地函深處的謎樣岩石斑塊

原文網址：http://msutoday.msu.edu/news/2017/puzzling-pockets-of-rock-deep-in-earths-mantle/

地函深處的謎樣岩石斑塊

密西根州立大學和亞利桑納州立大學的地質科學家利用電腦模擬，來尋求岩石如何在地核和地函邊界聚積成蕈狀斑塊的解釋。

這些較小的岩體大概位在地下2600公里深。由於地震波經過它們時會大幅變慢，因此又被稱作「超低速帶」(ultra-low velocity zones)。長久以來地質科學家認為這些區域處於部分熔融狀態；然而，假設中由炙熱岩石組成的這些區域，其中卻有許多經常被觀測到位於深部地函較冷的地區，為此地質科學家困惑了數十年之久。

此論文發表於當期的《自然通訊》(Nature Communications)。主要作者，亞利桑納州立大學地球與太空探索學院的助理教授Mingming Li表示：「過往認為這些小區域跟他們周圍的岩石相同，不過是處於部分熔融狀態。但是從它們在全球都有分布，且密度、形狀和大小有很多變化來看，它們的組成可能截然不同。」

Li是共同作者，密西根州立大學的地球與環境科學家Allen McNamara的研究生。其他共同作者還有亞利桑那州立大學的Edward Garnero和Shule Yu。

McNamara表示：「我們不知道超低速帶到底是什麼。它們可能跟一般地函一樣，只不過溫度比較高且處於熔融狀態；或者完全是由其他某些東西組成。事實上，這兩種可能性都跟地震波呈現出來的證據相符。因此，我們決定運用地函對流的電腦模型，來探討它們的形狀和外形是否可以提供給我們這項問題的解答。」

一年之前，Garnero和McNamara發表地球深處有兩處岩石形成的巨型構造，可能是跟地函其他部分不同的物質組成。他們將這些構造稱作熱化學堆(thermochemical piles)，或簡稱為「團塊」。雖然科學家還不清楚團塊的起源和成分，但他們猜測團塊擁有地球形成及運作方式的重要線索。

團隊的電腦模型解釋了小型而孤立的超低速帶岩石斑塊跟大上許多的團塊位置之間的關係，同時也呈現出大部分超低速帶的成分跟它們周圍的地函有所差異。

地震波的觀測結果顯示這些小岩石斑塊存在於大型熱化學堆的邊緣，在某些例子中，它們又跟高溫的大型團塊相距甚遠。因此團隊面臨的問題是要找出小區塊岩石的形成方法，可以用來解釋此觀測結果。

Garnero問說：「為何有些部分熔融的地函岩石可以零星分布在較冷的區域？如果它們在那裡的溫度都高到足以發生熔融，難道我們不該預期在溫度最高的地方可以看見巨大的熔融區域？但我們確實沒有觀察到這種東西。」

Li接著說團隊發現在核幔邊界上方，地函溫度最高的區域深處在團塊內部。這代表位在大團塊深處的某些小斑塊可能僅靠部分熔融作用產生。

Li的電腦模擬結果顯示成分截然不同的岩石區塊會從核幔邊界各處往大團塊的邊緣靠近。

McNamara表示：「熱化學堆的邊緣是地函對流模式的匯集之處，也造成這些地區變成密度較高岩石的集貨站。」

這類移動過程發生的背後因素是地函對流的動力來源――熱。

地函雖然是由高溫岩石組成，但它的行為比較像是爐子上慢火熬煮的糖漿。地函的熱是由地函岩石的放射性衰變，以及正下方中心溫度將近太陽表面的地核提供。地函岩石受到熱的作用而緩緩地攪拌――此動作稱為對流。

Li表示：「詳細情形還沒有全數釐清。」但模擬結果顯示不同成分的岩石受到對流作用時，成分相近的物質會逐漸匯聚在一起。這造成化學成分特異的小岩石斑塊往核幔邊界上方的高溫團塊邊緣移動。

Li表示：「我們運用高解析度的三維電腦模型，並發展新方法可以同時追蹤超低速帶的小斑塊，以及大上許多的熱化學堆的移動方式。這讓我們可以探討那些小斑塊是如何到處移動，以及它們的所處位置跟起源之間有何關聯。」

所以他們的模型有何不同之處？

McNamara表示：「我們用的是全新的方法，它對電腦運算來說也相當具有挑戰性。我們的模擬同時考慮了規模相差甚鉅的運動過程，從全球尺度的地函對流模式，再到下部地函的大型熱化學堆，然後是地函底部非常小尺度的超低速帶斑塊。」

McNamara繼續說道：「我們最後發現如果超低速帶是由跟一般地函完全一樣的物質熔融而成，那它就應該會處於地函溫度最高的熱化學堆深處。相反地，如果它們是由一些其他化學成分組成，即使它們形成時是在熱化學堆之外，但接下來還是會被地函對流運送到熱化學堆邊緣而聚集起來。這跟我們從地震波看到的觀測結果一致。」

那這些不同的物質最初是從哪裡來的？

Garnero說：「有幾種不同的可能性。這些物質有部分可能跟隱沒到非常深處，之前是玄武岩質的海洋地殼有關。或者是跟外核富含鐵的流體和地函矽酸鹽結晶產生的化學反應有關。」

超低速帶的起源目前仍未解開，但是讓這些物質聚集成小岩石斑塊的作用已經相當清楚。

Garnero表示：「可能有相當多種機制，比方說板塊作用，可以將不同化學成分的岩石從地球任何一角帶到地函最深的地方。不過這些不同成分的岩石一旦到達地底深處，對流作用就會接手然後將它們掃往高溫區域，也就是陸地大小的熱化學堆所在之處。」

Puzzling pockets of rock deep in Earth's mantle

Michigan State University and Arizona State University geoscientists have used computer modeling to find an explanation for how pockets of mushy rock have accumulated at the boundary between Earth's core and mantle.

The relatively small rock bodies reside roughly 1800 miles below the surface. Known as “ultra-low velocity zones” because seismic waves greatly slow down as they pass through them, they have been long thought to be partially molten. However, they have puzzled geoscientists for decades because many of these assumed hot rock zones are often observed in cooler regions of the deep mantle.

"These small regions have been assumed to be a partially molten version of rock that surrounds them," said Mingming Li, assistant professor in ASU's School of Earth and Space Exploration and lead author of the paper published in the current issue of Nature Communications. "But their global distribution and large variation of density, shape and size suggest that they have a different composition."

Li was a graduate student of Allen McNamara, coauthor and MSU earth and environmental scientist. The additional coauthors are Edward Garnero and Shule Yu of ASU.

“We don’t know what ultra-low velocity zones are,” McNamara said. “They are either hot, partially-molten portions of otherwise normal mantle or they are something else entirely, some other composition. In fact, the seismic evidence allows for both possibilities. We decided to perform computational modeling of mantle convection to investigate whether their shapes and positions can provide the answer to that question.”

A year ago, Garnero and McNamara reported that two gigantic structures of rock deep in the Earth are likely made of something different from the rest of the mantle. They called the structures thermochemical piles, or more simply "blobs." The origin and composition of the blobs were unknown, but the scientists suspected they held important clues as to how the Earth was formed and how it works.

The team’s computer modeling explained how these small, isolated pockets of rock, the ultra-low velocity zones, are associated with the location of the much bigger blobs. They also showed that most of these ultra-low velocity zones are different in composition than their surrounding mantle.

So the problem the team faced was finding a way to make small pockets of rock that explained these seismic observations, namely, that they existed near the margins of the larger thermochemical piles, and in some cases, far away from the hot massive blobs.

"How could partially melted mantle rock exist in the cold areas in tiny spots?” Garnero asked. “If it's hot enough to melt there, shouldn't we expect massive melt zones in the hottest regions? But we do not see that."

The team found that the hottest regions above the core-mantle boundary are well inside the blobs. This suggests that some pockets located well inside the large blobs may be caused by partial melting alone, Li added.

Li's computer modeling showed that pockets of distinctly different rock composition will migrate from anywhere on the core-mantle boundary towards the margins of the large blobs.

“The margins of the thermochemical piles,” McNamara said, “are where mantle flow patterns are converging, and therefore, these areas provide a collection depot for denser types of rock.”

The secret driving this movement is heat which powers mantle convection.

Earth's mantle is made of hot rock, but it behaves more like fudge simmering slowly on a stove. For the mantle, the heat is provided by both radioactivity of mantle rock and from the core which lies just below, the center of which is about as hot as Sun's surface. Mantle rock responds to this heat with a slow churning – convective – motion.

"The details are not completely clear," Li said. But the modeling shows that rocks of different composition respond to the convection in a way that gathers compositionally similar materials together. This moves the small pockets of chemically distinct rocks to the edges of the hotter blobs above the core-mantle boundary.

“We ran 3D high resolution computer modeling and we developed a method to track the movement of both the small pockets of ultra-low velocity zones and the much larger thermochemical piles,” Li said. “This allows us to study how the small pockets are moved around and how their locations can be related to their origin.”

What was different about their models?

“What was new and computationally challenging about our approach,” McNamara said, “is that our modeling simultaneously took into account vastly different scales of motion, from global mantle-scale convection patterns, to the large thermochemical piles in the lower mantle, to the very small-scale pockets of ultra-low velocity zone at the bottom.”

“What we ultimately found,” continues McNamara, “is that if ultra-low velocity zones are caused by melting of otherwise normal mantle, they should be located well inside of the thermochemical piles, where mantle temperatures are the hottest. If they are instead caused by some other composition, then although they could originate outside of the piles, they would continually be carried toward the edges of piles by mantle convection, where they collect. This is consistent with what we see in the seismic observations.”

Where do the different materials come from in the first place?

“There are several possibilities,” Garnero said. "Some of the material might be associated with former basaltic oceanic crust that got subducted deeply. Or it might be associated with chemical reactions between the outer core's iron-rich fluid and the crystalline silicate mantle."

The origin of ultra-low velocity zones is currently unsolved, but the process of collecting the material into small pockets of rock is clear.

"You can have various mechanisms, such as plate tectonics, that push rock of differing chemistries into the deepest mantle anywhere on Earth,” Garnero said. "But once these different rocks have gone down deep, convection wins and sweeps them to the hot regions, namely, where the continental-sized thermochemical piles reside. "

原始論文：Mingming Li, Allen K. McNamara, Edward J. Garnero, Shule Yu. Compositionally-distinct ultra-low velocity zones on Earth’s core-mantle boundary. Nature Communications, 2017; 8 (1) DOI: 10.1038/s41467-017-00219-x

引用自：Arizona State University. "Puzzling pockets of rock deep in Earth's mantle explained."