指出地函的碳來源為何的關鍵證物

原始網址：www.sciencedaily.com/releases/2016/11/161109132623.htm

指出地函的碳來源為何的關鍵證物

科學找到了關鍵證物可以指出從地函中發現的碳是否源自大陸地殼。這讓我們對地球表面地殼形成的歷史，以及不同地質年代的板塊移動速率有更深入的瞭解，進而探討地函的冷卻速率。

刊登在期刊《自然-地質科學》(Nature Geoscience)的新研究結果顯示在稱為碳酸岩(carbonatite)的火成岩當中，不同年代的岩石其硼同位素比例也不同。這項研究由聖母大學地球科學系和土木與環境工程學系的副教授Antonio Simonetti領導。

目前有三種理論來解釋從地函中發現的碳來源為何：其一認為這些碳是45.6億年前地球形成時便已存在的原始(primordial)成分；另一則認為它們是星體碰撞地球的產物；最後則是在板塊移動過程中，當一塊板塊下沉至另一塊之下而形成隱沒帶時，海洋或大陸板塊上的碳會於此循環回地函。

「我們最重要的發現是看到硼同位素的比例變化非常地大，代表在地函中，碳的來源會隨著地質時間經過而跟著改變。」Simonetti說。研究碳酸岩中的硼同位素比例，研究人員得以更精細地決定在地質歷史上的特定時刻應該適用哪一項理論。

「隱沒速率在過去45.6億年間不斷變化著。」 Simonetti說。「原先，在地球形成最初的20億年左右，地函溫度比今日高上許多。因此當隱沒作用開始發生時，由於地函溫度較高，造成下沉的板塊跟今日發生的隱沒作用相比並無法穿透至地球深處。在最近這20億年左右，地函變得較冷因此能讓隱沒板塊下沉至地函深部，並讓地殼循環進來的物質有機會儲存在地球深處，甚至有可能一路深達核幔邊界。」

這項探討地球歷史重要時刻產生的碳酸岩中，硼同位素組成性質的初步結果，讓Simonetti和他的團隊得以進一步去觀察這個性質的長期變化。這使得他們有機會對地殼形成如何隨時間變化有更清晰的瞭解，甚至可以追溯至數十億年以前的時光。

研究共同作者包括聖母大學土木、環境工程及地質學系的Samuel R.W. Hulett，石溪大學的E. Troy Rasbury，以及紐約市立大學皇后學院的N. Gary Hemming。

Key indicator of carbon sources in Earth's mantle

Scientists have found a key indicator in determining whether the presence of carbon, found in the Earth's mantle, is derived from continental crust -- a step toward better understanding the history of crustal formation on Earth's surface and the rate at which tectonic plates have moved throughout geologic time, which can be linked to the cooling of Earth's mantle.

Results of a new study published in the journal Nature Geoscience show evidence of varying ratios of boron isotopes in igneous rocks, known as carbonatites, of different ages. The research was led by Antonio Simonetti, associate professor in the Department of Civil and Environmental Engineering and Earth Sciences at the University of Notre Dame.

Three theories exist regarding the source of carbon found within the Earth's mantle: It is of primordial origin, formed during the creation of the planet 4.56 billion years ago; it is a result of planetary collision; or it had been present in marine environments or continental crust, and recycled back into the mantle in areas of subduction, where tectonic plates shifted, one diving beneath the other.

"Our most important finding is that the Boron isotope ratios are highly variable, indicating that the source of carbon within the mantle changed with geological time on Earth," Simonetti said. Studying the ratios of boron isotopes within carbonatites, researchers are closer to determining which hypothesis applies to specific moments in geological time.

"During the past 4.56 billion years, the subduction rate has varied," said Simonetti. "Early on, during the first 2 billion years or so, Earth's mantle was much hotter than it is today, so when subduction did occur, the diving plate did not penetrate as deep into the mantle as it does today because of the higher temperature. During the last 2 billion years or so, a cooler mantle has allowed the subducting plate to dive deeper into the mantle and provide the opportunity to store recycled crustal materials at greater depths, and possibly all the way down to the core-mantle boundary."

This preliminary investigation into the boron isotope compositions of carbonatites from significant periods in Earth's history allows Simonetti and his team to monitor long-term temporal variations -- creating a clearer picture of crustal formation over time, with the potential to go as far back as several billion years.

The study was co-authored by Samuel R.W. Hulett in the Department of Civil and Environmental Engineering and Earth Sciences at Notre Dame, E. Troy Rasbury of Stony Brook University N. Gary Hemming of Queens College -- CUNY.

原始論文：Samuel R. W. Hulett, Antonio Simonetti, E. Troy Rasbury, N. Gary Hemming. Recycling of subducted crustal components into carbonatite melts revealed by boron isotopes. Nature Geoscience, 2016; DOI: 10.1038/ngeo2831

引用自：University of Notre Dame. "Key indicator of carbon sources in Earth's mantle." ScienceDaily. ScienceDaily, 9 November 2016.