年輕的火山岩中有遠古地球留下的記號

原文網址：www.sciencedaily.com/releases/2017/04/170406143910.htm

年輕的火山岩中有遠古地球留下的記號

地球化學傳遞出的線索顯示夏威夷和薩摩亞的某些岩石，可能具有地球甫誕生時就形成的物質。

組成地球地函的岩石雖然堅硬但也會在數百萬年之間緩緩流動。某些地質學家推測這種緩慢循環會完全抹滅產生自地球歷史早期的地球化學痕跡。但馬里蘭大學的地質學家領導的新研究中，發現的新證據卻可以追溯至超過45億年前。

4月7日刊登於期刊《科學》(Science)的研究論文中，作者探討了近代由夏威夷和薩摩亞的火山所噴發的火山岩。這些岩石中含有的地球化學異常訊號令人十分訝異――此指紋存在的條件存在於地球剛形成時。

研究人員仍不確定地球地函是如何保存這些異常訊號。然而，此團隊的研究確實顯示出某些岩石含有的物質可能活過了整部地球歷史，而地球內部可能根本沒有混和得十分均勻。

「我們發現了必定得在將近45億年前才能產生的地球化學訊號。」此篇研究的主要作者，馬里蘭大學地質學系的博士後研究員Andrea Mundl表示。「令人特別興奮的是在如此年輕的岩石中找到這些異常訊號。雖然我們仍不明白它們是如何度過這麼漫長的時間，但我們已經有了一些想法。」

此異常訊號發現於兩種元素――鎢和氦的重要同位素之間的比例。

對擁有許多同位素的鎢來說，重要的是鎢-182跟鎢-184之間的比例。比較重的同位素鎢-184處於穩定狀態且從地球最初形成時就存在至今。另一方面，鎢-182則是從非常不穩定的鉿-182衰變而來。所有自然生成的鉿-182都在地球歷史的最初5000萬年內衰變殆盡，而在原處留下鎢-182。

在地球最初5000萬年期間，鉿和鎢的行為差異十分地大。鎢傾向跟金屬待在一起，所以它們大多數都會往地核遷移；另一方面，鉿則傾向跟矽酸鹽礦物待在一起，因此它們會留在地函和地殼當中。地球大多數岩石中，鎢-182跟鎢-184的比例都非常相似，所以此數字也被當作全球基準。地質學家可以從鎢-182含量特別高或低的岩石中得知許多事情，因為鎢-182代表了許久之前岩石中有多少鉿-182。

「這種異常值幾乎都是在太陽系形成後的最初5000萬年間產生。」Mundl表示，「高於正常值的鎢-182最有可能在許久以前具有大量鉿的岩石中看到。但是低含量的鎢-182卻很稀有，我們預期大概要在距地表相當深處，由金屬組成的地核附近或內部才能看到類似情況。」

十分肯定的是，Mundl和她的同事在夏威夷和薩摩亞的某些岩石中觀察到鎢-182的含量異常地低。單就鎢同位素來說，雖然這種比例十分令人感興趣，但卻不足以由此得到任何具備說服力的結論。然而，研究人員卻觀察到相同岩石中，含有的氦同位素比例也十分不尋常。

地球的氦-3十分稀有，它們常出現的岩石樣品為自地球最初形成後就再也沒有熔化或者是循環的岩石。另一方面，氦-4則能從鈾和釷的衰變過程中產生。岩石的氦-3對氦-4的比例較平常還高一般意味著自地球形成後，它就沒有受到重大改變且十分古老。

「科學家長久以來就已經知道氦同位素的變化，但卻沒有人把它跟其他地球化學參數互相比對。」此篇研究的共同作者，馬里蘭大學地質科學系主任Richard Walker教授表示，「氦-3對氦-4比例高的岩石通常被推測為具有『原始』地函物質，但究竟有多原始卻不得而知。我們的鎢元素數據顯示它們的確十分原始，其源區極有可能是在太陽系歷史的最初5000萬年內形成。」

Mundl、Walker和其他共同作者提出一些不同情境來解釋從夏威夷和薩摩亞的火山岩中，觀察到的鎢和氦異常是如何產生。一種情況是這些火山可能會抽取地核的物質。科學家預期地核的同位素比例傾向具有較少的鎢-182和較多的氦-3。

另外一種情境則是，地球的岩石外層可能是在廣袤的岩漿海中一塊一塊地形成。岩漿海中有些部分可能結晶後沉到地核和地函的交界處，因此保存了許久以前的鎢和氦訊號。

「兩種情境都有一些我們還無法解釋的矛盾之處。」Mundl表示，「但這篇研究的結果絕對是相當地振奮人心，足以催生許多有趣的新問題來研究。」

Ancient Earth's fingerprints in young volcanic rocks

Geochemical clues suggest that some rocks from Hawaii, Samoa may contain material formed during the planet's birth

Earth's mantle is made of solid rock that nonetheless circulates slowly over millions of years. Some geologists assume that this slow circulation would have wiped away any geochemical traces of Earth's early history long ago. But a new study led by University of Maryland geologists has found new evidence that could date back more than 4.5 billion years.

The authors of the research paper, published April 7 in the journal Science, studied volcanic rocks that recently erupted from volcanoes in Hawaii and Samoa. The rocks contain surprising geochemical anomalies -- the "fingerprints" of conditions that existed shortly after the planet formed.

The researchers are not yet sure how Earth's mantle preserved these anomalies. But the group's results suggest that some of these rocks contain material that survived through all of Earth's history -- and that the planet's interior may not be well mixed after all.

"We found geochemical signatures that must have been created nearly 4.5 billion years ago," said Andrea Mundl, a postdoctoral researcher in geology at UMD and the lead author of the study. "It was especially exciting to find these anomalies in such young rocks. We don't yet know how these signatures survived for so long, but we have some ideas."

The anomalous signatures are found in the ratios of key isotopes of two elements: tungsten and helium.

In the case of tungsten, which has many isotopes, the important ratio is tungsten-182 to tungsten-184. The heavier isotope, tungsten-184, is stable and has existed since the planet first formed. Tungsten-182, on the other hand, results from the decay of hafnium-182, which is highly unstable. All naturally occurring hafnium-182 decayed within the first 50 million years of Earth's history, leaving tungsten-182 in its place.

Tungsten and hafnium behaved very differently during the planet's first 50 million years. Tungsten tends to associate with metals, so most of it migrated to Earth's core, while hafnium, which tends to associate with silicate minerals, stayed in Earth's mantle and crust. Most of the rocks on Earth have a similar ratio of tungsten-182 to tungsten-184, and this ratio serves as a global baseline. Geologists can learn a lot from rocks with an unusually high or low amount of tungsten-182 -- which indicates how much hafnium-182 was present in the rock long ago.

"Nearly all of these anomalies formed within the first 50 million years after the solar system formed," Mundl said. "Higher than normal levels of tungsten-182 are seen in very old rocks that most likely contained a lot of hafnium long ago. But lower levels of tungsten-182 are rare, and resemble what we might expect to see deep beneath the surface, in or near the planet's metallic core."

Sure enough, Mundl and her colleagues observed an unusually low amount of tungsten-182 in some of the rocks from Hawaii and Samoa. On its own, the tungsten isotope ratio is interesting, but not enough to make any convincing conclusions. But the researchers also observed that the same rocks contain an unusual ratio of helium isotopes.

Helium-3 is extremely rare on Earth, and tends to show up in samples of rock that have not been melted or otherwise recycled since the planet first formed. Helium-4, on the other hand, can form from the radioactive decay of uranium and thorium. A higher than normal ratio of helium-3 to helium-4 typically indicates very old rocks that have not been significantly altered since the planet formed.

"Variations in the isotopic composition of helium have been long known, but have never been correlated with other geochemical parameters," said Richard Walker, professor and department chair of geology at UMD and a co-author of the paper. "Rocks with high helium-3 to helium-4 ratios have commonly been speculated to contain 'primitive' mantle material, but how primitive was not known. Our tungsten data show that it is very primitive indeed, with the source region most likely forming within the first 50 million years of solar system history."

Mundl, Walker and their co-authors suggest a few different scenarios that could have produced the tungsten and helium anomalies they observed in volcanic rocks from Hawaii and Samoa. Perhaps the volcanoes are drawing material from Earth's core, where the ratios are expected to favor low tungsten-182 and high helium-3.

Alternatively, the rocky outer surface of Earth might have formed in patches, with vast magma oceans in between. Parts of these magma oceans may have crystallized and sunk to the boundary between the mantle and the core, preserving the ancient tungsten and helium signatures.

"Each of these scenarios contain some inconsistencies that we can't yet explain," Mundl said. "But this is an exciting result that is sure to generate lots of interesting new research questions."

原始論文：Andrea Mundl, Mathieu Touboul, Matthew G. Jackson, James M. D. Day, Mark D. Kurz, Vedran Lekic, Rosalind T. Helz, Richard J. Walker. Tungsten-182 heterogeneity in modern ocean island basalts. Science, 2017; 356 (6333): 66 DOI: 10.1126/science.aal4179

引用自：University of Maryland. "Ancient Earth's fingerprints in young volcanic rocks: Geochemical clues suggest that some rocks from Hawaii, Samoa may contain material formed during the planet's birth." ScienceDaily. ScienceDaily, 6 April 2017.