關於地殼如何形成的新理論

原文網址：www.sciencedaily.com/releases/2017/05/170505121013.htm

關於地殼如何形成的新理論

實驗結果能指引科學家尋找有生命居住的地外星球

地球的大陸地殼有90%以上是由富含矽的礦物組成，像是長石和石英。但這些富含矽的礦物是從哪裡來的？尋找其他星球上的生命時它們又能帶給我們什麼線索？

傳統理論認為地球原始地殼的材料皆來自於火山活動。然而，加拿大麥基爾大學的地球科學家Don Baker和Kassandra Sofonio現在卻發表了一項嶄新的理論：這些材料的化學組成，有部分是從當時常處於雲霧繚繞的大氣中，沉降到地表而形成。

首先來點有關古代地球化學的歷史知識：科學家認為45億年前左右，有顆跟火星差不多大的小行星撞上了原始地球。地球因此熔化並成為一座岩漿海，同時也產生了大量岩屑而形成月亮。之後，地表逐漸冷卻成差不多是固體的狀態。Baker的新理論跟傳統理論一樣，都是建立於上述假設。

然而，撞擊過後的大氣會含有高溫蒸氣，足以溶解跟它直接接觸的地表岩石。Baker解釋：「就像砂糖溶解在咖啡一樣」。此為新引入的細微差異。「溶入空氣的礦物會升到大氣上層而冷卻下來，這些在地表溶化的矽酸鹽物質接著會開始從空氣分離出來然後落回地表。我們稱其為矽酸鹽雨。」

為了測試該理論，Baker和共同作者，麥基爾大學的碩士級研究助理Kassandra Sofonio花費數月發展並設計一系列的實驗室試驗，目的是要模仿早期地球充滿蒸氣的大氣。他們先在加熱到1550℃的空氣中，將等同地球矽酸鹽總體組成的物質跟水一起熔化，接著再磨成細粉。然後他們取微量粉末跟水一起封在由金和鈀做成的金屬囊中，將其置入高壓艙，提高溫度壓力至727℃與地表大氣壓力的100倍，藉此來模擬受到形成月球的撞擊大約100萬年之後，地球大氣的環境條件。在每次實驗之後，他們都會快速冷卻樣品，並分析溶化在高溫蒸氣中的物質。

Baker指出這項實驗是依循著先前其他科學家，在高壓之下進行岩石與水之間交互作用的實驗，以及麥基爾大學團隊自身的初步結算結果。即便如此，「我們還是很驚訝由實驗產生，溶解在空氣的矽酸鹽跟從地殼中找到的物質竟然那麼相似。」

他們的成果論文發表在期刊《地球與行星科學通訊》(Earth and Planetary Science Letters)。團隊假設的新理論認為今日我們所知最古老的岩石樣本中，部分是由「空中交代變質作用」(aerial metasomatism)形成。Sofonio創造此詞彙來描述發生在大約一百萬年之間，矽酸鹽礦物凝結並落回地面的過程。

Baker表示：「我們的實驗不但顯示出經由此作用形成的物質的化學成分，還可以告訴科學家重要線索來指出可能有能力安頓生物的地外星球。」

「探討地球早期歷史的這個時刻還是相當令人興奮。」他補充，「許多人認為在我們討論的事件發生不久之後，生命便開始成形。這可以說是地球準備孕育生命的前置階段。」

New theory on how Earth's crust was created

Experimental findings could guide search for exoplanets that may harbor life

More than 90% of Earth's continental crust is made up of silica-rich minerals, such as feldspar and quartz. But where did this silica-enriched material come from? And could it provide a clue in the search for life on other planets?

Conventional theory holds that all of the early Earth's crustal ingredients were formed by volcanic activity. Now, however, McGill University earth scientists Don Baker and Kassandra Sofonio have published a theory with a novel twist: some of the chemical components of this material settled onto Earth's early surface from the steamy atmosphere that prevailed at the time.

First, a bit of ancient geochemical history: Scientists believe that a Mars-sized planetoid plowed into the proto-Earth around 4.5 billion years ago, melting the Earth and turning it into an ocean of magma. In the wake of that impact -- which also created enough debris to form the moon -- the Earth's surface gradually cooled until it was more or less solid. Baker's new theory, like the conventional one, is based on that premise.

The atmosphere following that collision, however, consisted of high-temperature steam that dissolved rocks on the Earth's immediate surface -- "much like how sugar is dissolved in coffee," Baker explains. This is where the new wrinkle comes in. "These dissolved minerals rose to the upper atmosphere and cooled off, and then these silicate materials that were dissolved at the surface would start to separate out and fall back to Earth in what we call a silicate rain."

To test this theory, Baker and co-author Kassandra Sofonio, a McGill undergraduate research assistant, spent months developing a series of laboratory experiments designed to mimic the steamy conditions on early Earth. A mixture of bulk silicate earth materials and water was melted in air at 1,550 degrees Celsius, then ground to a powder. Small amounts of the powder, along with water, were then enclosed in gold palladium capsules, placed in a pressure vessel and heated to about 727 degrees Celsius and 100 times Earth's surface pressure to simulate conditions in the Earth's atmosphere about 1 million years after the moon-forming impact. After each experiment, samples were rapidly quenched and the material that had been dissolved in the high temperature steam analyzed.

The experiments were guided by other scientists' previous experiments on rock-water interactions at high pressures, and by the McGill team's own preliminary calculations, Baker notes. Even so, "we were surprised by the similarity of the dissolved silicate material produced by the experiments" to that found in the Earth's crust.

Their resulting paper, published in the journal Earth and Planetary Science Letters, posits a new theory of "aerial metasomatism" -- a term coined by Sofonio to describe the process by which silica minerals condensed and fell back to earth over about a million years, producing some of the earliest rock specimens known today.

"Our experiment shows the chemistry of this process," and could provide scientists with important clues as to which exoplanets might have the capacity to harbor life Baker says.

"This time in early Earth's history is still really exciting," he adds. "A lot of people think that life started very soon after these events that we're talking about. This is setting up the stages for the Earth being ready to support life."

原始論文：Don R. Baker, Kassandra Sofonio. A metasomatic mechanism for the formation of Earth's earliest evolved crust. Earth and Planetary Science Letters, 2017; 463: 48 DOI: 10.1016/j.epsl.2017.01.022

引用自：McGill University. "New theory on how Earth's crust was created: Experimental findings could guide search for exoplanets that may harbor life." ScienceDaily. ScienceDaily, 5 May 2017.