveforms are consistent with the presence of at least three ultralow‐velocity zones (ULVZs), on the core‐mantle boundary beneath northern Mexico and the southeastern United States. These ULVZs ring an area of high seismic wave speeds observed in tomographic models that has long been associated with past subduction. Waveform modeling using the PSVaxi method suggests that the ULVZs have S and P wave velocity decreases of 40% and 10%, respectively. These velocity decreases are likely best explained by a partially molten origin where the melt is generated through melting of mid‐ocean ridge basalt atop the subducted slab. Plain Language Summary We use a set of seismic observations recorded globally to investigate the lower mantle beneath Central America. The deepest mantle in this region has been associated with the final resting place of subducted slab material from subduction that initiated approximately 200 million years ago. This ancient subducted material is associated with high seismic wave speeds in the lowermost mantle just above the core‐mantle boundary. We find that patches of highly reduced seismic wave speeds, referred to as ultralow‐velocity zones (ULVZs), appear to be associated with the border of the high wave speed region, along the border of the subducted slab material. These ULVZ patches are consistent with being regions of partial melt. A possible scenario for their creation is that mid‐ocean ridge basalt (MORB), comprising the crust of the subducted slab material, has a low melting point at conditions in the deep earth and may be melting as the slabs reach the bottom of the mantle. Previous experimental work has suggested that MORB will likely partially melt in the deep mantle, yet little evidence for the existence of MORB partial melt has previously been found. Key Points Subducted slab material resting on the core‐mantle boundary is ringed by ultralow‐velocity zones A partially molten origin to these ultralow‐velocity zones is likely Melting of mid‐ocean ridge basalt may give rise to ultralow‐velocity zones