European Synchrotron Radiation Facility, 71, avenue des Martyrs, CS 40220, 38043 Grenoble Cedex 9, France.
Bayerisches Geoinstitut, Universität Bayreuth, D-95440 Bayreuth, Germany.
Nat Commun. 2017 Jul 19;8:15960. doi: 10.1038/ncomms15960.
The presence of carbonates in inclusions in diamonds coming from depths exceeding 670 km are obvious evidence that carbonates exist in the Earth's lower mantle. However, their range of stability, crystal structures and the thermodynamic conditions of the decarbonation processes remain poorly constrained. Here we investigate the behaviour of pure iron carbonate at pressures over 100 GPa and temperatures over 2,500 K using single-crystal X-ray diffraction and Mössbauer spectroscopy in laser-heated diamond anvil cells. On heating to temperatures of the Earth's geotherm at pressures to ∼50 GPa FeCO partially dissociates to form various iron oxides. At higher pressures FeCO forms two new structures-tetrairon(III) orthocarbonate FeCO, and diiron(II) diiron(III) tetracarbonate FeFeCO, both phases containing CO tetrahedra. FeCO is stable at conditions along the entire geotherm to depths of at least 2,500 km, thus demonstrating that self-oxidation-reduction reactions can preserve carbonates in the Earth's lower mantle.
来自超过 670km 深度的钻石包裹体中碳酸盐的存在，是碳酸盐存在于地球下地幔的明显证据。然而，碳酸盐的稳定范围、晶体结构以及脱碳过程的热力学条件仍受到很大限制。本文利用激光加热金刚石压腔中的单晶 X 射线衍射和穆斯堡尔谱学，研究了纯碳酸铁在超过 100GPa 压力和 2500K 以上温度下的行为。加热到地球地热温度和约 50GPa 的压力下，FeCO 部分分解形成各种氧化铁。在更高的压力下，FeCO 形成两种新的结构——四价铁(III)正交碳酸铁 FeCO，和二价铁(II)二价铁(III)四碳酸铁 FeFeCO，这两种相都含有 CO 四面体。FeCO 在整个地热梯度下的条件下稳定存在，深度至少达到 2500km，因此证明了自氧化还原反应可以在地幔下地幔中保存碳酸盐。
