地球下地幔中的四面体配位碳酸盐
Tetrahedrally coordinated carbonates in Earth's lower mantle
摘要：
碳酸盐是通过俯冲作用将碳带入地球深处的主要物种。它们是一种重要的岩石形成矿物，与地壳中的硅酸盐有根本区别，因为碳与三个氧原子结合，而硅与四个氧原子结合。在这里，我们提出的实验证据表明，下地幔中存在的足够高的压力和高温下，铁镁碳酸盐转化为四面体配位碳相。在80 GPa以上，原位同步辐射红外实验显示了（Mg，Fe）CO $_3$高压相的明确光谱特征。使用从头计算，我们分配的新的IR签名与四面体配位的碳与不对称的C-O键的C-O带。预期四面体配位的碳酸酯表现出与低压三重配位的碳酸酯显著不同的反应性，以及在液态下不同的化学性质。因此，这可能对碳库和碳通量以及全球地球动力学碳循环产生重大影响。
英文摘要：
Carbonates are the main species that bring carbon deep into our planet through subduction. They are an important rock-forming mineral group, fundamentally distinct from silicates in Earth's crust in that carbon binds to three oxygen atoms, while silicon is bonded to four oxygens. Here, we present experimental evidence that under the sufficiently high pressures and high temperatures existing in the lower mantle, ferromagnesian carbonates transform to a phase with tetrahedrally coordinated carbons. Above 80 GPa, in situ synchrotron infrared experiments show the unequivocal spectroscopic signature of the high-pressure phase of (Mg,Fe)CO$_3$. Using ab-initio calculations, we assign the new IR signature to C-O bands associated with tetrahedrally coordinated carbon with asymmetric C-O bonds. Tetrahedrally coordinated carbonates are expected to exhibit substantially different reactivity than low pressure three-fold coordinated carbonates, as well as different chemical properties in the liquid state. Hence this may have significant implications on carbon reservoirs and fluxes and the global geodynamic carbon cycle.
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