富含鐵的岩石為地球的星球歷史呈現出新的見解

 原文網址：https://news.rice.edu/news/2023/iron-rich-rocks-unlock-new-insights-earths-planetary-history

帶狀鐵礦的特徵在視覺上相當突出：它具有層層交疊的焦橙色、銀色、棕色和藍黑色條紋。萊斯大學的新研究指出這種沉積岩可能促成了地球史上幾次最大型的火山爆發。

這塊受到變形而有褶皺的帶狀鐵礦來自懷俄明州南部，年代大約為27億年。深色的的條帶為氧化鐵(磁鐵礦與赤鐵礦)；橘黃色的條帶為含有氧化鐵的燧石(碧玉)。圖片來源：Linda Welzenbach-Fries/Rice University

這種岩石含有的氧化鐵在許久之前沉到海底，層層堆疊越壓越密之後，最終成為了石頭。這週發表在《自然—地球科學》(Nature Geoscience)的研究提出這些富含鐵的岩層連結了許久以前地表出現的變化(像是光合作用生物的出現)以及行星的發展過程(像是火山活動和板塊運動)。

除了連接一般認為互不相關的行星發展過程之外，這項研究或許也重新構築了科學家對於地球早期歷史的認知，並幫助他們瞭解哪些過程或許可以讓太陽系之外的地外行星形成適合生命居住的環境。

「這些岩石逐字逐句地訴說了一顆星球的環境不停變化的故事，」研究主要作者Duncan Keller表示。他是萊斯大學地球、環境與行星科學系的博士後研究員。「它們具體呈現了大氣與海洋化學的變化。」

帶狀鐵礦是種化學沉積物，它們從古代溶有許多鐵的海水中直接沉澱出來。科學家認為微生物的代謝作用，像是光合作用有利於這些礦物的沉澱，使它們經年累月下來和燧石(微晶質的二氧化矽)交疊成層。規模最大的帶狀鐵礦形成於大約25億年前，氧氣開始在地球大氣累積的時候。

「我們知道形成這些岩石的古代海域在一段時間之後，就因為兩側的板塊構造作用而封閉起來，」Keller解釋。

Keller和共同研究人員假設帶狀鐵礦大量隱沒的區域或許有助於地函柱形成。在下部地函異常高溫的地區，上方會形成地函柱。這種高溫岩石的上升管道可以在地表形成巨大的火山，就像組成夏威夷群島的火山群一樣。「我們可以從震測數據看出在夏威夷底下有高溫地函上昇的管道，」Keller表示。「想像你的瓦斯爐有個特別高溫的地方。當鍋子裡的水煮滾，那附近就會有水上升而且泡泡特別多。地函柱就像是這種現象的巨大版本。」

「我們觀察帶狀鐵礦的沉積年代以及大型玄武岩噴發事件——稱為『大火成岩區域』——的發生年代，結果發現兩者之間有相關性，」Keller表示。「這些火成岩事件中最大的若發生10到15次，面積可能就足以蓋住整顆地球。它們之中有許多發生在帶狀鐵礦沉積後約2億4100萬年，加減1500萬年之間。如此強烈的相關性背後有個機制是很合理的。」

從帶狀鐵礦最初被拖進下部地函深處，然後影響熱對流，再驅動地函柱往上方數千公里地表湧動的過程，研究表示他們可以解釋所需的時間。

Keller在致力追尋帶狀鐵礦所經歷的旅程的途中，他也跨越了不同領域的邊界而遇見了意料之外的觀點。

「早期海洋中的微生物以化學方式改變了地表環境，如果在此之後所發生的事情最終能在2億5000萬年之後，讓地球某個地方噴發出大量岩漿，就代表這些過程彼此有關而且能互相『交談』，」Keller表示。「這也代表那些過程產生關連的時間尺度可能比人們預期的還要長上許多。為了得出這份論點，我們引用了許多不同領域的數據，包括了礦物學、地球化學、地球物理和沉積學。」

Keller希望此成果可以促成更加深入的研究。「這項研究觸及了許多不同的領域，希望結果可以激發這些領域的研究人員，」他說。「如果它可以讓人們從全然不同的角度討論地球系統的不同部分是如何產生連結，那就真的太棒了。」

Keller是「CLEVER Planets(智慧星球)」計畫(Cycles of Life-Essential Volatile Elements in Rocky Planets，岩石星球上生命必須的揮發性元素的循環)的成員之一。這個由多個學術單位合作的垮領域科學家團隊由萊斯大學的地球、環境與行星科學系的地球系統科學教授Rajdeep Dasgupta主持。

「這個跨越許多領域的合作計畫探討了生物學裡相當重要的揮發性元素，包括碳、氫、氮、氧、硫、磷在行星上的行為模式、行星如何得到這些元素、以及它們在讓行星變得適合生命居住的過程中扮演了什麼樣的腳色，」Keller表示。

「我們運用了我們擁有的最佳範例——地球，但我們也試著釐清一般而言，對行星來說擁有或缺少這些元素當中的一或多個可能代表了什麼，」他接著說道。

萊斯大學的地質、地球、環境與行星科學教授Cin-Ty Lee與Dasgupta是此研究的共同作者。其他共同作者還有包括了智利貝爾納多•奧希金斯大學的副教授Santiago Tassara、加拿大里賈納大學的助理教授Leslie Robbins(兩位之前都在耶魯大學進行博士後研究)、耶魯大學地球與行星科學的教授Jay Ague(也是Keller的博士指導教授)。

研究經費來自NASA(80NSSC18K0828)與加拿大自然科學暨工程研究委員會(RGPIN-2021-02523)。

 

Iron-rich rocks unlock new insights into Earth’s planetary history

Visually striking layers of burnt orange, yellow, silver, brown and blue-tinged black are characteristic of banded iron formations, sedimentary rocks that may have prompted some of the largest volcanic eruptions in Earth’s history, according to new research from Rice University.

The rocks contain iron oxides that sank to the bottom of oceans long ago, forming dense layers that eventually turned to stone. The study published this week in Nature Geoscience suggests the iron-rich layers could connect ancient changes at Earth’s surface — like the emergence of photosynthetic life — to planetary processes like volcanism and plate tectonics.

In addition to linking planetary processes that were generally thought to be unconnected, the study could reframe scientists’ understanding of Earth’s early history and provide insight into processes that could produce habitable exoplanets far from our solar system.

“These rocks tell — quite literally — the story of a changing planetary environment,” said Duncan Keller, the study’s lead author and a postdoctoral researcher in Rice’s Department of Earth, Environmental and Planetary Sciences. “They embody a change in the atmospheric and ocean chemistry.”

Banded iron formations are chemical sediments precipitated directly from ancient seawater rich in dissolved iron. Metabolic actions of microorganisms, including photosynthesis, are thought to have facilitated the precipitation of the minerals, which formed layer upon layer over time along with chert (microcrystalline silicon dioxide). The largest deposits formed as oxygen accumulated in Earth’s atmosphere about 2.5 billion years ago.

“These rocks formed in the ancient oceans, and we know that those oceans were later closed up laterally by plate tectonic processes,” Keller explained.

Keller and his co-workers posit that regions enriched in subducted iron formations might aid the formation of mantle plumes, rising conduits of hot rock above thermal anomalies in the lower mantle that can produce enormous volcanoes like the ones that formed the Hawaiian Islands. “Underneath Hawaii, seismological data show us a hot conduit of upwelling mantle,” Keller said. “Imagine a hot spot on your stove burner. As the water in your pot is boiling, you’ll see more bubbles over a column of rising water in that area. Mantle plumes are sort of a giant version of that.”

“We looked at the depositional ages of banded iron formations and the ages of large basaltic eruption events called large igneous provinces, and we found that there’s a correlation,” Keller said. “Many of the igneous events — which were so massive that the 10 or 15 largest may have been enough to resurface the entire planet — were preceded by banded iron formation deposition at intervals of roughly 241 million years, give or take 15 million. It’s a strong correlation with a mechanism that makes sense.”

The study showed that there was a plausible length of time for banded iron formations to first be drawn deep into the lower mantle and to then influence heat flow to drive a plume toward Earth’s surface thousands of kilometers above.

In his effort to trace the journey of banded iron formations, Keller crossed disciplinary boundaries and ran into unexpected insights.

“If what’s happening in the early oceans, after microorganisms chemically change surface environments, ultimately creates an enormous outpouring of lava somewhere else on Earth 250 million years later, that means these processes are related and ‘talking’ to each other,” Keller said. “It also means it’s possible for related processes to have length scales that are far greater than people expected. To be able to infer this, we’ve had to draw on data from many different fields across mineralogy, geochemistry, geophysics and sedimentology.”

Keller hopes the study will spur further research. “I hope this motivates people in the different fields that it touches,” he said. “I think it would be really cool if this got people talking to each other in renewed ways about how different parts of the Earth system are connected.

Keller is part of the CLEVER Planets: Cycles of Life-Essential Volatile Elements in Rocky Planets program, an interdisciplinary, multi-institutional group of scientists led by Rajdeep Dasgupta, Rice’s W. Maurice Ewing Professor of Earth Systems Science in the Department of Earth, Environmental and Planetary Sciences.

“This is an extremely interdisciplinary collaboration that’s looking at how volatile elements that are important for biology — carbon, hydrogen, nitrogen, oxygen, phosphorus and sulfur — behave in planets, at how planets acquire these elements and the role they play in potentially making planets habitable,” Keller said.

“We’re using Earth as the best example that we have, but we’re trying to figure out what the presence or absence of one or some of these elements might mean for planets more generally,” he added.

Cin-Ty Lee, Rice’s Harry Carothers Wiess Professor of Geology, Earth, Environmental and Planetary Sciences, and Dasgupta are co-authors on the study. Other co-authors are Santiago Tassara, an assistant professor at Bernardo O’Higgins University in Chile, and Leslie Robbins, an assistant professor at the University of Regina in Canada, who both did postdoctoral work at Yale University, and Yale Professor of Earth and Planetary Sciences Jay Ague, Keller’s doctoral adviser.

NASA (80NSSC18K0828) and the Natural Sciences and Engineering Research Council of Canada (RGPIN-2021-02523) supported the research.

原始論文：Duncan S. Keller, Santiago Tassara, Leslie J. Robbins, Cin-Ty A. Lee, Jay J. Ague, Rajdeep Dasgupta. Links between large igneous province volcanism and subducted iron formations. Nature Geoscience, 2023; DOI: 10.1038/s41561-023-01188-1

引用自：Rice University. "Iron-rich rocks unlock new insights into Earth's planetary history.”