Mantle convection and melt experiment located at the transition zone

Abstract

The mantle transition zone plays a significant role in mediating material transport between the upper mantle and lower mantle. Tight constraints on the mantle transition zone discontinuities reflect the locations and degree of such material transport. It is essential for a comprehensive understanding of geochemical reservoirs, hydration cycles, and the evolution of the Earth. We use P-to-S receiver functions calculated from an amphibious dataset in Cascadia to seismically image the underlying mantle transition zone. We find thinning of the mantle transition zone of 10 ± 6 km beneath the ridges and 8 ± 4 km behind the slab. We do not find evidence of thinning directly beneath the Cobb Hotspot. We find the 660 discontinuity topography is the largest signal in terms of magnitude and lateral scale and dominates the pattern of the mantle transition zone thickness, indicating it plays a major role in dominating material transport beneath the Cascadia ridges and subduction zones. The 410 is characterised by smaller variations, with locations of depressions away from locations of those where the 660 is uplifted. However, the depressions occur near slow seismic velocity anomalies imaged in the upper mantle and are accompanied by supra-410 melt phases. This may suggest that upper mantle convection plays a role in material transfer between the MTZ and the upper mantle, potentially entraining hydration from the shallow MTZ to the upper mantle that results in melt. We develop a new discontinuity imaging approach using deconvolved SS precursor phases. We demonstrate its effectiveness by applying it to synthetic seismograms and the global seismic dataset. Moho depths as shallow as 20 km and mantle transition zone discontinuities at 410 km and 660 km are shown to be resolvable. We image the Moho at 21 – 67 km depth beneath continental regions in 77 % of all continental bins, within 10 km of Crust 1.0. The approach shows broad promise for imaging mantle discontinuities. Finally, we apply the new approach to the global broadband seismic data set to image the mantle transition zone globally. The topographies of the mantle transition zone are tested with different migration models, including a 1-D reference model and seismic velocity tomography models. We obtain robust features across different migration models. We find the mantle transition zone is thickened beneath subduction zones with a thickness of ~ 255 – 270 km over 1800 - 3000 km laterally. The locations of thickening are coincident with the locations of inferred cold regions by imaged high-seismic-velocity anomalies in tomography models. We also find thinning beneath oceans, but noapparent systematic anomalous thinning beneath hotspot regions is resolved. Similar to the observations beneath the Cascadia, the 660 topographies closely resemble the mantle transition zone thickness perturbations. This suggests the 660 topography likely dominates the variation of the mantle transition zone thickness, likely because slabs generally accumulate there, increasing the intensity and length scales of the associated anomalies. In other words, the 660 plays a major role in material transfer to the lower mantle. The lack of expected anticorrelations between the 410 topography and the 660 topography may be because 410 variations occur at a smaller scale, laterally or in-depth, than the resolution of SS precursor imaging. This also suggests that the dynamics of material transport across the mantle transition zone are more complex than simple vertical ascending beneath hotspots and descending beneath subduction zones.

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Identifiers

ORCID for Yuhang Dai: orcid.org/0000-0001-9965-7106

ORCID for Nicholas Harmon: orcid.org/0000-0002-0731-768X

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