地球或許比之前認為的還早就擁有讓大陸地殼、甚至是生命出現的條件
科學家表示地殼可能在地球僅有3.5億歲的時候就形成了
By Louise Lerner
根據芝加哥大學領導的科學家團隊進行的研究顯示，早期地球開始適合生命居住的時間或許比過去認為的還要早上許多。
在計算加拿大北部岩石當中的鍶原子數目之後，他們發現的證據顯示地球形成大陸地殼的時間比過去認為的還要早數億年。大陸地殼擁有的主要礦物比年輕的火山岩還多，使得它們對於生命的形成更加有利。
「我們找到的證據和包括澳洲西部岩石在內的新證據一致，顯示早期地球可以在太陽系形成後的3.5億年以內就形成大陸地殼。」研究第一作者，芝加哥大學地球物理科學系的博士後研究員Patrick Boehnke表示。「這改變了傳統看法中，在地殼形成後有超過5億年的時間它都是處於又乾又熱，如地獄一般的狀態。」
在地質學上有一個懸而未解的問題是，有些本來由年代較輕的火山岩組成的地殼，是在什麼時候透過什麼方式轉變成我們所知且喜好的大陸地殼，其密度較輕且含有更多的矽。讓解開這項課題變得更加困難的是，隨著數百萬年經過之後證據會不斷熔化並改變形貌。在地球某些地方可以發現從地球最為古老的時期留下的些許地殼，它們以微小的磷灰石碎片鑲嵌在年輕的岩石之中。
對科學家來說幸運的是，較為「年輕」(年代仍然有39億年左右)的礦物中有一些是鋯石――某種相當堅硬、可以抵抗風化的礦物，看起來有點像鑽石。Boehnke表示：「鋯石是地質學家的最愛，因為它們是地球最初3億至4億年的歲月中所留下來的唯一紀錄。鑽石並非恆久遠――鋯石才是。」
此外，鋯石本身還可以定年。「它們就像是有標註日期的時間膠囊一樣。」研究共同作者，芝加哥大學地球物理科學系的主任Andrew Davis教授表示。
科學家通常藉著觀察元素的不同形貌――同位素――來講述岩石歷經的故事。研究團隊想要利用鍶提供的線索，來得知岩石形成時環境中有多少矽。唯一的問題是這些鋯石顆粒極其微小，大概只有五微米寬，也就是一根蜘蛛絲的直徑，所以必須要一顆一顆地數有多少鍶原子。
去年才開始運作的一個特殊儀器可以完成這項任務：芝加哥大學雷射離子儀，簡稱CHILI。這具探測器所用的雷射經過調整後可以特地把鍶挑出來使其游離。當他們利用CHILI計算加拿大努夫雅吉圖克岩石中的鍶同位素後，他們發現其同位素比例顯示岩石形成時環境中充滿了矽。
此發現的重要之處在於地殼的組成會直接影響到大氣組成、海洋成分，以及任何剛萌芽的生命想要在地球上成功繁衍可以得到多少養分。此外，由於隕石撞擊會讓大陸地殼難以形成，因此這項發現也意味著當時撞擊地球的隕石數目可能比以往認為的還要少。
「地球在如此早期就擁有大陸地殼，從很多方面都改變了我們對早期地球的想像。」同時身為恩里科•費米研究所的教授Davis表示，「現在我們需要找出一種地質作用可以在如此快的時間內就形成陸地，這可能需要水分還有溫度低了華氏600度左右(約攝氏315度)的岩漿參與其中。」
此研究也跟最近一篇由Davis和Boehnke的同事Nicolas Dauphas發表的論文互相契合，其找到的證據顯示在25億年前就有雨水落在陸地上了，比過往認為的還要更早。
Earth could have supported
continental crust, life earlier than thought
Scientists say crust could
have formed when Earth was just 350 million years old
The early Earth might have been
habitable much earlier than thought, according to new research from a group led
by University of Chicago scientists.
Counting strontium atoms in rocks from
northern Canada, they found evidence that the Earth’s continental crust could
have formed hundreds of millions of years earlier than previously thought.
Continental crust is richer in essential minerals than younger volcanic rock,
which would have made it significantly friendlier to supporting life.
“Our evidence, which squares with
emerging evidence including rocks in western Australia, suggests that the early
Earth was capable of forming continental crust within 350 million years of the
formation of the solar system,” said Patrick Boehnke, the T.C. Chamberlin
Postdoctoral Fellow in the Department of Geophysical Sciences and the first
author on the paper. “This alters the classic view, that the crust was hot, dry
and hellish for more than half a billion years after it formed.”
One of the open questions in geology is
how and when some of the crust — originally all younger volcanic rock — changed
into the continental crust we know and love, which is lighter and richer in
silica. This task is made harder because the evidence keeps getting melted and
reformed over millions of years. One of the few places on Earth where you can
find bits of crust from the very earliest epochs of Earth is in tiny flecks of
apatite imbedded in younger rocks.
Luckily for scientists, some of these
“younger” minerals (still about 3.9 billion years old) are zircons — very hard,
weather-resistant minerals somewhat similar to diamonds. “Zircons are a
geologist’s favorite because these are the only record of the first three to
four hundred million years of Earth. Diamonds aren’t forever — zircons are,”
Boehnke said.
Plus, the zircons themselves can be
dated. “They’re like labeled time capsules,” said Prof. Andrew Davis, chair of
the Department of Geophysical Sciences and a coauthor on the study.
Scientists usually look at the different
variants of elements, called isotopes, to tell a story about these rocks. They
wanted to use strontium, which offers clues to how much silica was around at
the time it formed. The only problem is that these flecks are absolutely tiny —
about five microns across, the diameter of a strand of spider silk — and you
have to count the strontium atoms one by one.
This was a task for a unique instrument
that came online last year: the CHicago Instrument for Laser Ionization, or
CHILI. This detector uses lasers that can be tuned to selectively pick out and
ionize strontium. When they used CHILI to count strontium isotopes in rocks
from Nuvvuagittuq, Canada, they found the isotope ratio suggested plenty of
silica was present when it formed.
This is important because the makeup of
the crust directly affects the atmosphere, the composition of seawater, and
nutrients available to any budding life hoping to thrive on planet Earth. It
also may imply there were fewer meteorites than thought pummeling the Earth at
this time, which would have made it hard for continental crust to form.
“Having continental crust that early
changes the picture of early Earth in a number of ways,” said Davis, who is
also a professor with the Enrico Fermi Institute. “Now we need a way for the
geologic processes that make the continents to happen much faster; you probably
need water and magma that’s about 600 degrees Fahrenheit less hot.”
The study is also confluent with a
recent paper by Davis and Boehnke’s colleague Nicolas Dauphas, which found
evidence for rain falling on continents 2.5 billion years ago, earlier than
previously thought.
原始論文：Patrick
Boehnke, Elizabeth A. Bell, Thomas Stephan, Reto Trappitsch, C. Brenhin Keller,
Olivia S. Pardo, Andrew M. Davis, T. Mark Harrison, Michael J. Pellin. Potassic,
high-silica Hadean crust. Proceedings of the National Academy of
Sciences, 2018; 201720880 DOI: 10.1073/pnas.1720880115
引用自：University of Chicago. "Earth could have
supported continental crust, life earlier than thought."
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