Structural Modifications of Single-Crystal Aragonite CaCO3 Beginning at ~15 GPa: In Situ Vibrational Spectroscopy and X-Ray Diffraction Evidence

Jing Gao,Wen Su,Yungui Liu,Yingxin Liu,Xueyin Yuan,Xiang Wu

doi:10.3390/min10100924

Abstract

The structural chemistry of carbonates under mantle conditions facilitates our understanding of carbon recycling pathways in the earth’s interior. It also has impacts on the dynamics of mantle–slab interactions. Aragonite is a common calcium carbonate mineral in pelagic marine sediments. The structural chemistry of single-crystal aragonite during successive compression and the behavior of a structural H+ have been investigated by micro-vibrational spectroscopy and synchrotron X-ray diffraction techniques in diamond anvil cells. We describe a reduction of the b-axial compressibility beginning at ~15 GPa, and the related discontinuities in the first-order derivatives of the vibrational modes. The structural modifications of aragonite are manifested by mutations occurring in the pressure relations of the wavenumbers of the O-C-O bending modes, and of the bandwidth and band intensities of the measured internal and external modes. These anomalies are indicative of changes occurring in the force constant of the C-O bonds, and possibly a second-order phase transition. Besides, the [CaO9] polyhedra begin to deform, possibly with some Ca-O bonds becoming elongated and the others shortening. An increase in the co-ordination number for the Ca2+ sites could be expected under higher pressures. Additionally, the weakening of the OH modes may imply H+-loss from the aragonite lattice above 11.5 GPa.

Highlights

Plate tectonics theory outlines the subduction process of the oceanic lithosphere at the interface of convergent plates [1]
The single-crystal X-ray diffraction (XRD) patterns of aragonite have been collected with pressure increasing up to
A least-square fit of the compression data to the third-order Birch–Murnaghan equation of state (BM EoS) yields the values of the bulk modulus of the volume to be B0 = 71(5) GPa, B0 0 = 4.3(4) and V 0 = 228.3(8) Å3, in accordance with the values obtained by XRD [33,38] and Brillouin spectroscopy measurements [39]

Summary

Introduction

Plate tectonics theory outlines the subduction process of the oceanic lithosphere at the interface of convergent plates [1]. Geophysical observations [2] and mineralogical evidence [3] extend the subduction region to the 660 km discontinuity and depict the retention of slabs at the topmost of the lower mantle. Sedimentary carbonates are transported into the earth. Studies on the high-pressure behaviors of carbonates provide fundamental information for our understanding on decarbonation reactions under upper mantle conditions [6]. Existence of carbonates in the Earth’s mantle decreases the melting point of mantle peridotite and promotes partial melting. This provides a predisposition for geochemical and geophysical anomalies locally or planet differentiation on a large scale [7]. In consideration of the relative coldness of the subducted slabs at subarc depths [8], a certain amount of carbonates would

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Conclusion