科學家對地球大陸地殼最初的形成過程有了新見解

原文網址：www.sciencedaily.com/releases/2016/09/160919121941.htm

科學家對地球大陸地殼最初的形成過程有了新見解

科學家精確定出地球現存最古老的岩石單位於42億年前形成，這讓他們能解開地球地殼的最初歷史中一些仍然未解的基本問題。由阿爾伯塔大學主持的發現認為早期地球的表面大多由類似海洋地殼的物質包覆。

「這給了我們關於原始陸地是如何形成的重要資訊，」第一作者Jesse Reimink表示。「由於陸地的形成要追溯到很遠之前，因此我們必須盡量掌握任何一片能找到的證據。而我們並沒有多少資料點可以用來推測當時地球上發生了何種事物。」事實上，全世界只有三個地方擁有年代早於40億年的岩石或礦物，他們分別為來自魁北克北方的岩石、西澳產出的礦物顆粒以及這項研究的探討對象：位於加拿大西北地區的岩層。

雖然已知此地最古老的岩石形成於40億年前，Reimink的岩石上的特殊紋路含有保存良好的鋯石(zircon)顆粒，使科學家可以精準無誤地定出其形成年代。他們調查的樣品是由Reimink的指導博士Tom Chacko在加拿大黃刀鎮(Yellowknife)北邊約300公里的地區進行野外考察時發現。Reimink最近完成他在阿爾伯塔大學的博士學位，現今則於美國華盛頓的卡內基科學研究院進行博士後研究。

「封存於鋯石中的不僅有年代方面的資訊，還有這篇研究中我們運用到的眾多地球化學訊息，」Reimink繼續說道。「我們結合岩石和鋯石能得到的訊息遠比它們個別呈現出來的還要多。鋯石擁有的化學訊號以及紀錄下來的年代資訊並不會被之後的地質事件重置，而岩石本身則能保存鋯石顆粒未能記下的化學訊息。」

他解釋岩石本身的化學性質看起來跟今日於冰島形成的岩石有相似之處。冰島是海洋地殼跟大陸地殼之間的過渡帶。事實上，有些假說認為可以用冰島來類比大陸地殼最初是如何開始形成。

「我們分析岩石本身的諸多訊號來探討岩漿以何種方式入侵至周圍的岩石當中，」其中有道特別訊號記錄到在地殼中曾發生了岩漿同化作用(assimilation)。「當岩漿冷卻的同時會加熱並且熔化周圍岩石，而我們有證據顯示曾經發生這類事件。」

Reimink表示這塊岩石缺乏大陸地殼應有的訊號，跟預期中早期陸塊會呈現的樣貌相異，造成它帶來的問題比回答的還要更多。Reimink說身為一名地質學家的最大挑戰，便是當我們越深入追溯地球的過往，我們能獲得的證據量與品質就下降的越多。「地球一直以來都在持續地進行自身物質的循環，隨著地殼被扭曲變形或者熔化，過往的歷史也就從此化為烏有。」 Reimink如此強調。

「陸地矗立於水面之上而暴露在空氣當中，對地球大氣的化學性質以及生命的出現與否都具有重大意涵。地球的陸塊大小對地球深部(地核和地函)以及表面(大氣層與生物圈)發生的化學作用會有相當影響。在化學作用以及地質過程之間有持續發生的循環回饋作用。雖然整個圖像仍然有許多尚未知曉的部分，這項研究又再一次傳達出地球所有事物都是環環相扣的。」此研究刊登於十月號的《自然—地質科學》(Nature Geoscience)，題名為「No evidence for Hadean continental crust within Earth's oldest evolved rock unit」。

引用自：University of Alberta. "Scientists uncover insights into the formation of Earth’s oldest continental crust." ScienceDaily. ScienceDaily, 19 September 2016. 

原始論文J. R. Reimink, J. H. F. L. Davies, T. Chacko, R. A. Stern, L. M. Heaman, C. Sarkar, U. Schaltegger, R. A. Creaser, D. G. Pearson. No evidence for Hadean continental crust within Earth’s oldest evolved rock unit. Nature Geoscience, 2016; DOI: 10.1038/ngeo2786

Scientists uncover insights into the formation of Earth’s oldest continental crust

Addressing fundamental unknowns about the earliest history of Earth's crust, scientists have precisely dated the world's oldest rock unit at 4.02 billion years old. Driven by the University of Alberta, the findings suggest that early Earth was largely covered with an oceanic crust-like surface.

"It gives us important information about how the early continents formed," says lead author Jesse Reimink. "Because it's so far back in time, we have to grasp at every piece of evidence we can. We have very few data points with which to evaluate what was happening on Earth at this time." In fact, only three locations worldwide exist with rocks or minerals older than 4 billion years old: one from Northern Quebec, mineral grains from Western Australia, and the rock formation from Canada's Northwest Territories examined in this new study.

While it is well known that the oldest rocks formed prior to 4 billion years ago, the unique twist on Reimink's rock is the presence of well-preserved grains of the mineral zircon, leaving no doubt about the date it formed. The sample in question was found during fieldwork by Reimink's PhD supervisor, Tom Chacko, in an area roughly 300 kilometres north of Yellowknife. Reimink recently completed his PhD at the University of Alberta before starting a post-doctoral fellowship at the Carnegie Institute for Science in Washington, D.C.

"Zircons lock in not only the age but also other geochemical information that we've exploited in this paper," Reimink continues. "Rocks and zircon together give us much more information than either on their own. Zircon retains its chemical signature and records age information that doesn't get reset by later geological events, while the rock itself records chemical information that the zircon grains don't."

He explains that the chemistry of the rock itself looks like rocks that are forming today in modern Iceland, which is transitional between oceanic and continental crust. In fact, Iceland has been hypothesized as an analog for how continental crusts started to form.

"We examined the rock itself to analyze those signatures to explore the way that the magma intrudes into the surrounding rock." One signature in particular recorded the assimilation step of magma from Earth's crust. "While the magma cooled, it simultaneously heated up and melted the rock around it, and we have evidence for that."

Reimink says that the lack of signatures of continental crust in this rock, different from what the early continents were expected to look like, leads to more questions than answers. Reimink says one of the biggest challenges as a geologist is that as we travel back in time on Earth, the quantity and quality of available evidence decreases. "Earth is constantly recycling itself, the crust is being deformed or melted, and pre-history is being erased," remarks Reimink.

"The presence of continents above water and exposed to the atmosphere has huge implications in atmospheric chemistry and the presence or absence of life. The amount of continents on Earth has a large chemical influence both on processes in the deep Earth (mantle and core) and at the Earth's surface (atmosphere and biosphere). There are constant feedback loops between chemistry and geology. Though there are still a lot of unknowns, this is just one example that everything on Earth is intertwined." "No evidence for Hadean continental crust within Earth's oldest evolved rock unit" appears in the October issue of Nature Geoscience.