從全新觀點看待地球內部
By Felix Würsten
目前科學家的理解是地函擁有頗為均勻的化學成分。但是蘇黎世聯邦理工學院的研究人員最近進行的實驗顯示這種看法太過簡化。他們的結果能夠解決地球科學正面臨的重要問題――同時也讓一些新的問題浮上檯面。
蘇黎世聯邦理工學院的研究人員利用一連串複雜的實驗來探討地球深處的岩石行為。樣本位在照片中央的方形儀器裡面。(圖片來源：M. Murakami, ETH Zurich)
有些地方是我們永遠到不了的，地球內部正是其中之一。不過，我們確實有方法可以讓我們更加了解這處未知的世界。比方說，地震波可以讓我們界定地球的內部結構，以及我們腳下深處的物質有著什麼樣的物理性質，這些資訊都相當重要。再來，地表某些地方還有從地球深處噴上來的火山岩，它們可以提供關於地函化學成分的重要線索。最後，我們還能在實驗室進行實驗，從較小的尺度來模擬地球內部的環境條件。
最近實驗礦物物理學的教授村上元彥和他的團隊發表在期刊《美國國家科學院院刊》(PNAS)的文章，顯示了這類實驗可以帶來極大的啟發。他們的發現暗示了許多地球科學家對於地球內部的理解可能過於簡化。
戲劇性的變化
在地球只有幾公里厚的地殼之下便是地函。地函同樣是由岩石組成，一直延伸到我們腳下2900公里處，然後把地球的核心包圍起來。透過地震波訊號，我們知道地函在大約660公里深的地方發生了劇烈變化：此處是上部地函和下部地函交會的地方，由於岩石的力學性質開始改變，造成地震波前進的速度在此邊界變化得十分劇烈。
不過，我們仍不清楚這道邊界是否只標示了物理性質的變化，或者岩石的化學成分也在這個地方有所改變。許多地球科學家假設地函整體來說頗為一致，是由富含鎂的岩石組成，和地球表面發現的橄欖岩有著類似的成分。橄欖岩是地函上部的使者，它們經由火山爆發這類事件來到地表，成分裡鎂和矽的比值大約為1.3。
「地函組成大抵一致的假設是根據相當簡單的假說而來，」村上如此解釋。「亦即地函裡面強力的物質對流不僅推動了地表的板塊運動，同時也不停地在攪拌整個地函。不過這樣的觀點可能太過簡化。」
矽去哪裡了？
這項假說裏頭有個十分基本的缺陷。科學家一般同意地球是在45億年前由隕石吸積而成，因為這些隕石都是形成自原始太陽系星雲，所以它們的整體成分會一模一樣。地球分化成地核、地函和地殼則是後續過程的一部份。
排除現為地核組成一部份的鐵和鎳之後，就能明顯看出地函應該會比橄欖岩含有更多的矽。根據計算結果，地函鎂和矽的比例應該要接近於1，而非1.3。
這讓地球科學家不禁要問：消失的矽去哪了？有個顯然易見的答案是：地函的矽會這麼少，是因為它們跑到地核裡了。不過村上得出的結論卻完全不同：他認為矽是待在下部地函，這也意味著下部地函的成分和上部地函有所差異。
迂迴而得的假說
村上推導假說的過程有些迂迴曲折：首先，我們已經可以精準得出地震波在地函裡行進的速度；其次，實驗室試驗顯示下部地函的組成礦物大多為矽質的矽酸鹽鈣鈦礦(bridgmanite)和富含鎂的鐵方鎂石(ferropericlase)。第三，我們知道地震波在岩石中行進的速度，取決於組成礦物的彈性。因此，如果我們能得到這兩種礦物的彈力性質，就有辦法計算出兩者之間比例應該是多少，才能對應觀測到的地震波波速，接著還能推導出下部地函必須要有什麼樣的化學組成。
雖然我們已經知道鐵方鎂石的彈力性質，卻還不知道矽酸鹽鈣鈦礦的。這是因為此礦物的彈力性質很大一部份取決於它的化學成分，確切來說，會隨著鐵含量而改變。
耗時許久的測量
村上最近和團隊在他的實驗室對成分不同的矽酸鹽鈣鈦礦進行了高壓試驗。過程是先把一小塊樣本夾在兩枚鑽石的尖端之間，然後再用特殊的設備把兩者壓向對方，這會讓中間的樣本承受極高的壓力，就像下部地函的環境一樣。
團隊接著把一道雷射光射向樣本，然後測量光線從另一頭色散出來的的波譜。利用波譜的位移量，他們就能測定矽酸鹽鈣鈦礦在不同壓力下的彈力性質。「這項測量需要花費非常久的時間，」村上的報告中表示。「由於矽酸鹽鈣鈦礦的含鐵量越高，光線就越難穿透，
因此我們每一次的測量最多得用上15天才能完成。」
找出矽在哪裡
村上接著利用測量得到的數值，模擬什麼樣的礦物組成可以和地震波的色散模式有最佳的對應關係。結論證實了下部地函的成分和上部地函有所不同。「我們估計下部地函的組成中有百分之88到93是矽酸鹽鈣鈦礦，」村上說。「進而得出這個區域的鎂和矽的比例大約為1.1。」因此村上的假說解決了矽消失的謎題。
不過這也造成了新的問題。舉例來說，我們知道在某些隱沒帶，地球的地殼會被推到地函深處，有時甚至能深入到地核的邊界，代表地函上部和下部實際上並不是完全隔開的個體。因此要讓地函的這兩個區域有不同的化學成分，它們之間要有什麼樣的交互作用，以及地球內部動力學的確切運作方式是什麼，都是有待解答的問題。
A new way of looking at the Earth’s
interior
Current understanding is that the
chemical composition of the Earth’s mantle is relatively homogeneous. But
experiments conducted by ETH researchers now show that this view is too simplistic.
Their results solve a key problem facing the geosciences – and raise some new
questions.
There are places that will always be beyond our
reach. The Earth’s interior is one of them. But we do have ways of gaining an
understanding of this uncharted world. Seismic waves, for instance, allow us to
put important constraints about the structure of our planet and the physical
properties of the materials hidden deep within it. Then there are the volcanic
rocks that emerge in some places on the Earth’s surface from deep within and
provide important clues about the chemical composition of the mantle. And
finally there are lab experiments that can simulate the conditions of the
Earth’s interior on a small scale.
A new publication by Motohiko Murakami, Professor of
Experimental Mineral Physics, and his team was featured recently in the journal
PNAS and shows just how illuminating
such experiments can be. The researchers’ findings suggest that many
geoscientists’ understanding of the Earth’s interior may be too simplistic.
Dramatic change
Below the Earth’s crust, which is only a few
kilometres thick, lies its mantle. Also made of rock, this surrounds the
planet’s core, which begins some 2,900 kilometres below us. Thanks to seismic
signals, we know that a dramatic change occurs in the mantle at a depth of
around 660 kilometres: this is where the upper mantle meets the lower mantle
and the mechanical properties of the rock begin to differ, which is why the
propagation velocity of seismic waves changes dramatically at this border.
What is unclear is whether this is merely a physical
border or whether the chemical composition of the rock also changes at this
point. Many geoscientists presume that the Earth’s mantle as a whole is
composed relatively consistently of magnesium-rich rock, which in turn has a
composition similar to that of peridotite rock found on the Earth’s surface.
These envoys from the upper mantle, which arrive on the Earth’s surface by way
of events like volcanic eruptions, exhibit a magnesium-silicon ratio of ~1.3.
“The presumption that the composition of the Earth’s
mantle is more or less homogeneous is based on a relatively simple hypothesis,”
Murakami explains. “Namely that the powerful convection currents within the
mantle, which also drive the motion of the tectonic plates on the Earth’s surface,
are constantly mixing it through. But it’s possible that this view is too
simplistic.”
Where’s the
silicon?
There really is a fundamental flaw in this
hypothesis. It is generally agreed that the Earth was formed around 4.5 billion
years ago through the accretion of meteorites that emerged from the primordial
solar nebula, and as such has the same overall composition of those meteorites.
The differentiation of the Earth into core, mantle and crust happened as part
of a second step.
Leaving aside the iron and nickel, which are now part
of the planet’s core, it becomes apparent that the mantle should actually
contain more silicon than the peridotite rock. Based on these calculations, the
mantle should have a magnesium-silicon ratio closer to ~1 rather than ~1.3.
This moves geoscientists to ask the following
question: where is the missing silicon? And there is an obvious answer: the
Earth’s mantle contains so little silicon because it is in the Earth’s core.
But Murakami reaches a different conclusion, namely that the silicon is in the
lower mantle. This would mean that the composition of the lower mantle differs
to that of the upper mantel.
Winding
hypothesis
Murakami’s hypothesis takes a few twists and turns:
First, we already know precisely how fast seismic waves travel through the
mantle. Second, lab experiments show that the lower mantle is made mostly of
the siliceous mineral bridgmanite and the magnesium-rich mineral
ferropericlase. Third, we know that the speed the seismic waves travel depends
on the elasticity of the minerals that make up the rock. So if the elastic
properties of the two minerals are known, it is possible to calculate the
proportions of each mineral required to correlate with the observed speed of
the seismic waves. It is then possible to derive what the chemical composition
of the lower mantle must be.
While the elastic properties of ferropericlase are
known, those of bridgmanite are as yet not. This is because this mineral’s
elasticity depends greatly on its chemical composition; more specifically, it
varies according to how much iron the bridgmanite contains.
Time-consuming
measurements
In his lab, Murakami and his team have now conducted
high-pressure tests on this mineral and experimented with different
compositions. The researchers began by clamping a small specimen between two
diamond tips and using a special device to press them together. This subjected
the specimen to extremely high pressure, similar to that found in the lower
mantle.
The researchers then directed a laser beam at the
specimen and measured the wave spectrum of the light dispersed on the other
side. Using the displacements in the wave spectrum, they were able to determine
the mineral’s elasticity at different pressures. “It took a very long time to
complete the measurements,” Murakami reports. “Since the more iron bridgmanite
contains the less permeable to light it becomes, we needed up to fifteen days
to complete each individual measurement.”
Silicon
discovered
Murakami then used the measurement values to model
the composition that best correlates with the dispersal of seismic waves. The
results confirm his theory that the composition of the lower mantle differs to
that of the upper mantel. “We estimate that bridgmanite makes up 88 to 93
percent of the lower mantle,” Murakami says, “which gives this region a
magnesium-silicon ratio of approximately 1.1.” Murakami’s hypothesis solves
the mystery of the missing silicon.
But his findings raise new questions. We know for
instance that within certain subduction zones, the Earth’s crust gets pushed
deep into the mantle – sometimes even as far as the border to the core. This
means that the upper and lower mantles are actually not hermetically separated
entities. How the two areas interact and exactly how the dynamics of the
Earth’s interior work to produce chemically different regions of mantle remains
to be seen.
原始論文：Izumi Mashino,
Motohiko Murakami, Nobuyoshi Miyajima, Sylvain Petitgirard. Experimental
evidence for silica-enriched Earth’s lower mantle with ferrous iron dominant
bridgmanite. Proceedings of the National Academy of Sciences,
2020; 201917096 DOI: 10.1073/pnas.1917096117
引用自：ETH Zurich. "A new way of looking at the
Earth's interior."
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