科學家闡明沉入深部地球的碳的性質

原文網址：http://www.esrf.eu/home/news/general/content-news/general/scientists-shed-light-on-carbons-descent-into-the-deep-earth.html

科學家闡明沉入深部地球的碳的性質

探究地球內部狀態的重要性不只在於讓我們有機會一探地球過往的歷史，也能使我們瞭解現今環境與其未來走向。

刊登在《自然通訊》(Nature Communications)的這篇研究解釋了碳沉入地球深部之後的行為。德國拜羅伊特大學的Leonid Dubrovinsky表示：「要瞭解深部碳循環以及地球深處在全球碳循環中扮演的腳色，碳酸鹽的穩定區域是關鍵。」主要作者Valerio Cerantola強調：「歐洲同步輻射裝置(ESRF)產生的強力X射線讓我們可以達到整個地函內部所處的極端條件。」他之前在拜羅伊特大學攻讀博士，現在則為ESRF的博士後研究員。

在上個世紀大氣中的二氧化碳濃度迅速上升與隨之觀察到的氣候變遷，使得科學家越來越關注地表的碳循環過程和演變。然而碳循環也會延伸至地表下方：最近的估計認為地球多達90%的碳是儲存於地函和地核內部。由於板塊擁有不停移動、對流並隱沒的特性，因此地球表面和深處的碳也會持續不斷的循環流通。

在此研究中，研究團隊聚焦在碳酸鹽的不同狀態。碳酸鹽為地函深部含碳礦物的主要類型之一，這群礦物含有碳酸根離子(CO₃^2-)以及金屬離子，像是鐵或鎂。科學家研究了一種鐵碳酸鹽――FeCO₃ (稱作菱鐵礦)在地函的行為。整個地函普遍處於溫度超過2500 K和壓力超過100 GPa(兆帕)，大約等同於大氣壓力100萬倍的極端高溫高壓環境。

拜羅伊特大學的Elena Bykova表示：「我們對鐵碳酸鹽特別感興趣是因為它在下部地函的環境下會發生自旋轉變(spin transition)而處於穩定狀態。此外在高壓環境下碳酸鹽的晶體化學也跟周圍環境有很大的差異。」

為了研究FeCO₃的穩定性，研究人員在ESRF的三條光束線中進行高溫高壓實驗，它們分別是：ID27、ID18和ID09a (現為ID15b)。Cerantola表示：「我們結合多種技術而得到的獨特數據，最終使我們發現了地函深處攜帶碳的新物質，並得出了其形成過程背後的機制。」其中一項實驗是在美國先進光子源的光束線13 ID-D中執行。

將FeCO₃在將近50 GPa的壓力下加熱至地球於此壓力時的溫度，FeCO₃ 會部分分解而形成多種鐵氧化物。在更高的壓力，大約是75 GPa時，科學家發現出現了兩種新型化合物，分別是正碳酸三鐵(tetrairon (III) orthocarbonate)Fe₄³⁺C₃O₁₂以及四碳酸二亞鐵二鐵(diiron (II) diiron (III) tetracarbonate)Fe₂²⁺Fe₂³⁺C₄O₁₃。

Bykova特別指出：「雖然曾有理論預測高溫下碳酸鹽的結構會是如何，但迄今為止透過實驗得到的資訊仍相當有限(實際上也有爭議)而難以推測碳酸鹽的晶體化學。我們的數據顯示跟Fe₂²⁺Fe₂³⁺C₄O₁₃同樣的晶體結構可以在矽酸鹽中發現，不過自然界中卻沒有發現跟Fe₄³⁺C₃O₁₂有類似結構的物質。」

他們也發現其中一個型態，四碳酸鹽Fe₄C₄O₁₃的結構具有前所未見的穩定性，在沿著地溫曲線而壓力增長的過程中，至少到2500公里深處它都可以一直維持原有構造，此時已經接近於地函和地核的交界。這顯示自我氧化還原作用可以使地球的下部地函仍然擁有碳酸鹽。拜羅伊特大學的Catherine McCammon指出：「這項研究顯示氧化還原作用對深部碳循環的重要性，而此勢必也會牽涉到其他易發揮物質的循環過程，像是氧元素。」

Scientists shed light on carbon's descent into the deep Earth

Examining conditions within the Earth’s interior is crucial not only to give us a window back to Earth’s history but also to understand the current environment and its future.

This study, published in Nature Communications, offers an explanation of carbon’s descent into the deep Earth. “The stability regions of carbonates are key to understanding the deep carbon cycle and the role of the deep Earth in the global carbon cycle.” says Leonid Dubrovinsky, from the University of Bayreuth. “The intense X-rays from the ESRF allow us to access the extreme conditions within the entire Earth’s mantle.” underlines Valerio Cerantola, lead author, former PhD student at the University of Bayreuth and now postdoctoral scientist at the ESRF.

In the last century, the rapid increase in the amount of CO₂ in the atmosphere together with the observed climate change have increasingly focused scientists’ attention on the carbon cycle and its evolution at the Earth’s surface. The carbon cycle also extends below the surface: recent estimations locate up to 90% of the Earth’s carbon budget in the Earth’s mantle and core. Due to the dynamic nature of tectonic plate movements, convection and subduction, there is a constant recycling of carbon between the Earth’s surface and its deep interior.

In this study, the research team focused on carbonate phases, which are one of the main carbon-bearing minerals in the deep mantle. Carbonates are a group of minerals that contain the carbonate ion (CO₃^2-) and a metal, such as iron or magnesium. The scientists studied the behaviour of a pure iron carbonate, FeCO₃ (called siderite), at extreme temperature and pressure conditions covering the entire Earth’s mantle, meaning over 2500 K and 100 GPa, which corresponds to roughly one million times the atmospheric pressure.

“This iron carbonate is of particular interest because of its stability at lower mantle conditions due to spin transition. Moreover the crystal chemistry of the high-pressure carbonates is dramatically different from that at ambient conditions.” explains Elena Bykova, from the University of Bayreuth.

In order to study the stability of FeCO₃, the research team performed high pressure and high temperature experiments at three ESRF beamlines: ID27, ID18 and ID09a (now ID15b). “The combination of the multiple techniques gave us unique datasets that ultimately allowed us to uncover new C-carriers inside the deep Earth and show the mechanism behind their formation” says Cerantola. One experimental run was carried out at beamline 13 ID-D at APS.

Upon heating FeCO₃ to Earth geotherm temperatures at pressures up to about 50 GPa, FeCO₃ partially dissociated and formed various iron oxides. At higher pressures, above ~75 GPa, the scientists discovered two new compounds – tetrairon (III) orthocarbonate, Fe₄³⁺C₃O₁₂, and diiron (II) diiron (III) tetracarbonate, Fe₂²⁺Fe₂³⁺C₄O₁₃.

"There were some theoretical predictions, but so far experimental information about structures of high pressure carbonates have been too limited (and indeed controversial) to speculate about carbonate crystal chemistry. Our data show that while crystal structure of Fe₂²⁺Fe₂³⁺C₄O₁₃  could be found in silicates, no analogues of Fe₄³⁺C₃O₁₂ are found in nature", underlines Bykova.

​They also found out that one phase, the tetracarbonate Fe₄C₄O₁₃, shows unprecedented structural stability and keeps its structure even at pressures along the entire geotherm to depths of at least 2500 km, which is close to the boundary between the mantle and the core. It thus demonstrated that self-oxidation-reduction reactions can preserve carbonates in the Earth’s lower mantle. “The study shows the importance of redox reactions in the deep carbon cycle, which are inevitably linked to other volatile cycles such as oxygen”, underlines Catherine McCammon, from the University of Bayreuth.

原始論文：Valerio Cerantola, Elena Bykova, Ilya Kupenko, Marco Merlini, Leyla Ismailova, Catherine McCammon, Maxim Bykov, Alexandr I. Chumakov, Sylvain Petitgirard, Innokenty Kantor, Volodymyr Svitlyk, Jeroen Jacobs, Michael Hanfland, Mohamed Mezouar, Clemens Prescher, Rudolf Rüffer, Vitali B. Prakapenka, Leonid Dubrovinsky. Stability of iron-bearing carbonates in the deep Earth’s interior. Nature Communications, 2017; 8: 15960 DOI: 

引用自：European Synchrotron Radiation Facility. "Scientists shed light on carbon's descent into the deep Earth."