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

Abstract

AbstractThis 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), ∂KS/∂T = −0.020(2) GPa/K, and ∂G/∂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.