Large-scale depressions on the 660 seismic discontinuity beneath Europe: signature of the role of akimotoite in subducting slabs

Abstract:

The upper and lower mantle are delineated by a seismic discontinuity around 660 km depth. This discontinuity is regularly associated with the dissociation of ringwoodite to bridgmanite and periclase, which has a negative Clapeyron slope, and appears to play a role in global mantle dynamics. Here we study the topography of this discontinuity and the one around 410 km, using receiver functions and investigate the effect of subducting and ponding slabs beneath Europe.

We collected ~150,000 receiver functions across Europe over a time period from 2000 to 2014. After quality control, we use ~28,000 in a common conversion point stack. Corrections for lateral velocity variations are applied using the P- and S-velocity models in EU60 from Zhu et al. (2015). While northern Europe shows an average mantle transition zone, strong anomalous depressions of 30 km are seen in the topography of the 660 beneath central Europe and around the Mediterranean. There is no (anti-)correlated topographic anomaly on the 410 in these regions.

The observed depressions beneath central Europe correlate with elevated seismic velocities above 660 km, and are therefore probably caused by cold subducted slab material. Temperature effects on the ringwoodite-bridgmanite transition alone cannot explain such a depression. Several wt% water deepens the 660 and increases the Clapeyron slope, but other geophysical observations, e.g. elevated Vp/Vs ratio, attenuation and electric conductivity, are not seen in these regions in various studies. Our preferred alternative hypothesis is the suggestion of Yu et al. (2011) that ringwoodite dissociates into akimotoite and periclase at temperatures beneath ~1400 K within the slab The subsequent transition of akimotoite to bridgmanite has a strong negative Clapeyron slope, which explains the deep discontinuity seen here in combination with cold temperatures, as well as providing a mechanism for slabs to pond in the mantle transition zone.