Temperature-induced amorphization in CaCO3 at high pressure and implications for recycled CaCO3 in subduction zones

Mingqiang Hou,Yingwei Fei,Renbiao Tao,Wenge Yang,Ho-Kwang Mao,Yoshio Kono,Hong Liu,Qian Zhang,Bin Chen

doi:10.1038/s41467-019-09742-5

Abstract

Calcium carbonate (CaCO3) significantly affects the properties of upper mantle and plays a key role in deep carbon recycling. However, its phase relations above 3 GPa and 1000 K are controversial. Here we report a reversible temperature-induced aragonite-amorphization transition in CaCO3 at 3.9–7.5 GPa and temperature above 1000 K. Amorphous CaCO3 shares a similar structure as liquid CaCO3 but with much larger C-O and Ca-Ca bond lengths, indicating a lower density and a mechanism of lattice collapse for the temperature-induced amorphous phase. The less dense amorphous phase compared with the liquid provides an explanation for the observed CaCO3 melting curve overturn at about 6 GPa. Amorphous CaCO3 is stable at subduction zone conditions and could aid the recycling of carbon to the surface.

Highlights

Calcium carbonate (CaCO3) significantly affects the properties of upper mantle and plays a key role in deep carbon recycling
This study aims to clarify the nature of the transition from aragonite to the disordered crystal phase or new phase mentioned in Suito et al.[20] and Litasov et al.[25], respectively, and provide its structure information by in situ measurements up to 7.5 GPa and 1723 K using Paris–Edinburgh press techniques coupled with energy-dispersive X-ray diffraction (EDXRD)
The bond length comparison and the negative slope of the melting curve support that amorphous CaCO3 is less dense than the liquid counterpart

Summary

Introduction

Calcium carbonate (CaCO3) significantly affects the properties of upper mantle and plays a key role in deep carbon recycling. The less dense amorphous phase compared with the liquid provides an explanation for the observed CaCO3 melting curve overturn at about 6 GPa. Amorphous CaCO3 is stable at subduction zone conditions and could aid the recycling of carbon to the surface. A disordered calcite crystal phase (resembling calcite-IV or -V) was reported according to energy-dispersive X-ray diffraction (EDXRD) patterns[20], while Litasov et al.[25] suggested that it might be a new phase Both studies did not provide any detailed structure information of the high-PT phase. This study aims to clarify the nature of the transition from aragonite to the disordered crystal phase or new phase mentioned in Suito et al.[20] and Litasov et al.[25], respectively, and provide its structure information by in situ measurements up to 7.5 GPa and 1723 K using Paris–Edinburgh press techniques coupled with EDXRD. Correlations between properties of the amorphous phase and the mechanism of largescale CaCO3 dissolution in subduction zone are subsequently presented

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Methods

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Conclusion