Ultralow velocities of amorphous CaCO3 and the origin of seismic anomalies in the Earth's upper mantle
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稿件编号：218 访问权限：仅限参会人
更新：2021-06-07 16:31:31 浏览：895次
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摘要
Volatiles (H2O, CO2, etc.) modulate the global climate in geological time. Hydrous minerals and fluids have been predominantly believed to be the major cause of the seismic low velocity, electrical conductivity abnormalities, deep earthquake, partial melting, and magma chamber for volcanoes in the upper mantle. By contrast, the effect of CO2 carriers has largely been underestimated. Calcium carbonate (CaCO3) is a major long-term host for CO2 and the nature of CaCO3 at extreme pressure-temperature (P-T) conditions are indispensable to decipher its circulation between the Earth's interior and its surface via volcanoes and subduction. Using the pulse-echo-overlap method in a Paris-Edinburgh press coupled with X-ray diffraction, we explored the seismic velocities of CaCO3 and observed an anomalously drop associated with aragonite to amorphous phase transition under P-T conditions relevant to subducting slabs in the upper mantle. Compared to aragonite, the velocity reductions of the partially amorphous CaCO3 are approximately 35% and 52% for compressional (VP) and shear (VS) wave velocities, respectively, at 3.5–5.5 GPa and 1,073–1,373 K. The VP and VS values of the partially amorphous CaCO3 are about 1/2 and 1/3 of those of the upper mantle minerals, respectively. Amorphous CaCO3-bearing domains would be expected to dwell atop the subducting slabs and naturally account for many enigmatic characteristics of the low-velocity layers in circum-Pacific subduction zones.
关键字
deep carbon recycle,amorphization,high pressure,low velocity zone,sound velocity
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