New study zeros in on plate
tectonics' start date
新研究聚焦於板塊構造運動的起源之時
Analysis of trace elements places the onset of plate
tectonics about 3 billion years ago
分析稀有元素的結果將板塊構造運動的起始時間定在約莫30億年前
Earth has
some special features that set it apart from its close cousins in the solar
system, including large oceans of liquid water and a rich atmosphere with just
the right ingredients to support life as we know it. Earth is also the only
planet that has an active outer layer made of large tectonic plates that grind
together and dip beneath each other, giving rise to mountains, volcanoes,
earthquakes and large continents of land.
地球擁有某些特色使得她跟其他太陽系內關係最近的親戚們截然不同，眾所皆知這包括了由液態水組成的廣袤海洋，以及成分恰到好處而能供養生命的豐厚大氣層。地球也是唯一擁有由大型板塊組成活躍外層的行星，這些板塊彼此之間的摩擦和傾沒，形成了山脈、火山、地震和巨大的陸塊。
Geologists have long debated when these processes, collectively known as plate tectonics, first got underway. Some scientists propose that the process began as early as 4.5 billion years ago, shortly after Earth's formation. Others suggest a much more recent start within the last 800 million years. A study from the University of Maryland provides new geochemical evidence for a middle ground between these two extremes: An analysis of trace element ratios that correlate to magnesium content suggests that plate tectonics began about 3 billion years ago. The results appear in the January 22, 2016 issue of the journal Science.
地質學家長期以來爭論著這些合稱為板塊構造的作用於何時開始。有些科學家提出45億年前地球形成不久之後便有這些作用了。其他人則提倡較為近代的開始時間，約莫小於8億年前。馬里蘭大學的研究則以新的地球化學證據而在這兩端之間取了較為中庸的說法：分析與鎂含量相關的稀有元素比例，結果顯示板塊構造運動約起始於30億年前。這項結果刊登在2016年1月22日發行的期刊《科學》之中。
"By linking crustal composition and plate tectonics, we have provided first-order geochemical evidence for the onset of plate tectonics, which is a fundamental Earth science question," said Ming Tang, a graduate student in geology at UMD and lead author of the study. "Because plate tectonics is necessary for the building of continents, this work also represents a further step in understanding when and how Earth's continents formed."
「藉由連接地殼成分和板塊構造運動之間的關係，我們提供了地球化學方面的初步證據可以指引板塊運動啟動的時刻。這在地球科學之中可是個相當重要的問題。」這篇研究的第一作者，馬里蘭大學的研究生Ming Tang說。「由於板塊構造運動是陸地形成過程中不可或缺的要素，因此這件工作同樣能讓我們更進一步了解地球的陸地是在何時及如何形成。」
The study zeros in on one key characteristic of Earth's crust that sets it apart geochemically from other terrestrial planets in the solar system. Compared with Mars, Mercury, Venus and even our own moon, Earth's continental crust contains less magnesium. Early in its history, however, Earth's crust more closely resembled its cousins, with a higher proportion of magnesium.
此篇研究聚焦於地球地殼的某一項重要特性，這項特性使得地球跟其他太陽系的類地行星在地球化學特性上有所區隔。與火星、水星、金星甚至是和我們的月亮相比，地球的大陸地殼含有較少的鎂。然而，在地球歷史的早期，地球地殼和他的表親們較現今更為相似，有著比較高比例的鎂。
At some point, Earth's crust evolved to contain more granite, a magnesium-poor rock that forms the basis of Earth's continents. Many geoscientists agree that the start of plate tectonics drove this transition by dragging water underneath the crust, which is a necessary step to make granite.
於某個時間點，地球的地殼演變成含有較多的花崗岩，這種岩石是構成地球陸地的基礎，其鎂含量相當少。許多地質學家同意板塊構造運動發生時連帶將海水拖入地殼下方，這道形成花崗岩的關鍵步驟使得地殼成分發生轉變。
"You can't have continents without granite, and you can't have granite without taking water deep into the Earth," said Roberta Rudnick, former chair of the Department of Geology at UMD and senior author on the study. Rudnick, who is now a professor of earth sciences at the University of California, Santa Barbara, conducted this research while at UMD. "So at some point plate tectonics began and started bringing lots of water down into the mantle. The big question is when did that happen?"
「沒有花崗岩就沒有陸地，而沒有將水分帶入地球深處就不會形成花崗岩。」此研究的第二作者Roberta Rudnick說。他於馬里蘭大學地質科學系就任系主任時指導了這篇研究，現今則在加州大學聖芭芭拉分校地球科學系擔任教授一職。「因此板塊構造運動在某個時刻開始運作，並攜帶大量海水到下方的地函。而最重要的問題是：這究竟是在什麼時候發生的？」
A logical approach would be to look at the magnesium content in ancient rocks formed across a wide span of time, to determine when this transition toward low-magnesium crustal rocks began. However, this has proven difficult because the direct evidence--magnesium--has a pesky habit of washing away into the ocean once rocks are exposed to the surface.
一種合理的作法是分別檢驗過往一大段時期中不同年代岩石的鎂含量，來確立地殼岩石的成分何時開始轉變成鎂含量較低的狀態。然而，這個做法已被證明出來相當困難，因為達成這個目標所需的直接證據，也就是鎂，有個十分麻煩的特性：一旦岩石露出地表，其中的鎂很容易就會被水沖到海裡。
Tang, Rudnick and Kang Chen, a graduate student at China University of Geosciences on a one and a half-year research visit to UMD, sidestepped this problem by looking at trace elements that are not soluble in water. These elements--nickel, cobalt, chromium and zinc--stay behind long after most of the magnesium has washed away. The researchers found that the ratios of these elements hold the key: higher ratios of nickel to cobalt and chromium to zinc both correlate to higher magnesium content in the original rock.
Tang、Rudnick和Kang Chen(於馬里蘭大學進行為期一年半學術交流的中國地質大學研究生)探討不溶於水的稀有元素而迴避了上述問題。這些元素，包含鎳、鈷、鉻和鋅，在大多數的鎂被沖刷殆盡後仍能留存在岩石中很長一段時間。研究人員發現這些元素彼此之間的比例就是關鍵所在：當岩石的鎳鈷比和鉻鋅比較高時，意味著岩石的最初鎂含量也較高。
"To our knowledge, we are the first to discover this correlation and use this approach," Tang said. "Because the ratios of these trace elements correlate to magnesium, they serve as a very reliable 'fingerprint' of past magnesium content."
「就我們所知，我們是第一個發現這種相關性並將之利用的團隊。」Tang說。「由於這些稀有元素之間的比例跟鎂的含量有關，因此可將其視作過去鎂含量留下的清晰「指紋」。」
Tang and his coauthors compiled trace element data taken from a variety of ancient rocks that formed in the Archean eon, a time period between 4 and 2.5 billion years ago, and used it to determine the magnesium content in the rocks when they were first formed. They used these data to construct a computer model of the early Earth's geochemical composition. This model accounted for how magnesium (specifically, magnesium oxide) content in the crust changed over time.
Tang和他的同僚彙整了於太古宙(40億年前至25億年前)形成的各類岩石中的稀有元素含量，並利用這些資料來判定這些岩石形成當時的鎂含量。接著再用這些資料建構出早期地球化學成分的電腦模型。這個模型可以呈現地殼中鎂的含量(特別是氧化鎂)如何隨著時間變化。
The results suggest that 3 billion years ago, the Earth's crust had roughly 11 percent magnesium oxide by weight. Within a half billion years, that number had dropped to about 4 percent, which is very close to the 2 or 3 percent magnesium oxide seen in today's crust. This suggested that plate tectonics began about 3 billion years ago, giving rise to the continents we see today.
結果顯示30億年前，地球地殼的總重中約有百分之11是氧化鎂。但在短短5億年之內，這個數值便驟降到百分之4左右，跟現今地殼中氧化鎂只占百分之2到3的情況十分類似。這顯示板塊構造運動大概在30億年前開始，並形成了今日我們所見的陸地。
"It's really kind of a radical idea, to suggest that continental crust in Archean had that much magnesium," said Rudnick, pointing out that Tang was the first to work out the correlation between trace element ratios and magnesium. "Ming's discovery is powerful because he found that trace insoluble elements correlate with a major element, allowing us to address a long-standing question in Earth history."
「聲稱太古宙的大陸地殼有更多的鎂其實是種相當激進的說法。」Rudnick說。他並且指出Tang是第一個發現稀有元素比例和鎂含量之間有關係的人。「Ming的發現強而有力之處在於他找到了非可溶性的稀有元素跟主要元素之間有所關連，而讓我們可以解答地球歷史中懸宕已久的未解之謎。」
"Because the evolution of continental crust is linked to many major geological processes on Earth, this work may provide a basis for a variety of future studies of Earth history," Tang said. "For example, weathering of this magnesium-rich crust may have affected the chemistry of the ancient ocean, where life on Earth evolved. As for the onset of plate tectonics, I don't think this study will close the argument, but it certainly adds a compelling new dimension to the discussion."
「因為地球大陸地殼的演變跟許多重大地球化學作用都有牽扯，這項成果或許也能做為未來諸多地球歷史相關研究的基礎。」Tang說。「舉例來說，古代海洋是地球生命演化而成之處，而它的化學性質可能會被富含鎂的地殼受到的風化作用影響。至於板塊構造運動的起源問題，我不認為這起研究會終結有關這方面的爭論，但肯定會在這場激辯中佔有新的一席之地。」
引用自：University of Maryland. "New study zeros in on plate tectonics' start date: Analysis of trace elements places the onset of plate tectonics about 3 billion years ago." ScienceDaily. ScienceDaily, 21 January 2016.
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