P17-Hou Mingqiang

Thermal-induced amorphization in deep recycled CaCO3: New engine for intermediate-depth earthquakes

Mingqiang Hou1,2*, Qian Zhang1, Renbiao Tao3, Hong Liu4, Yingwei Fei3*, Yoshio Kono5*,Ho-kwang Mao1,3, Wenge Yang1

1Center for High Pressure Science and Technology Advanced Research, Shanghai 201203, China.

2The Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA

3Geophysical Laboratory, Carnegie Institution of Washington, Washington, DC 20015, USA

4CEA Key Laboratory of Earthquake Prediction, Institute of Earthquake Science, China Earthquake Administration, Beijing 100036, China

5HPCAT, Carnegie Institution of Washington, Advanced Phonon Source, Argonne National Laboratory, Argonne, IL 60439, USA

Calcite carbonate, a maternal mineral of carbon for deep carbon recycling, has never been connected to intermediate-depth earthquakes (IDE). Here we proposed an alternative mechanism for triggering IDE by recycled sedimentary carbonate besides the existing models of dehydration embrittlement, thermal shear runaway instability, and solid-solid crystal phase transition. Thermal-induced amorphization in CaCO3 was experimentally observed at 3.9-7.5 GPa above 1073 K. The amorphous CaCO3 shared similar structure as liquid CaCO3 but with much lower density compared with that of liquid- or aragonite-CaCO3. The soft property1 and large positive volume expansion (∆VV>14%) during aragonite-amorphous phase transition will decrease the effective pressure on existing or potential planes of weakness to trigger IDE by production of a pore pressure. The accumulation of amorphous CaCO3 on the top surface of subducting slab might explain seismic low velocity zone.  On the other hand, the low density of amorphous CaCO3 compared with surrounding materials endows it ultra-buoyancy to escape from subducting slabs and ascend to magmas as a source and engine for deep carbon recycle which could balance the insufficiency between input and output carbon by subduction and volcanism.