Sound velocity anomalies of limestone at high pressure and implications for the mantle wedge

ABSTRACT Limestone mainly consisting of CaCO3 is one of the most abundant carbonates on Earth's surface. The sound velocities of limestone at high pressure were determined at room temperature using ultrasonic interferometry in a multi-anvil apparatus. Softening and discontinuities in compressional (P) and shear (S) wave velocities have been observed at around 1.45 GPa due to the phase transition of CaCO3-I to CaCO3-II, and the coexistence of CaCO3-IIIb and CaCO3-III stay up to ∼5 GPa. Limestone under CaCO3-I and CaCO3-III phase has much lower velocities and higher V P/V S ratio than most crustal and mantle minerals and the PREM model. Phase transitions from CaCO3-I to CaCO3-II cause the abrupt reduction of V P, V S, and V P/V S. The low velocities and high/low V P/V S ratio are well consistent with the seismology observed in the mantle wedge. The result suggests that the subduction of limestone into Earth's interior would cause low-velocity anomalies in mantle wedges.