超深鑽石提供了瞭解超大陸如何成長的管道

原文網址：https://carnegiescience.edu/superdeep-diamonds-provide-window-supercontinent-growth

幫助古代岡瓦納大陸從底部開始成長並往上浮的地函岩石，在鑽石當中留下了它們的痕跡

幫助古代岡瓦納大陸從底部開始成長並往上浮的地函岩石，在鑽石當中留下了它們的痕跡。這篇新研究的科學團隊主持人為伯恩大學的Suzette Timmerman(之前任職於阿爾伯塔大學)，成員包含了卡內基研究所的Steven Shirey、Michael Walter、 Andrew Steele。他們發表於《自然》(Nature)的這項成果證實了超深鑽石可以成為一扇穿越時間與空間的窗口，讓我們瞭解超大陸形成與成長的過程。

從巴西茹伊納地區的金伯利岩筒Collier-4挖掘出來的超深鑽石。圖片來源：Sarah Milne，阿爾伯塔大學

數十億年來，板塊運動不斷撕裂地球的陸塊之後再把它們撞在一塊，每過一段時間就會形成巨大的超大陸。形成超大陸的作用源自於地函的大尺度對流。然而，這些事件的紀錄保存情況卻很差，因為海洋地殼的年代不夠古老，且會持續被隱沒作用拉進地球內部；而大陸地殼只能對地球深部的運作方式提供片面觀點。

讓人驚訝的是，研究團隊成功證明在地下300到700公里深形成的超深鑽石，可以透露出物質如何添加到曾經非常巨大的超大陸底部。

「這些鑽石讓我們看見深處的板塊構造作用和超大陸循環之間的關聯，」Shirey表示。

科學家認為岡瓦納超大陸形成於距今8億到5.5億年前的新元古代。從現今的南極所在地開始，其囊括了組成今日南美洲、非洲、中東、印度與澳洲的陸塊。

「藉著找出造成讓岡瓦納成長的地質作用，科學家可以更加瞭解塑造地球歷史的力量，以及陸地穩定存在的原因——後者無疑是生命最後能在地球成功立足的必要因素，」Walter補充。

在地表之下大約40到250公里深的地方，稱為地函龍骨的地質結構形成了大陸地殼的基礎。組成地函龍骨的物質在陸地下方逐漸增厚、穩定、降溫之後形成了堅固且具有浮力的結構，而能夠抵禦地球的構造活動持續不斷地破壞。

超深鑽石內部隱含了微小的矽酸鹽與硫化物包裹體，從中可以發現幫助這些龍骨形成的地函岩石殘留下來的物質。包裹體在一般用做珠寶的鑽石中被視為缺陷，但對地科學家來說卻是最好的朋友。他們找出包裹體並加以分離，探討它們的結晶學特性，再用放射性定年來判定其地質年代。

進行這項研究的為阿爾伯塔大學與卡內基科學研究所的研究人員，還有阿姆斯特丹自由大學、布里斯托大學和帕多瓦大學專門研究鑽石的團隊。研究需要的步驟非常多，包括數次把鑽石運到不同國家，以使用現有最精密的質譜儀和X光繞射儀。

「要用許多種測量技術來研究如此稀少的樣品需要龐大的團隊合作。但更令人讚嘆的是，透過仔細分析如此微小的樣品，可以揭曉地球最大的陸塊是如何演變，」Timmerman解釋。

「從這些包裹體的年代得到的紀錄，顯示具有浮力的地函物質是在什麼時候從下方加入岡瓦納超大陸，進而構築、支撐並使其成長，」Shirey接著表示。

接著，大概在距今1億2000萬年前，因為含有這些鑽石的岩石而曾經往上浮的岡瓦納超大陸開始分裂，最後在經過3000萬年以後，也就是距今9000萬年前，含有鑽石的金伯利岩岩漿造成的劇烈火山爆發，將這些鑽石連同其中的包裹體一起噴往地表。

今日，藉由實驗室分析加上現有的構造運動與陸地遷移模型，研究人員可以利用這些經過漫長旅程的鑽石，了解陸地的碎片是如何結合到無比巨大的陸塊底部，使其能更夠保持穩定。

 Superdeep diamonds provide a window on supercontinent growth

Diamonds contain evidence of the mantle rocks that helped buoy and grow the ancient supercontinent Gondwana from below

Diamonds contain evidence of the mantle rocks that helped buoy and grow the ancient supercontinent Gondwana from below, according to new research from a team of scientists led by Suzette Timmerman—formerly of the University of Alberta and now at the University of Bern—and including Carnegie’s Steven Shirey, Michael Walter, and Andrew Steele. Their findings, published in Nature, demonstrate that superdeep diamonds can provide a window through space and time into the supercontinent growth and formation process.

For billions of years, Earth’s landmasses have been ripped apart and smashed back together by plate tectonics, periodically forming giant supercontinents. This formation process results from large-scale convection of the planet’s mantle. But the records of these events are poorly preserved, because the oceanic crust is young and continually sinks beneath the planet’s surface by a process called subduction, while the continental crust only provides a limited view of Earth’s deep workings.

Surprisingly, the research team was able to show that superdeep diamonds that formed between 300 and 700 kilometers below Earth’s surface can reveal how material was added to the base of a once-mighty supercontinent.

“These diamonds allow us to see how deep plate tectonic processes relate to the supercontinent cycle,” Shirey said.

The supercontinent Gondwana is thought to have formed between 800 and 550 million years ago in Neoproterozoic times. Starting over the present-day location of the South Pole, it incorporated the landmasses that make up present day South America, Africa, the Middle East, India, and Australia. 

“By revealing the geological processes that contributed to Gondwana’s growth, scientists can better understand the forces that shaped Earth’s history and phenomenon of continental stability, which is—of course—fundamental to the eventual success of life on our planet,” added Walter. 

About 40 to 250 kilometers beneath the surface, geologic formations called mantle keels act as the foundation of the continental crust. The material that forms these keels thickened, stabilized, and cooled under the continental blocks to form strong, buoyant structures that can resist the relentless destructive forces of Earth’s tectonic activity.

Remnants of the mantle rocks that helped form the keel can be found in tiny silicate and sulfide inclusions hidden inside these superdeep diamonds. Typically flaws in normal gem diamonds, these inclusions are the best friends of a geoscientist. They were identified, isolated, studied crystallographically, and then radiometrically dated to determine their geologic ages.

This work was carried out by researchers at the University of Alberta and the Carnegie Institution for Science, as well as by other teams of diamond specialists at the Vrije Universiteit Amsterdam, University of Bristol, and the University of Padua. It required many steps, including shipping the diamonds around the world several times, and deployed some of the most precise mass spectrometers and X-ray diffractometers available.

“The study of such rare samples with a variety of measurement techniques required major teamwork. But even more remarkable is how careful analyses of such minute amounts of material can shed light on the evolution of Earth’s largest continental landmasses,” Timmerman explained.

“The age of these inclusions provides a record of when buoyant mantle was added to Gondwana from below, thereby scaffolding, underpinning, and growing the supercontinent” added Shirey.

Then, about 120 million years ago, the supercontinent once buoyed by the rocks that housed these diamonds started to break up and, eventually, 30 million years later—around 90 million years ago—the diamonds—and the inclusions trapped inside them—were brought to the Earth’s surface in violent volcanic eruptions of diamond-bearing kimberlite magma.

Now, by combining their lab analysis with existing models of tectonic movement and continent migration, the researchers can use these remarkably well-traveled diamonds to understand how material welds continental fragments together from below, stabilizing such a super-sized continental landmass.

原始論文：Suzette Timmerman, Thomas Stachel, Janne M. Koornneef, Karen V. Smit, Rikke Harlou, Geoff M. Nowell, Andrew R. Thomson, Simon C. Kohn, Joshua H. F. L. Davies, Gareth R. Davies, Mandy Y. Krebs, Qiwei Zhang, Sarah E. M. Milne, Jeffrey W. Harris, Felix Kaminsky, Dmitry Zedgenizov, Galina Bulanova, Chris B. Smith, Izaac Cabral Neto, Francisco V. Silveira, Antony D. Burnham, Fabrizio Nestola, Steven B. Shirey, Michael J. Walter, Andrew Steele, D. Graham Pearson. Sublithospheric diamond ages and the supercontinent cycle. Nature, 2023; DOI: 10.1038/s41586-023-06662-9

引用自：Carnegie Institution for Science. "Superdeep diamonds provide a window on supercontinent growth."