為什麼地函中層的岩石會緩緩流動？

原文網址：www.sciencedaily.com/releases/2017/06/170605155932.htm

為什麼地函中層的岩石會緩緩流動？

數十年來，研究人員一直是利用地震產生的地震波來研究地球內部。最近由亞利桑那州立大學地球與太空探索學院的副教授Dan Shim主持的研究中，研究人員在實驗室重建了地球深處的環境條件，而發現了在我們腳下遙遠之處的地函，主要組成礦物的特殊性質。

Shim和他的團隊結合了美國國家能源部實驗室同步輻射設施的X光技術，以及亞利桑那州立大學的原子分辨電子顯微鏡，找出地球內部600英里(1000公里)深處及其下方，岩石的特殊流動模式成因為何。他們的研究成果發表於期刊《美國國家科學院院刊》(Proceedings of the National Academy of Sciences)。

地深之處的緩慢流動

地球的構造層層相疊，包括了表面的地殼，以及下方的地函和地核。地核產生的熱能使得組成地函的矽酸鹽質岩石緩緩攪動，就像是火爐上緩緩滾沸的法吉糖一樣。這種輸送帶般的運動造成地表的板塊互相碰撞。在地球45億年的歷史中，此作用至少從一半以前就持續至今。

Shim的團隊將重心放在循環流動過程中令人感到疑惑的一環：為什麼攪動模式在地表下方600至900英里處會突然慢下來？

Shim表示：「近期的地球物理研究顯示流動模式突然改變，是因為在此深度的地函岩石比較難以流動。」「但背後的成因為何？岩石在此處的成分發生改變了嗎？或者岩石在此深度和壓力下，突然變得比較黏稠？沒有人知道究竟是為什麼。」

為了在實驗室中探討這項問題，Shim的團隊開始研究bridgmanite，過往的研究顯示這種含鐵礦物是地函最主要的成分。

Shim表示：「我們發現預估在1000至1500公里深處產生的壓力下，bridgmanite會有所變化。這些變化導致bridgmanite的黏滯性――也就是流動的難易度增加。」

團隊在實驗室中合成bridgmanite的樣品後，再讓它們承受地函不同深度之下的高壓環境。

礦物是地函性質的關鍵

團隊進行的實驗顯示，在深度1000公里之上和1700公里之下，bridgmanite含有的鐵其氧化態和還原態數量幾乎相等。但在這兩個深度之間的壓力範圍，bridgmanite歷經的化學變化最終會讓它的鐵含量大幅降低。

此作用會先逐出bridgmanite含有的氧化鐵。氧化鐵接著會消耗散佈在地函中，有如蛋糕中的罌粟籽一般的微量金屬鐵。總結來說，此化學反應會移除金屬鐵，讓這處地函中的關鍵層位擁有比較多的還原鐵。

還原鐵去哪了？Shim的團隊表示答案是，它們會進入存在於地函中的另一種礦物――鐵方鎂石(ferropericlase)，此礦物具有很容易吸收還原鐵的化學性質。

Shim解釋：「結果便是在深層地函中，此處的bridgmanite鐵含量較低。」他強調這是此層位產生這種行為的關鍵。

Shim表示：「bridgmanite失去鐵的時候，黏滯性也會跟著上升。這可以解釋地震波的觀測結果中，在此深度的地函為何流動速度比較慢。」

Why rocks flow slowly in Earth's middle mantle

For decades, researchers have studied the interior of the Earth using seismic waves from earthquakes. Now a recent study, led by Arizona State University's School of Earth and Space Exploration Associate Professor Dan Shim, has re-created in the laboratory the conditions found deep in the Earth, and used this to discover an important property of the dominant mineral in Earth's mantle, a region lying far below our feet.

Shim and his research team combined X-ray techniques in the synchrotron radiation facility at the U.S. Department of Energy's National Labs and atomic resolution electron microscopy at ASU to determine what causes unusual flow patterns in rocks that lie 600 miles and more deep within the Earth. Their results have been published in the Proceedings of the National Academy of Sciences.

Slow flow, down deep

Planet Earth is built of layers. These include the crust at the surface, the mantle and the core. Heat from the core drives a slow churning motion of the mantle's solid silicate rocks, like slow-boiling fudge on a stove burner. This conveyor-belt motion causes the crust's tectonic plates at the surface to jostle against each other, a process that has continued for at least half of Earth's 4.5 billion-year history.

Shim's team focused on a puzzling part of this cycle: Why does the churning pattern abruptly slow at depths of about 600 to 900 miles below the surface?

"Recent geophysical studies have suggested that the pattern changes because the mantle rocks flow less easily at that depth," Shim said. "But why? Does the rock composition change there? Or do rocks suddenly become more viscous at that depth and pressure? No one knows."

To investigate the question in the lab, Shim's team studied bridgmanite, an iron-containing mineral that previous work has shown is the dominant component in the mantle.

"We discovered that changes occur in bridgmanite at the pressures expected for 1,000 to 1,500 km depths," Shim said. "These changes can cause an increase in bridgmanite's viscosity -- its resistance to flow."

The team synthesized samples of bridgmanite in the laboratory and subjected them to the high-pressure conditions found at different depths in the mantle.

Mineral key to the mantle

The experiments showed the team that, above a depth of 1,000 kilometers and below a depth of 1,700 km, bridgmanite contains nearly equal amounts of oxidized and reduced forms of iron. But at pressures found between those two depths, bridgmanite undergoes chemical changes that end up significantly lowering the concentration of iron it contains.

The process starts with driving oxidized iron out of the bridgmanite. The oxidized iron then consumes the small amounts of metallic iron that are scattered through the mantle like poppy seeds in a cake. This reaction removes the metallic iron and results in making more reduced iron in the critical layer.

Where does the reduced iron go? The answer, said Shim's team, is that it goes into another mineral present in the mantle, ferropericlase, which is chemically prone to absorbing reduced iron.

"Thus the bridgmanite in the deep layer ends up with less iron," explained Shim, noting that this is the key to why this layer behaves the way it does.

"As it loses iron, bridgmanite becomes more viscous," Shim said. "This can explain the seismic observations of slowed mantle flow at that depth."

原始論文：Sang-Heon Shim, Brent Grocholski, Yu Ye, E. Ercan Alp, Shenzhen Xu, Dane Morgan, Yue Meng, and Vitali B. Prakapenka. Stability of ferrous-iron-rich bridgmanite under reducing midmantle conditions. PNAS, June 5, 2017 DOI: 10.1073/pnas.1614036114

引用自：Arizona State University. "Why rocks flow slowly in Earth's middle mantle." ScienceDaily. ScienceDaily, 5 June 2017.