Low-Velocity Structure of Subducted Oceanic Crust in the Upper Mantle: Insights From High Pressure and Temperature Elasticity Measurements of Aragonite

This study examines the effects of water and carbon on the velocity profiles of subducted oceanic crust in the upper mantle. High pressure and temperature Brillouin measurements were conducted to determine the single-crystal elasticity of aragonite (CaCO3) up to 20 GPa and 600 K, respectively. Using the finite-strain method, we determined the elastic parameters: KS0 = 70.7 GPa, G0 = 36.1(3) GPa with KS ' = 5.0(1), G ' = 1.3(1), partial differential KS/ partial differential T = -0.020(2) GPa/K, and partial differential G/ partial differential T = -0.015(1) GPa/K. When combined with literature results, our findings reveal that neither hydrous minerals nor carbonate alone can explain the observed 3%-4% and 3%-7% low velocity anomalies in the compressional (VP) and shear-wave (VS) velocities of subducted oceanic crust in the circum-Pacific region at 150-250 km depth. Considering the combined effect of water and carbonate, the addition of 5.9-6.9 vol.% aragonite together with 10 vol.% lawsonite in the oceanic crust can produce a 3%-4% and 6.1%-8.1% low velocity anomalies in the VP and VS at 150-250 km depth, respectively, consistent with the seismic observations in the region. Complete dehydration of lawsonite accompanied by the gradual decarbonization of the subducted oceanic crust explains the absence of low-velocity anomalies below 300-km depth. Our findings help for a better understanding of the possible influence of carbon and water on seismic velocities of the mantle. As a result, the circulation of carbon and water may be better understood while taking the complex velocity structure of subduction zones and mineral physics findings into account. The subducting slab generally exhibits higher wave velocities than the normal mantle due to its lower temperatures. However, seismological studies have detected the existence of 3%-4% and 3%-7% low compressional (VP) and shear-wave (VS) velocity anomalies in the subducted oceanic crust at 150-250 km depth in the circum-Pacific region, respectively, which cannot be explained by water (lawsonite) alone. Our new experimental constraints on the single-crystal elasticity of aragonite up to 20 GPa and 600 K indicate that aragonite, along with water, can significantly reduce the velocity of mid-ocean ridge basalts which constitute a major component of the subducted oceanic crust. The presence of 5.9-6.9 vol.% aragonite together with 10 vol.% lawsonite can make the VP and VS of subducted oceanic crust 3%-4% and 6.1%-8.1% lower than surrounding mantle, respectively. Below 300-km depth, subducted oceanic crust will be seismically detected as a high-velocity structure due to complete dehydration of lawsonite and gradual decarbonization with aragonite content less than 8 vol.%. These findings contribute to our understanding of the complex relationship between carbon, water, and depth-dependent velocity anomalies in the subduction process. Single-crystal elasticity of aragonite was determined to 20 GPa and 600 K, showing the lowest VP and VS compared to magnesite and dolomiteThe obtained results were used to model the velocity profiles of the subducted oceanic crust in the upper mantleThe low-velocity anomalies within the subducted oceanic crust at 150-250 km depth should be a net effect of water and carbon