從澳洲西部傑克丘找到的古老鋯石，精準定出讓地球適合生命居住的重要事件發生在什麼時候
史密森尼美國國立自然史博物館的地質學家Michael Ackerson領導的科學團隊提出的新證據，顯示目前的板塊運動類型――這項讓地球能養育生命的關鍵特徵大約是出現在36億年前。
鋯石顆粒。圖片來源：Michael Ackerson，史密森尼基金會
地球是人類所知唯一有複雜生命的星球，這份能力部分源自於另一項讓地球獨一無二的特徵：板塊運動。科學界目前已知的其他星體都沒有和地球一樣活躍的地殼，它們分成許多陸塊，從遠古以來便持續移動、碎裂並且互相碰撞。板塊運動提供了地球內部與表面的化學反應互相連結的管道，產生了像是大氣中的氧氣以及可以調節氣候的二氧化碳，這些物質打造出讓今日的人們得以安居樂業的地球。但是板塊運動的開始時間和過程仍是一道未解之謎，塵封在數十億年的地質時間當中。
5月14發表在期刊⟪地質化學前瞻通訊⟫(Geochemical Perspective Letters)的文章中，作者利用在地球發現到的最老礦物――鋯石――來追溯地球的遠古時光。
這項研究中最古老的鋯石取自於澳洲西部的傑克丘，年代大約為43億年，代表這些幾乎堅不可摧的礦物在地球本身還是嬰兒時(大約為2億歲)就已經形成了。從傑克丘採集到的其他遠古鋯石年代一直延續到30億年前，涵蓋了地球最初的歷史。這些礦物聯合起來組成了地球剛出生時的化學紀錄，是研究人員迄今擁有的同類紀錄中最接近連續的。
「我們正在重建地球從一顆由融化的岩石與金屬組成的球體轉變成現今這樣的過程，」Ackerson 表示。「別的星球都沒有陸地、液體海洋或者生命。某種層面上來說，我們也在試著解開為什麼地球是如此特別的原因，而這些鋯石可以讓我們得到一定程度的答案。」
為了回溯地球數十億年前的時光，Ackerson 和研究團隊從傑克丘採集了15塊葡萄柚大小的岩石。接著他們利用稱為「花栗鼠」的機器把岩石磨成沙粒，目的是得到岩石最小的組成單元――礦物。幸運的是，由於鋯石的密度很高，因此運用類似掏金的方法就能把它們和其他的沙子分離。
團隊試驗了超過3500顆鋯石，每一個的寬度大概都只有人類的頭髮兩倍粗。他們用雷射燃燒這些鋯石之後再用質譜儀測量它們的化學成分。這些實驗可以顯示每一顆鋯石的年代以及其中的化學成分。由於這些鋯石在數十億年前形成之後便飽受摧殘，因此在數千個樣本當中，大概只有200個符合研究的標準。
「要解開封鎖在這些礦物中的秘密並不容易，」Ackerson 表示。「雖然我們得分析數千顆礦物來得到一些資料點，但是每一個樣品都有可能揭發全新的事物，使我們對地球的起源有所改觀。」
科學家能以相當高的精確度測出鋯石的年代，這是因為每顆鋯石裡面都含有鈾。鈾是一種廣為人知的天然放射性元素，由於鈾的衰變速率已經被定量得非常準確，因此科學家可以藉此反推這些礦物已經形成多久。
研究團隊對於鋯石中的鋁含量也很感興趣。現代鋯石的試驗結果顯示鋁含量高的鋯石只能經由某些方式製造出來，因此研究人員便能透過鋁的多寡來推測鋯石形成的當下(從地質上來說)發生了什麼作用。
Ackerson 和共同作者試驗了數千顆鋯石，從中挑選出幾百個有用的加以分析之後，他們辨認出大概在36億年前鋁的濃度有顯著的增加。
「成分改變可能標示了現代板塊運動類型的起源，或許也能視作地球生命出現的訊號，」Ackerson 表示。「但是我們還需要進行非常多的研究來判定這種地質變化與生命起源之間的關聯。」
鋁含量高的鋯石與活躍的地殼開始出現之間有因果關係的立論如下：鋁含量高的鋯石只能透過少數方式產生，其中之一便是地球更深部的岩石發生融化。
「由於化學鍵的關係，要讓鋁進到鋯石裡面非常困難，」Ackerson 表示。「這需要相當極端的地質條件才能發生。」
Ackerson 將更深處的岩石熔融的跡象，解讀成當時的地殼開始變厚並逐漸冷卻，而地殼變厚則意味著地球的板塊運動正在轉變成現今的類型。
先前對於加拿大北部年代為40億年的阿卡斯塔片麻岩進行的研究，也顯示地殼當時正在變厚，使得地球更深處的岩石發生融化。
「阿卡斯塔片麻岩的結果讓我們對於傑克丘鋯石的解讀更有信心，」Ackerson表示。「雖然這兩個地方目前相隔了數千英里，但它們訴說的故事卻相當一致，也就是36億年前左右全球發生了某個重大事件。」
這項成果是自然史博物館新的研究方針「獨一無二的地球」的一部分。這項公私合作的計畫資助的研究探討了是什麼讓地球如此特別的問題中，最重大且由來已久的那些問題。未來其他研究還會探討地球液體海洋的起源，以及礦物如何幫助點燃生命的火花。
Ackerson 表示以這項成果為基礎，接下來他想繼續研究傑克丘的古老鋯石是否有生命留下來的跡象，以及其他極為古老的岩層中，是否也有證據顯示地殼開始變厚的時間是在36億年前左右。
本研究的經費與贊助來自史密森尼基金會與美國國家航空暨太空總署。
Earth’s oldest minerals date onset of plate tectonics to 3.6 billion years ago
Ancient zircons from the Jack Hills of Western Australia hone date of an event that was crucial to making the planet hospitable to life
Scientists led by Michael Ackerson, a research geologist at the Smithsonian’s National Museum of Natural History, provide new evidence that modern plate tectonics, a defining feature of Earth and its unique ability to support life, emerged roughly 3.6 billion years ago.
Earth is the only planet known to host complex life and that ability is partly predicated on another feature that makes the planet unique: plate tectonics. No other planetary bodies known to science have Earth’s dynamic crust, which is split into continental plates that move, fracture and collide with each other over eons. Plate tectonics afford a connection between the chemical reactor of Earth’s interior and its surface that has engineered the habitable planet people enjoy today, from the oxygen in the atmosphere to the concentrations of climate-regulating carbon dioxide. But when and how plate tectonics got started has remained mysterious, buried beneath billions of years of geologic time.
The study, published May 14 in the journal Geochemical Perspective Letters, uses zircons, the oldest minerals ever found on Earth, to peer back into the planet’s ancient past.
The oldest of the zircons in the study, which came from the Jack Hills of Western Australia, were around 4.3 billion years old—which means these nearly indestructible minerals formed when the Earth itself was in its infancy, only roughly 200 million years old. Along with other ancient zircons collected from the Jack Hills spanning Earth’s earliest history up to 3 billion years ago, these minerals provide the closest thing researchers have to a continuous chemical record of the nascent world.
“We are reconstructing how the Earth changed from a molten ball of rock and metal to what we have today,” Ackerson said. “None of the other planets have continents or liquid oceans or life. In a way, we are trying to answer the question of why Earth is unique, and we can answer that to an extent with these zircons.”
To look billions of years into Earth’s past, Ackerson and the research team collected 15 grapefruit-sized rocks from the Jack Hills and reduced them into their smallest constituent parts—minerals—by grinding them into sand with a machine called a chipmunk. Fortunately, zircons are very dense, which makes them relatively easy to separate from the rest of the sand using a technique similar to gold panning.
The team tested more than 3,500 zircons, each just a couple of human hairs wide, by blasting them with a laser and then measuring their chemical composition with a mass spectrometer. These tests revealed the age and underlying chemistry of each zircon. Of the thousands tested, about 200 were fit for study due to the ravages of the billions of years these minerals endured since their creation.
“Unlocking the secrets held within these minerals is no easy task,” Ackerson said. “We analyzed thousands of these crystals to come up with a handful of useful data points, but each sample has the potential to tell us something completely new and reshape how we understand the origins of our planet.”
A zircon’s age can be determined with a high degree of precision because each one contains uranium. Uranium’s famously radioactive nature and well-quantified rate of decay allow scientists to reverse engineer how long the mineral has existed.
The aluminum content of each zircon was also of interest to the research team. Tests on modern zircons show that high-aluminum zircons can only be produced in a limited number of ways, which allows researchers to use the presence of aluminum to infer what may have been going on, geologically speaking, at the time the zircon formed.
After analyzing the results of the hundreds of useful zircons from among the thousands tested, Ackerson and his co-authors deciphered a marked increase in aluminum concentrations roughly 3.6 billion years ago.
“This compositional shift likely marks the onset of modern-style plate tectonics and potentially could signal the emergence of life on Earth,” Ackerson said. “But we will need to do a lot more research to determine this geologic shift’s connections to the origins of life.”
The line of inference that links high-aluminum zircons to the onset of a dynamic crust with plate tectonics goes like this: one of the few ways for high-aluminum zircons to form is by melting rocks deeper beneath Earth’s surface.
“It’s really hard to get aluminum into zircons because of their chemical bonds,” Ackerson said. “You need to have pretty extreme geologic conditions.”
Ackerson reasons that this sign that rocks were being melted deeper beneath Earth’s surface meant the planet’s crust was getting thicker and beginning to cool, and that this thickening of Earth’s crust was a sign that the transition to modern plate tectonics was underway.
Prior research on the 4 billion-year-old Acasta Gneiss in northern Canada also suggests that Earth’s crust was thickening and causing rock to melt deeper within the planet.
“The results from the Acasta Gneiss give us more confidence in our interpretation of the Jack Hills zircons,” Ackerson said. “Today these locations are separated by thousands of miles, but they’re telling us a pretty consistent story, which is that around 3.6 billion years ago something globally significant was happening.”
This work is part of the museum’s new initiative called Our Unique Planet, a public–private partnership, which supports research into some of the most enduring and significant questions about what makes Earth special. Other research will investigate the source of Earth’s liquid oceans and how minerals may have helped spark life.
Ackerson said he hopes to follow up these results by searching the ancient Jack Hills zircons for traces of life and by looking at other supremely old rock formations to see if they too show signs of Earth’s crust thickening around 3.6 billion years ago.
Funding and support for this research were provided by the Smithsonian and the National Aeronautics and Space Administration (NASA).
原始論文：M.R. Ackerson, D. Trail, J. Buettner. Emergence of peraluminous crustal magmas and implications for the early Earth. Geochemical Perspectives Letters, 2021; 17: 50 DOI: 10.7185/geochemlet.2114
引用自：Smithsonian. "Earth's oldest minerals date onset of plate tectonics to 3.6 billion years ago
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