鑽石告訴我們地球適合居住的關鍵之一：穩定的陸地如何形成？
地球的陸地在板塊構造運動的摧殘下仍然可以存在許久，是生命出現在地球的重要地質條件之一，而這種穩定性取決於連在陸塊下方的地函性質。美國卡內基研究所、美國寶石學院和加拿大阿爾伯塔大學的地質學家團隊最新進行的研究顯示，鑽石可以闡明某些陸地下方具有浮力的地函是如何增厚，使得上方的陸塊可以長期處於穩定狀態。
採自獅子山的鑽石，具有含硫礦物的包裹體。圖片來源：美國寶石學院
「從古代火山爆發中噴出的硫，有極為少數可以進到地函，最後跑到鑽石裡面。我們新發現的方法可以運用這些硫，來證實一種特定的造陸過程。」美國寶石學院的Karen
Smit表示。他是研究團隊本周發表在《科學》(Science)的論文主要作者。「我們的方法顯示形成非洲大陸西部的地質作用，是海洋地殼下沉到地函的板塊構造運動。」
鑽石也許深受珠寶收藏家的喜愛，不過它們其實是地質學家最好的朋友。雖然鑽石內部的微小礦物顆粒在珠寶市場通常不受青睞；但是，因為它們形成於地球深處，所以可以透露出鑽石形成時的環境細節。
研究共同作者，卡內基研究所的Steve
Shirey加以解釋：「由此看來，鑽石就像是地球深處派來的礦物特使。」
稱為「地函龍骨」(mantle keel)的層位位在地下150至200公里，具有穩定大陸地殼的功能。它的組成物質必須要有足夠的厚度和穩定度且溫度較低，才能形成堅固並具有浮力的龍骨來支撐地表的陸塊，使其不被地球毫不間斷且破壞力十足的構造活動摧毀。但是科學界對於地函龍骨的形成過程仍然沒有定論。
「解開這項謎題是瞭解下列問題的關鍵：陸地如何演變至現今的狀態，以及陸地如何存活於地質活躍的行星之上。」Shirey解釋，「地球是唯一已知擁有活躍板塊運動的岩石行星，因此瞭解形成陸地的地質作用，是解開地球為什麼適合居住的一個關鍵。」
有些科學家認為地函龍骨的起源是隱沒作用，當地表的海洋板塊滑到另一個板塊下方，發生的隱沒作用會讓板塊沉入地球深處。另外一些科學家則認為地函龍骨的成因是一種垂直進行的作用，跟高溫的岩漿柱從地球極為深處上湧有關。
要解決這項爭議可以透過地球化學工具的協助，它可以偵測出地函龍骨的組成物質究竟是來自於地表的板塊或是上湧的深部地函。幸運地，地函龍骨擁有鑽石形成的理想條件；代表科學家可以研究在地函龍骨形成的鑽石內部的包裹體，藉此查出地函龍骨的起源。
研究團隊分析了開採自獅子山的鑽石，其中富含硫的礦物(硫化物)包裹體指出這個地區的歷史中，曾經發生了兩次隱沒事件。
他們判斷的根據是這些硫化物礦物顆粒的化學性質，只有在早於25億年前，也就是地球大氣充滿氧氣之前的地表礦物樣本中才能見到這種化學性質。代表這些礦物包裹體中的硫必定曾經待在地表，接著才被隱沒作用拖到地函當中。
研究團隊比較波札那的鑽石，也得到了類似證據指出地函龍骨是由隱沒作用形成。但是，加拿大北部鑽石內的硫卻沒有呈現出同樣的化學性質，意謂形成該區地函龍骨的作用，並沒有地表物質參與其中。
研究團隊的發現顯示西非大陸下方地函龍骨增厚與穩定的原因，是地函和沉下來的海床物質碰撞並擠壓所導致。但這種地函龍骨增厚，使得陸地穩定的方式，並無法解釋加拿大北部下方的地函龍骨如何形成。加拿大鑽石內的含硫礦物無法告訴研究人員此區的地函龍骨如何形成，只能顯示出並非透過上述方式。
Smit總結：「我們的研究成果顯示鑽石內的含硫包裹體，是一種可以用來探討造陸作用的強力工具。」
Diamonds reveal how continents are
stabilized, key to Earth’s habitability
The longevity of Earth’s continents in the face of
destructive tectonic activity is an essential geologic backdrop for the
emergence of life on our planet. This stability depends on the underlying
mantle attached to the landmasses. New research by a group of geoscientists
from Carnegie, the Gemological Institute of America, and the University of
Alberta demonstrates that diamonds can be used to reveal how a buoyant section
of mantle beneath some of the continents became thick enough to provide
long-term stability.
“We’ve found a
way to use traces of sulfur from ancient volcanoes that made its way into the
mantle and eventually into diamonds to provide evidence for one particular
process of continent building,” explained Karen Smit of the Gemological
Institute of America, lead author on the group’s paper, which appears this week
in Science. “Our
technique shows that the geologic activity that formed the West African
continent was due to plate tectonic movement of ocean crust sinking into the
mantle.”
Diamonds may be
beloved by jewelry collectors, but they are truly a geologist’s best friend.
Because they originate deep inside the Earth, tiny mineral grains trapped
inside of a diamond, often considered undesirable in the gem trade, can reveal
details about the conditions under which it formed.
“In this way,
diamonds act as mineralogical emissaries from the Earth’s depths,” explained
Carnegie co-author Steve Shirey.
About 150 to 200 kilometers,
93 to 124 miles, beneath the surface, geologic formations called mantle keels
act as stabilizers for the continental crust. The material that comprises them
must thicken, stabilize, and cool under the continent to form a strong,
buoyant, keel that is fundamental for preserving the surface landmass against
the relentless destructive forces of Earth’s tectonic activity. But how this is
accomplished has been a matter of debate in the scientific community.
“Solving this
mystery is key to understanding how the continents came to exist in their
current incarnations and how they survive on an active planet,” Shirey
explained. “Since this is the only tectonically active, rocky planet that we
know, understanding the geology of how our continents formed is a crucial part
of discerning what makes Earth habitable.”
Some scientists
think mantle keels form by a process called subduction, by which oceanic plates
sink from the Earth’s surface into its depths when one tectonic plate slides
beneath another. Others think keels are created by a vertical process in which
plumes of hot magma rise from much deeper in the Earth.
A geochemical
tool that can detect whether the source of a mantle keel’s makeup originated
from surface plates or from upwelling of deeper mantle material was needed to
help resolve this debate. Luckily, mantle keels have the ideal conditions for
diamond formation. This means scientists can reveal a mantle keel’s origin by
studying inclusions from diamonds that formed in it.
The research
group’s analysis of sulfur-rich minerals, called sulfides, in diamonds mined in
Sierra Leone indicate that the region experienced two subduction events during
its history.
They were able to
make this determination because the chemistry of the sulfide mineral grains is
only seen in samples from Earth’s surface more than 2.5 billion years
ago—before oxygen became so abundant in our planet’s atmosphere. This means
that the sulfur in these mineral inclusions must have once existed on the
Earth’s surface and was then drawn down into the mantle by subduction.
The team’s
comparison to diamonds from Botswana showed similar evidence of keel-creation
through subduction. But comparison to diamonds mined from northern Canada does
not show the same sulfur chemistry, meaning that the mantle keel in this region
originated in some way that did not incorporate surface material.
The group’s
findings suggest that thickening and stabilization of the mantle keel beneath
the West African continent happened when this section of mantle was squeezed by
collision with the sinking ocean floor material. This method of keel thickening
and continent stabilization is not responsible for forming the keel under a
portion of northern Canada. The sulfide minerals inside Canadian diamonds do
not tell the researchers how this keel formed, only how it didn’t.
“Our work shows
that sulfide inclusions in diamonds are a powerful tool to investigate
continent construction processes,” Smit concluded.
原始論文：Karen V. Smit, Steven B. Shirey, Erik H. Hauri,
Richard A. Stern. Sulfur isotopes in diamonds reveal differences in
continent construction. Science, 2019 DOI: 10.1126/science.aaw9548
引用自：Carnegie Institution for Science. "Diamonds reveal how
continents are stabilized, key to Earth's habitability.”
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