改寫地球歷史的遠古水滴
利用古代海洋留下的微小水滴重建地球開始進行板塊運動的時間。
圖中的科馬提岩來自於標準產地――南非巴伯頓山脈的科馬提河谷。這塊科馬提岩顯示出獨有的鬣刺結構(spinifex
texture)，為板狀的橄欖石晶體排列得像是交雜的樹枝一樣。越往熔岩流上方(圖片上方)的晶體越小，排列方向也更加不規則。圖片來源：維基百科
科學家利用古代海洋留下的微小水滴重建出地球板塊運動的開始時間，因而改寫地球的演化史。
板塊運動是地球相當重要且獨有的持續循環過程。地球幾乎每項機能都受到板塊運動直接或間間的影響，包括大氣的組成狀況、造山運動(形成陸地)、火山與地震之類的自然災害、礦床的形成、維持海洋存在......等。板塊運動造成地球上的大型陸塊持續移動，並讓地球最表層(地殼)循環回地函，再由火山活動等作用產生新的地殼取而代之。
科學家之前認為板塊運動大概始於27億年前，不過最近一組國際科學團隊利用地球深部地函的一滴古代海水――為板塊運動從地表帶來的――指出板塊運動的開始時間還要再早個6億年。他們的研究證明了板塊運動在33億年前就已經開始。論文於7月16日發表在高影響力的學術期刊《自然》(Nature)。
南非金山大學地球科學院的教授Allan
Wilson是研究團隊的成員之一。他說：「板塊運動讓地球的物質持續循環更新。如果沒有板塊運動，地球就會變得跟火星一樣。」
「我們的研究顯示板塊運動的開始時間為33億年前，跟地球上生命開始發展的時期一致。這讓我們知道地球的起源以及演變過程。」
地球是太陽系裡頭唯一受到板塊運動影響的星球。如果沒有板塊運動地球就會變得無法居住。
研究團隊進行這項研究時分析了一小塊稱為科馬提岩的熔岩。科馬提岩的標準產狀位在南非普馬蘭加省巴伯頓鎮附近的科馬提河，因而得其名。它們是有史以來最高溫的岩漿凝固留下的岩石，形成於地球生命的最初四分之一(太古宙)。雖然大多數科馬提岩都因為暴露在大氣之中，或是受到其他變化而難以解讀原本的樣貌，但是岩石內部一種稱為橄欖石的礦物卻能保存微量熔岩，使得團隊可以研究完好保存下來的遠古岩漿。
「我們檢視了一滴直徑只有10微米(0.01釐米)的岩漿，並分析它含有的化學指標，像是水含量、氯、氘氫比......等。結果發現地球開始擁有循環作用的時間，比原先認為的還要早了將近6億年。」Wilson表示。「我們發現海水運到地函深處之後，會在核幔邊界經由岩漿柱重新回到地表。」
研究結果讓科學家更加瞭解板塊運動的初始階段，以及穩定的大陸地殼如何形成。
Wilson說：「這項研究另外一個引人注目的地方是，今年剛好是金山大學的教授Morris和Richard
Viljoen兄弟，在巴伯頓山脈發現科馬提岩的50周年。」
Ancient drop of water rewrites Earth's
history
The remains of a microscopic drop of
ancient seawater was used to re-establish the date that plate tectonics started
on Earth.
The remains of a microscopic drop of ancient seawater
has assisted in rewriting the history of Earth’s evolution when it was used to
re-establish the time that plate tectonics started on the planet.
Plate tectonics is Earth’s vital – and unique –
continuous recycling process that directly or indirectly controls almost every
function of the planet, including atmospheric conditions, mountain building
(forming of continents), natural hazards such as volcanoes and earthquakes,
formation of mineral deposits and the maintenance of our oceans. It is the
process where the large continental plates of the planet continuously move, and
the top layers of the Earth (crust) are recycled into the mantle and replaced
by new layers through processes such as volcanic activity.
Where it was previously thought that plate tectonics
started about 2.7 billion years ago, a team of international scientists used
the microscopic leftovers of a drop of water that was transported into the
Earth’s deep mantle – through plate tectonics – to show that this process
started 600 million years before that. An article on their research that proves
plate tectonics started on Earth 3.3 billion years ago was published in the
high impact academic journal, Nature,
on 16 July.
“Plate tectonics constantly recycles the planet’s
matter, and without it the planet would look like Mars,” says Professor Allan
Wilson from the Wits School of Geosciences, who was part of the research team.
“Our research showing that plate tectonics started
3.3 billion years ago now coincides with the period that life started on Earth.
It tells us where the planet came from and how it evolved.”
Earth is the only planet in our solar system that is
shaped by plate tectonics and without it the planet would be uninhabitable.
For their
research, the team analysed a piece of rock melt, called komatiite – named
after the type occurrence in the Komati river near Barberton in Mpumalanga –
that are the leftovers from the hottest magma ever produced in the first
quarter of Earth’s existence (the Archaean). While most of the komatiites were
obscured by later alteration and exposure to the atmosphere, small droplets of
the molten rock were preserved in a mineral called olivine. This allowed the
team to study a perfectly preserved piece of ancient lava.
“We examined a piece of melt that was 10 microns
(0.01mm) in diameter, and analysed its chemical indicators such as H2O
content, chlorine and deuterium/hydrogen ratio, and found that Earth’s
recycling process started about 600 million years earlier than originally thought,”
says Wilson. “We found that seawater was transported deep into the mantle and
then re-emerged through volcanic plumes from the core-mantle boundary.”
The research allows insight into the first stages of
plate tectonics and the start of stable continental crust.
“What is exciting is that this discovery comes at the
50th anniversary of the discovery of komatiites in the Barberton Mountain Land
by Wits Professors, the brothers Morris and Richard Viljoen,” says Wilson.
原始論文：Alexander V.
Sobolev, Evgeny V. Asafov, Andrey A. Gurenko, Nicholas T. Arndt, Valentina G.
Batanova, Maxim V. Portnyagin, Dieter Garbe-Schönberg, Allan H. Wilson, Gary R.
Byerly. Deep hydrous mantle reservoir provides evidence for crustal
recycling before 3.3 billion years ago. Nature, 2019; 571
(7766): 555 DOI: 10.1038/s41586-019-1399-5
引用自：University of the Witwatersrand. "Drop of
ancient seawater rewrites Earth's history.”
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