原文網址：https://www.sciencedaily.com/releases/2020/11/201116075717.htm
研究得到的線索顯示沉入地函深處的板塊會有什麼樣的命運
By Jade Boyd
一篇新研究讓「跌入谷底」有了新的意義。地球表面是由許多塊石板組合而成，它們的底側稱為岩石圈。地震學家在中國東北方發現有塊岩石圈因為板塊隱沒作用的緣故，而被拉到地球內部400多英里(約650公里)深的地方。
上圖簡單顯示了驅動地球板塊運動(黑色箭頭)的熱對流循環(紅色箭頭)。熱流會經由地函最上層的軟流圈往隱沒帶流動。萊斯大學的電腦模型發現軟流圈在局部地方可以牽動上方的板塊，而非許多人認為的只會阻礙板塊的運動。圖片來源：Surachit/Wikimedia Commons
中國和美國組成的研究團隊在《自然―地球科學》(Nature Geoscience)發表了這項研究。他們呈現了新的證據，顯示富含水的海洋板塊被拖到陸地下方的地函之後會發生什麼事情。
萊斯大學的地震學家Fenglin
Niu是共同通訊作者。他說這項研究首度給出了高解析度的震測影像，顯示出地函過渡帶裡板塊岩石圈的上下邊界。地函過渡帶大約從地表下方410公里處延伸至660公里處，是地函裡面相當重要的區域。
Niu表示：「很多研究提出在地函過渡帶，隱沒板塊實際上已經變形得相當嚴重，原因是在此它們會變軟，因此十分容易變形。」板塊變形或是維持原狀的程度有多高非常重要，因為這可以解釋它們是否和地函有所混和，以及混和程度有多高，此外也能解釋它們能對地函造成什麼樣的降溫效果。
地函就像烤箱中的熱氣一樣會產生對流。從地核來的熱能會隨著地函一路往上升到海洋中央，並在此處形成板塊。接著熱能會繼續跟著地函往陸地的方向流動，同時溫度也逐漸下降，然後在陸地下方往地核沉沒。最後它們又會重新汲取地核的熱能，再次往上升而完成整個對流循環。
Niu和共同通訊作者，中國科學院的陳棋福領導的中國地震局團隊互相合作，運用了中國東北313個區域地震站測量到的67000多筆資料來得到這項成果。過往的研究雖然有在地函中偵測到隱沒板塊的邊界，但是很少有人能看到深度200公里以下的地方，而且解析度都完全不及這篇研究。
研究探討的基本問題關乎數十億年來塑造地球表面的過程，也就是驅動地球板塊運動的地函對流。地球表面是由許多堅硬的岩塊――板塊彼此扣在一起而成。由於它們漂浮在地函最上層，同時也是地球內部流動性最高的地方――軟流圈，因此會不停運動。
在板塊相遇的地方它們會彼此碰撞或產生摩擦，把能量以震波的形式釋放出來。在極端情形下，這會造成具有破壞性的地震或海嘯，但是大部分震波造成的運動對人類來說都相當微弱，需要儀器才能感覺得到。透過地震儀，科學家可以偵測這些地震波擾動的規模以及發生地點。同時，因為地震波在某些種類的岩石中前進得比較快，某些岩石中比較慢，因此科學家可以利用這點來創造出地球內部的影像，如同醫生可以利用超音波來得到病患身體內部的影像一樣。
萊斯大學地球、環境與行星科學系的教授Niu二十多年來在震波成像這方面一直位居領導地位。當他20多年前在日本受訓成為博士的時候，研究人員正在利用密集的地震觀測網，對隱沒板塊的外型蒐集第一批精確的影像，他們的對象和本周發表的研究一樣――都是太平洋板塊。
「太平洋板塊在日本的所在位置之下，到達了大約100公里深的地方，」Niu表示。「隱沒的太平洋板塊裡面有很多水分，因此產生了很多部分熔融。這造成了許多島弧火山，形成了日本的一部份。不過，關於在這個深度水份是否就已經完全釋放出來，仍有很多爭議。有越來越多證據指出一部分的水可以待在板塊內部，進到非常非常深的地方。」
探討這項說法是否正確的時候，中國東北有一項極為有利的優勢。日本海溝是太平洋板塊開始隱沒到地球內部之處，而中國東北跟日本海溝的距離為1000公里左右。2009年，美國國家科學基金會和其他機構提供經費，讓Niu和德州大學奧斯汀分校、中國地震局、東京大學地震研究所、京都大學地震和火山預警研究中心的科學家開始在中國東北架設寬頻地震儀。
「我們在該區放置了140個測站。不過測站越多理所當然地解析度就越好，」Niu說。「中國科學院又再放置了更多測站，因此他們可以得到品質更好、細節更多的影像。」
從新研究的測站數據可以看出在地函過渡帶，太平洋板塊的上下邊界是以25度向下傾斜。板塊在過渡帶的位置對於研究地函對流來說相當重要，這是因為過渡帶比軟流圈更深，剛好是壓力增強到特定的地函礦物會產生劇烈相變的位置。在震測剖面當中礦物的不同相會展現出很大的差異，就像液態的水和固態的冰雖然都是由同樣的分子組成，卻有很大的差別一樣。由於在地函過渡帶只有壓力和溫度達到特定數值時才會發生相變，因此地質學家可以把相變當作溫度計來測量地函的溫度。
中國東北的震測成像顯示出隱沒板塊的頂部(X1)和底部(X2)。這個板塊之前是太平洋底的一部份，之後被往下拉到地球表面下方410至660公里處的地函過渡帶。圖片來源：F.
Niu/萊斯大學
Niu表示還能看到隱沒板塊的上下邊界，是它還沒有和周遭的地函完全混和的證據。此外，在隱沒板塊下方的部份地函可以看到部分熔融產生的高溫訊號，間接證明了隱沒板塊中一部份的水可以被帶到過渡帶。
「難點在於解釋這些高溫的物質如何墜入地函深處，」Niu說。「這項問題仍有待解決，因為它們的溫度較高，所以浮力也較強。」
圖中顯示的過程表示當隱沒的太平洋板塊有洞時，熱能就可以往上脫逃，使得中國與北韓交界的長白山地區發生火山活動，於此同時板塊仍然可以繼續往地函深處下沉。圖片來源：F.
Niu/萊斯大學
照理來說浮力應該會像救生圈一樣，把正在下沉的板塊底部往上抬。Niu說這項問題的答案，可能是因為變形的板塊裡面有洞，使得這些高溫的熔融物質往上升的同時，板塊還能繼續往下沉。
「如果隱沒板塊有洞的話，熔融物質就能跑出去，」他說。「這是我們認為板塊可以繼續往下沉的原因。」
隱沒板塊裡的洞也能解釋中國和北韓交界為什麼會有長白山等火山出現在這裡。
「此處距離板塊邊界有1000公里遠，」Niu說。「我們還沒有完全瞭解這類火山的形成機制。不過從隱沒板塊裡的洞往上升的熔融物質，或許是一種可能的解釋。」
Former piece of Pacific Ocean floor
imaged deep beneath China
Study offers clues about the fate of
tectonic plates that sink deep in Earth’s mantle
In a study
that gives new meaning to the term “rock bottom,” seismic researchers have
discovered the underside of a rocky slab of Earth’s surface layer, or
lithosphere, that has been pulled more than 400 miles beneath northeastern
China by the process of tectonic subduction.
The study, published by a team of Chinese and U.S.
researchers in Nature Geoscience,
offers new evidence about what happens to water-rich oceanic tectonic plates as
they are drawn through Earth’s mantle beneath continents.
Rice University seismologist Fenglin Niu, a
co-corresponding author, said the study provides the first high-resolution seismic
images of the top and bottom boundaries of a rocky, or lithospheric, tectonic
plate within a key region known as the mantle transition zone, which starts
about 254 miles (410 kilometers) below Earth’s surface and extends to about 410
miles (660 kilometers).
“A lot of studies suggest that the slab actually
deforms a lot in the mantle transition zone, that it becomes soft, so it’s
easily deformed,” Niu said. How much the slab deforms or retains its shape is
important for explaining whether and how it mixes with the mantle and what kind
of cooling effect it has.
Earth’s mantle convects like heat in an oven. Heat
from Earth’s core rises through the mantle at the center of oceans, where
tectonic plates form. From there, heat flows through the mantle, cooling as it
moves toward continents, where it drops back toward the core to collect more
heat, rise and complete the convective circle.
Previous studies have probed the boundaries of
subducting slabs in the mantle, but few have looked deeper than 125 miles (200
kilometers) and none with the resolution of the current study, which used more
than 67,000 measurements collected from 313 regional seismic stations in
northeastern China. That work, which was done in collaboration with the China
Earthquake Administration, was led by co-corresponding author Qi-Fu Chen from
the Chinese Academy of Sciences.
The research probes fundamental questions about the
processes that shaped Earth’s surface over billions of years. Mantle convection
drives the movements of Earth’s tectonic plates, rigid interlocked pieces of
Earth’s surface that are in constant motion as they float atop the
asthenosphere, the topmost mantle layer and the most fluid part of the inner
planet.
Where tectonic plates meet, they jostle and grind
together, releasing seismic energy. In extreme cases, this can cause
destructive earthquakes and tsunamis, but most seismic motion is too faint for
humans to feel without instruments. Using seismometers, scientists can measure
the magnitude and location of seismic disturbances. And because seismic waves
speed up in some kinds of rock and slow in others, scientists can use them to
create images of Earth’s interior, in much the same way a doctor might use
ultrasound to image what’s inside a patient.
Niu, a professor of Earth, environmental and planetary
sciences at Rice, has been at the forefront of seismic imaging for more than
two decades. When he did his Ph.D. training in Japan more than 20 years ago,
researchers were using dense networks of seismic stations to gather some of the
first detailed images of the submerged slab boundaries of the Pacific plate,
the same plate that was imaged in study published this week.
“Japan is located about where the Pacific plate
reaches around 100-kilometer depths,” Niu said. “There is a lot of water in
this slab, and it produces a lot of partial melt. That produces arc volcanoes
that helped create Japan. But, we are still debating whether this water is
totally released in that depth. There is increasing evidence that a portion of
the water stays inside the plate to go much, much deeper.”
Northeastern China offers one of the best vantage
points to investigate whether this is true. The region is about 1,000
kilometers from the Japan trench where the Pacific plate begins its plunge back
into the planet’s interior. In 2009, with funding from the National Science
Foundation and others, Niu and scientists from the University of Texas at
Austin, the China Earthquake Administration, the Earthquake Research Institute
of Tokyo University and the Research Center for Prediction of Earthquakes and
Volcanic Eruptions at Japan’s Tohoku University began installing broadband
seismometers in the region.
“We put 140 stations there, and of course the more
stations the better for resolution,” Niu said. “The Chinese Academy of Sciences
put additional stations so they can get a finer, more detailed image.”
In the new study, data from the stations revealed
both the upper and lower boundaries of the Pacific plate, dipping down at a
25-degree angle within the mantle transition zone. The placement within this
zone is important for the study of mantle convection because the transition
zone lies below the asthenosphere, at depths where increased pressure causes
specific mantle minerals to undergo dramatic phase changes. These phases of the
minerals behave very differently in seismic profiles, just as liquid water and
solid ice behave very different even though they are made of identical
molecules. Because phase changes in the mantle transition zone happen at
specific pressures and temperatures, geoscientists can use them like a
thermometer to measure the temperature in the mantle.
Niu said the fact that both the top and bottom of the
slab are visible is evidence that the slab hasn’t completely mixed with the
surrounding mantle. He said heat signatures of partially melted portions of the
mantle beneath the slab also provide indirect evidence that the slab
transported some of its water into the transition zone.
“The problem is explaining how these hot materials
can be dropped into the deeper part of the mantle,” Niu said. “It’s still a
question. Because they are hot, they are buoyant.”
That buoyancy should act like a life preserver,
pushing upward on the underside of the sinking slab. Niu said the answer to
this question could be that holes have appeared in the deforming slab, allowing
the hot melt to rise while the slab sinks.
“If you have a hole, the melt will come out,” he
said. “That’s why we think the slab can go deeper.”
Holes could also explain the appearance of volcanos
like the Changbaishan on the border between China and North Korea.
“It’s 1,000 kilometers away from the plate boundary,”
Niu said. “We don’t really understand the mechanism of this kind of volcano.
But melt rising from holes in the slab could be a possible explanation.”
原始論文：Xin Wang,
Qi-Fu Chen, Fenglin Niu, Shengji Wei, Jieyuan Ning, Juan Li, Weijun Wang,
Johannes Buchen, Lijun Liu. Distinct slab interfaces imaged within the
mantle transition zone. Nature Geoscience, 2020; DOI: 10.1038/s41561-020-00653-5
引用自：Rice University. "Former piece of Pacific
Ocean floor imaged deep beneath China: Study offers clues about the fate of
tectonic plates that sink deep in Earth's mantle."
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