從鑽石得到的發現證實地球的海洋地殼會循環回地函

原文網址：https://www.folio.ca/diamond-discovery-proves-earths-oceanic-crust-gets-recycled/

從鑽石得到的發現證實地球的海洋地殼會循環回地函

首度在地表附近的鑽石中發現通常在地底700公里深才能找到的礦物。

By Jennifer-Anne Pascoe

科學家首度在地表找到地球含量第四豐富的礦物――矽酸鈣鈣鈦礦。

Graham Pearson表示：「從來沒人有辦法可以讓此物質在地表維持穩定狀態。」他是加拿大阿爾伯塔大學地球和海洋大氣科學系的教授，同時也是加拿大卓越研究講座教授的得主。他解釋這種礦物發現於地函內部深達700公里的地方。

「要讓這種礦物存於地表的唯一可能方式便是將它包裹在堅不可摧的容器當中，像是鑽石。」他解釋，「根據我們的發現，這種類型的鈣鈦礦在地球深部可能有多達1皆噸(zettaton，10²¹噸)這麼多。」

Pearson參與的研究團隊的科學家來自義大利的帕多瓦大學和帕維亞大學、英屬哥倫比亞大學和其他英國和南非的大學。他們發現的這顆藏有矽酸鈣鈣鈦礦的鑽石開採自南非科立南礦坑距地表不到一公里深的地方。此礦坑的著名之處在於英國女王皇冠上其中兩顆最大的鑽石即是來自此處。Pearson解釋從科立南礦坑開採出的鑽石不僅在世上具有極高的商業價值，在科學上也具有非凡的意義，因為這些鑽石可以讓我們一窺地底最深的地方。

他說研究中的這顆鑽石特殊之處為曾經承受超過240億帕斯卡的壓力，相當於大氣壓力的24萬倍。這顆鑽石形成的深度大概位於地表之下700公里深的位置，而大多鑽石通常形成於地表之下150到200公里深的地方。

Pearson表示：「鑽石確實是一種獨門管道能讓我們看見地球深處發生的事物。在這顆鑽石中含有的鈣鈦礦包裹體，其特有成分清楚顯示海洋地殼可以循環回到地球的下部地函。這提供了十分重要的證據顯示海洋板塊下沉至地球深處時所遭受的命運。」

他說這項發現再次強調出鑽石的獨特性質：它能夠保存以其他任何方式皆無法觀察到的事物。

Pearson表示：「這也是一個良好的例子呈現出科學運作的方式。先根據理論作出預測――在此例中是根據地震學――接著在偶然之間得到一個決定性的觀察結果證明該理論的確可行。」

Pearson身為世上研究鑽石的科學家最有名的其中一位，他也參與了2014年的一篇重要發現。他們在某顆鑽石中發現的尖晶橄欖石(ringwoodit，地球含量第五豐富的礦物)指出地函內部某個地方有相當大量的水跟矽酸岩結合在一起。

與Pearson共事的國際研究團隊中包括來自義大利帕多瓦大學的Fabrizio Nestola，他是世上最優秀的X光晶體學者之一；另外還有來自美國華盛頓特區深碳觀測組織的科學家。Pearson也和其他英屬哥倫比亞大學的科學家組成一個計畫：鑽石探勘研究與訓練學院――其隸屬於加拿大自然科學暨工程研究委員會的合作研究與經驗培育計畫――目的為培養下一代具備優秀技術的鑽石探勘人員。

這項名為「CaSiO₃ Perovskite in Diamond Indicates the Recycling of Oceanic Crust Into the Lower Mantle」將會刊登在3月8日的《自然》期刊。

Diamond discovery proves Earth’s oceanic crust gets recycled

Mineral usually found at depths of 700 km found ensconced within diamond near Earth’s surface for the first time.

For the first time, scientists have found Earth’s fourth most abundant mineral—calcium silicate perovskite—at Earth’s surface.

“Nobody has ever managed to keep this mineral stable at the Earth’s surface,” said Graham Pearson, a professor in the University of Alberta’s Department of Earth and Atmospheric Sciences and Canada Excellence Research Chair Laureate. He explained the mineral is found deep inside Earth’s mantle, at 700 kilometres.

“The only possible way of preserving this mineral at the Earth’s surface is when it’s trapped in an unyielding container like a diamond,” he explained. “Based on our findings, there could be as much as zettatonnes (10²¹) of this perovskite in deep Earth.”

Pearson, working with a team of colleagues from the universities of Padua and Pavia, the University of British Columbia and universities in the U.K. and South Africa, found the calcium silicate perovskite within a diamond mined from less than one kilometre beneath the Earth’s crust, at South Africa’s famous Cullinan Mine, best known as the source of two of the largest diamonds in the British Crown Jewels. Pearson explained that the diamonds from the mine are not only among the most commercially valuable in the world, but are also the most scientifically valuable, providing insight into the deepest parts of Earth’s core.

He said the particular diamond in question would have sustained more than 24 billion pascals of pressure, equivalent to 240,000 atmospheres. The diamond originated roughly 700 kilometres below Earth’s surface, whereas most diamonds are formed at depths of 150 to 200 kilometres.

“Diamonds are really unique ways of seeing what’s in the Earth,” said Pearson. “And the specific composition of the perovskite inclusion in this particular diamond very clearly indicates the recycling of oceanic crust into Earth’s lower mantle. It provides fundamental proof of what happens to the fate of oceanic plates as they descend into the depths of the Earth.”

He said the discovery once again highlights the uniqueness of diamonds being able to preserve things we otherwise would never be able to see.

“And it’s a nice illustration of how science works. That you build on theoretical predictions—in this case, from seismology—and that once in a while you’re able to make a clinching observation that really proves that the theory works,” said Pearson.

One of the best-known diamond researchers in the world, Pearson was also behind the major 2014 discovery of ringwoodite—Earth’s fifth most abundant mineral—in a diamond that pointed to a vast reservoir of water bound to silicate rocks in Earth’s mantle.

Pearson worked with an international team of researchers, including one of the best X-ray crystallographers in the world, Fabrizio Nestola from Padova, Italy, as well as scientists from the Deep Carbon Observatory in Washington, D.C. Pearson also teamed up with colleagues from the University of British Columbia who together lead a program—the Diamond Exploration Research and Training School, part of NSERC’s Collaborative Research and Training Experience—to train the next generation of highly qualified diamond explorers.

The study, “CaSiO₃ Perovskite in Diamond Indicates the Recycling of Oceanic Crust Into the Lower Mantle,” will be published in the March 8 issue of Nature.

原始論文：F. Nestola, N. Korolev, M. Kopylova, N. Rotiroti, D. G. Pearson, M. G. Pamato, M. Alvaro, L. Peruzzo, J. J. Gurney, A. E. Moore, J. Davidson. CaSiO₃ perovskite in diamond indicates the recycling of oceanic crust into the lower mantle. Nature, 2018; 555 (7695): 237 DOI: 10.1038/nature25972

引用自：University of Alberta. "Diamond discovery under pressure: Super-deep diamond provides first evidence in nature of Earth's fourth most abundant mineral, indicating the very deep recycling of oceanic crust."