新理論解釋地球鐵的特性

原文網址：http://www.geologypage.com/2017/03/new-theories-nature-earths-iron.html

新理論解釋地球鐵的特性

新研究挑戰了現行理論中，認為地球鐵的特殊性質是由數十億年前的地核形成過程所導致。

這項研究導出了另一項具有競爭力的理論來解釋為何地球上某些較重的鐵，也就是某些鐵同位素的含量會比其他太陽系天體中的還要多。現今盛行的理論將地球奇異的鐵組成歸因於地球地核的形成過程，但2月20日刊登於《自然通訊》(Nature Communications)中的這篇研究，卻認為地球特殊的鐵同位素訊號是在地球歷史中較晚才發展出來，可能是地球與其他行星碰撞使得較輕的鐵同位素蒸發，或是攪動地函而造成較重的鐵同位素以不同比例從地函進入至地殼之內。

鐵是太陽系中豐度最高的元素之一，因此了解它的性質對於釐清地球和其他天體的形成過程來說至關重要。研究人員取得地球和地外天體的岩石，包括了來自月球、火星和遠古隕石中的岩石，並比較它們較重的鐵同位素Fe-56和較輕的鐵同位素Fe-54之間的比例。他們發現地球岩石中的鐵同位素比例，跟數值全部一致的地外天體岩石比起來明顯地高出許多。他們的研究試著去解釋這種現象如何發生。

「地核的形成或許是對地球歷史中影響力最大的事件。」本篇論文的共同作者，德州大學奧斯汀分校的地質科學教授Jung-Fu Lin表示。「在這項研究中，我們認為地球鐵同位素的異常必定有別的來源，而非由地球的形成過程導致。」

共同作者，芝加哥大學地球物理的Louis Block教授Nicolas Dauphas表示此研究的突破之處在於「分析材料的合成方法，以及測量和處理數據的技術」

作者重現了在地核形成時地球所處的高壓環境特性。研究人員利用鑽石加壓砧(diamond anvil cell)來達成他們的目標，這種儀器可以創造出存在於星球深處的高壓環境，讓他們可以進行用別的方法無法達成的合成作用。

「過往已經有人利用鑽石加壓砧來進行類似研究，但難處在於取得正確的數據。」Dauphas說，「由於鑽石砧給出的訊號相當微弱，因此取得並處理數據時需要非常仔細。要解讀測量結果必須用上相當複雜的數學技巧，而這需要一個夢幻團隊才能達成。」

他們利用實驗嘗試去證明地函較重的鐵同位素含量較高可能是地核形成過程導致的結果。但測量數據卻顯示這個說法行不通。Dauphas說：「因此要找到這項謎題的解答勢必得另闢蹊徑。」

仍需更多研究才能了解地核的形成過程，以及地球鐵同位素的特殊訊號該如何解釋。

New theories about nature of Earth’s iron

New research challenges the prevailing theory that the unique nature of Earth’s iron was the result of how its core was formed billions of years ago.

The study opens the door to competing theories about why levels of certain heavy forms of iron, known as isotopes, are higher on Earth than in other bodies in the solar system. The prevailing view attributes Earth’s anomalous iron composition to the formation of the planet’s core. But the study published Feb. 20 in Nature Communications suggests that the peculiar iron’s isotopic signature developed later in Earth’s history, possibly created by a collision between Earth and another planetary body that vaporized the lighter iron isotopes, or the churning of Earth’s mantle, drawing a disproportionate amount of heavy iron isotopes to Earth’s crust from its mantle.

Iron is one of the most abundant elements in the solar system, and understanding it is key to figuring out how Earth and other celestial bodies formed. The researchers compared the ratio of the heavier iron isotope Fe-56 to the lighter Fe-54 for Earth and extraterrestrial rocks, including those from the moon, Mars and ancient meteorites. They found that the ratio is significantly higher for Earth rocks than for extraterrestrial rocks, all of which have an identical ratio. Their research attempts to explain how that happened.

“Earth’s core formation was probably the biggest event affecting Earth’s history,” said Jung-Fu Lin, professor of geosciences at the University of Texas at Austin and co-author of the paper. “In this study we say that there must be other origins than Earth’s formation for this iron isotopic anomaly.”

Co-author Nicolas Dauphas, the Louis Block Professor of Geophysical Sciences at the University of Chicago, called the research groundbreaking “because of the synthesis of the materials analyzed, the technique to take the measurements and the data treatment.”

The authors recreated the high pressure that characterized the conditions on Earth during the formation of its core. To do this, the researchers used a diamond anvil cell — a device capable of recreating pressures that exist deep inside planets — and were able to synthesize processes that would not be discernible otherwise.

“The diamond anvil cell has been used in this way before, but the difficulty is getting correct numbers,” Dauphas said. “That requires great care in data acquisition and treatment because the signal the diamond anvil gives off is very small. One has to use sophisticated mathematical techniques to make sense of the measurements, and it took a dream team to pull this off.”

The experiment sought to show that the high levels of heavy iron isotopes in Earth’s mantle likely occurred during the formation of Earth’s core. But the measurements show that it does not work, “so the solution to this mystery must be sought elsewhere,” Dauphas said.

More research is needed to understand the core’s formation and the reasons for Earth’s unique iron isotopic signature.

原始論文：Jin Liu, Nicolas Dauphas, Mathieu Roskosz, Michael Y. Hu, Hong Yang, Wenli Bi, Jiyong Zhao, Esen E. Alp, Justin Y. Hu, Jung-Fu Lin. Iron isotopic fractionation between silicate mantle and metallic core at high pressure. Nature Communications, 2017; 8: 14377 DOI: 10.1038/ncomms14377