由蒙納許大學的地質學家領導的國際研究，對於地球第一座陸地如何形成提出了新的見解。
參與這項研究的還有墨爾本大學、蘇格蘭聖安德魯斯大學以及澳洲西部地質調查局的研究人員。成果發表在《自然通訊》(Nature Communications)。
雖然地球是在45億年前形成，但是地球保存至今最古老的大陸地殼只能追溯到40到36億年前。
「在了解地球陸地起源的問題中，為什麼這些古老的穩定地殼需要5億年以上的時間才能形成，仍然是項還沒解答的重要部分，」主要作者，蒙納許大學地球大氣與環境科學院的研究員Jack
Mulder博士表示。
為了解答這項問題，Mulder和研究團隊探討了澳洲西部尤岡古陸塊年代為42億至32億年前的微小鋯石顆粒。
地殼基本上可以經由兩種不同的作用形成：一種是先前就有的地殼再次融化，另一種則是地球內部的地函產生新鮮的岩漿。
利用鋯石顆粒的鉿同位素組成可以追查這些作用。
「我們蒐集的新數據顯示尤岡古陸塊鋯石的鉿同位素組成在37億5000萬年前發生了重大改變，」Mulder博士表示。
「年代較老的鋯石，都是產生自更老的地殼重新熔融所產生的岩漿，」他說。
「但是從37億5000萬年前開始，含有鋯石的岩漿的來源，至少有一部份是從地函產生。」
「今日保存在尤岡古陸塊中年代最老的地殼的形成年代，與這些鋯石時光膠囊中紀錄的同位素變化的發生時間精準的重合在一起，這點十分重要。」
Mulder博士與研究共同作者認為可以用一種簡單的因果關係，來解釋兩個事件為什麼會同時發生：當岩漿從地函提煉出來，殘留在地殼下方的深部地函就會變得比較乾燥且堅硬——最重要的是，它們會往上浮，就像把水擠掉之後的海綿一樣。
「這些殘留下來、徒留骨架的地函缺乏熔融物質且帶有浮力，它們可能就像『救生筏』一樣可以保護上方剛形成的最古老陸塊，使其不會重新沉入地殼深處，」Mulder博士表示。
「這些結果強調出地殼的形成過程在37億5000萬年前有了本質上的改變，幫助地球形成了獨一無二的穩定地殼。」
Mulder博士表示今日的陸地是以這些古老的地殼為核心而成長，它們對於早期地球的氣候、大氣、海洋化學來說都有深遠的影響，並且為生命的崛起打下了基礎。
International study reveals new
insight into how continents were formed
An international study led by Monash
University geologists provides a new insight into how Earth’s first continents
formed.
The study involving researchers from Melbourne
University, University of St. Andrews (Scotland), and the Geological Survey of
Western Australia is published in Nature
Communications.
Although the Earth formed over 4.5 billion years ago,
the oldest preserved parts of our planet’s continental crust date back to
4.0—3.6 billion years ago.
“Why it took over half a billion years for these
ancient, stable pieces of crust to start forming remains a key unknown in our
understanding of the origin of Earth’s continents,” said lead study author, Dr
Jack Mulder, a Research Fellow with the Monash University School of Earth,
Atmosphere and Environment.
To address this question, Dr Mulder and the research
team studied 4.2- to 3.2-billion-year-old microscopic grains of the mineral
zircon from the Yilgarn Craton of Western Australia.
Crust can form through two fundamentally different
processes: remelting of pre-existing crust or through fresh melting of Earth’s
underlying mantle.
The hafnium isotopic composition of zircon grains can
track these processes.
“The new data we collected shows a dramatic shift in
the hafnium isotopic composition of Yilgarn Craton zircons 3.75 billion years
ago,” Dr Mulder said.
“Older zircons formed in magmas that were derived
solely though re-melting of older crust,” he said.
“From 3.75 billion years ago onwards, zircon-bearing
magmas began to be sourced, at least in part, from Earth’s mantle.
“Importantly, the isotope shift recorded in the
zircon time-capsules, coincides precisely with the time when the oldest
preserved crust in today’s Yilgarn Craton formed.”
Dr Mulder and the study co-authors attribute this
coincidence to a simple, causal relationship: When magma is extracted from
Earth’s mantle, the deep residue underpinning the crust is dry, rigid, and most
importantly buoyant— analogous to squeezing water from a sponge.
“These buoyant keels of melt-depleted residual mantle
may have served as ‘life-rafts’ that protected the new, overlaying earliest
continents from diving back into the deep Earth,” Dr Mulder said.
“These results highlight a fundamental shift in the
nature of crust formation 3.75 billion years ago, which facilitated the
formation of Earth’s unique, stable continental crust.”
Dr Mulder said these ancient crustal nuclei around
which today’s continents grew, had a profound influence on the climate,
atmosphere, and ocean chemistry of the early Earth, paving the way for
establishment of life.
原始論文：Jacob
A. Mulder, Oliver Nebel, Nicholas J. Gardiner, Peter A. Cawood, Ashlea N.
Wainwright, Timothy J. Ivanic. Crustal rejuvenation
stabilised Earth’s first cratons. Nature
Communication 12, 2021. DOI: 10.1038/s41467-021-23805-6
引用自：Monash
University. “International study reveals new insight into how continents were
formed.”
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