由科隆、柏林與約翰尼斯堡的大學組成的科學團隊發表在《美國國家科學院院刊》(PNAS)的文章表示，地球最初的地殼其根部仍然殘留在地函裡面，並且在過去的地球歷史當中持續為噴發到地表的岩漿提供原料。
在這篇國際合作的成果當中，科隆大學與柏林自由大學的地球科學家發現某些從地函深處到達地表並噴發出來的岩漿，其來源處的地函還殘留著地球最初的地殼。這些古老的物質勢必待在一座低溫且古老的地殼「墳場」超過了四十億年，也許從形成月球的巨型撞擊事件之後就一直待在該處。這項發現確實令人感到意外，因為地球的板塊運動體系會經由大尺度的地函對流，在小很多的時間尺度中把地殼逐漸回收，所以科學家過往推測地球早期的地質作用留下的痕跡，只能在其他類地行星(水星、金星、火星)、小行星和月球上找到類似的比較對象。然而，根據他們最近發表在《美國國家科學院院刊》的研究「長期保留在地函的冥古宙原始地殼」，透過地球從古至今噴發出來的岩漿岩帶有的訊號，還是能詳細解讀出一些訊息，像是最初的地殼有什麼樣的特性、在地函最深處的墳場保存了許久、並且可以透過年代較近的火山活動而復甦。
地質學家在進行研究的時候探討了採自南非、年代最老到35.5億年前的岩石。分析得出這些岩石的鎢(W)同位素組成，也就是182W的相對含量有微小的異常。和此種異常有關的地質作用，只有在45億多年前地球形成不久之後才會發生。
作者進行模擬計算出來的結果顯示他們觀測到的同位素182W的分布模式，最好的解釋方法是地球最初的地殼融入地函的物質之後，經由地函柱從下部地函往上升，接著產生岩漿從地表噴發，使這些最初的地殼得以回到地表。有趣的是，研究顯示在現代一種特別的火山岩(洋島玄武岩)中可以看到類似的同位素分布模式，意味地球最初的地殼仍然埋在地函的最底部。
「我們推測地殼的下層——也就是原始陸地的根部——會因為地質上的成熟過程而逐漸變得比周圍還重，使它們沉到下方的地函裡面，就和熔岩燈一樣，」科隆大學地質與礦物研究所的地球化學家Jonas
Tusch博士強調。「這道有趣的見解提出了幼年地球會留下什麼樣的地球化學訊號，使得我們可以更加瞭解地球歷史中的大型陸塊如何出現。此外，這也解釋了現今含有大量氧氣的大氣層如何演化出來，而成為複雜生命誕生的基礎，」柏林自由大學的Elis
Hoffmann補充。
早期地球的地球化學訊號也可以和太空任務從其他星球獲得的訊息比較。比方說，火星任務以及研究火星隕石獲得的數據顯示，火星因為缺乏板塊運動所以表面仍然相當古老，而它的成份或許便相當於幼年的地球。
Sampling the deep
graveyard of Earth’s earliest crust
Writing in PNAS, a team of scientists from the universities of Cologne, Berlin
and Johannesburg show that remnants of the roots of Earth’s first crust are
still present in the terrestrial mantle and contribute to magmas erupted at the
surface over Earth’s history.
In an international collaboration, Earth scientists
at the University of Cologne and Freie Universität Berlin discovered that some
magmas on Earth, which made their way through the deep terrestrial mantle and
erupted at Earth’s surface, originate from mantle portions that contain
remnants of Earth’s earliest crust. This ancient material must have been buried
in a ‘graveyard’ of old and cold crust more than 4 billion years ago and
survived since then, maybe since the giant impact event forming the Moon. This
finding is unexpected because the plate tectonic regime of our planet
progressively recycles crustal material via large-scale mantle convection at
much smaller time scales. Therefore, it has been assumed that vestiges of early
geological processes on Earth can only be found as analogues, on other
terrestrial planets (Mercury, Venus, and Mars), asteroids, or the Moon.
However, according to their study ‘Long-term preservation of Hadean protocrust
in Earth’s mantle’, which has recently appeared in the Proceedings of the National Academy of Sciences (PNAS), magmatic
rocks that erupted throughout Earth’s history can still carry signatures that
provide detailed information about the nature of the first crust, its long-term
preservation in a graveyard in the lower-most mantle, and its resurrection via
younger volcanism.
For their study, the geologists investigated up to
3.55 billion years old rocks from southern Africa. The analysis of these rocks
revealed small anomalies in the isotope composition of the element tungsten
(W). The origin of these isotope anomalies, namely the relative abundance of 182W,
relates to geological processes that must have occurred immediately after the
formation of the Earth more than 4.5 billion years ago.
Model calculations by the authors show that the
observed 182W isotope patterns are best explained by the recycling
of Earth’s earliest crust into mantle material that ascends via plumes from the
lower mantle to generate lavas erupting at Earth’s surface. Intriguingly, the
study shows that similar isotope patterns can be observed in distinct types of
modern volcanic rocks (ocean island basalts), which demonstrates that Earth’s
earliest crust is still buried in the lowermost mantle.
‘We assume that the lower layers of the crust – or
the roots of the primordial continents – became heavier than their surroundings
due to a geological maturation process and therefore sank into the Earth’s
underlying mantle. Similar to a lava lamp,’ geochemist Dr Jonas Tusch from the
University of Cologne’s Institute of Geology and Mineralogy remarked. ‘This
fascinating insight provides a geochemical fingerprint of the young Earth,
allowing us to better understand how large continents formed over the history
of our planet. It also explains how our current, oxygen-rich atmosphere evolved
– setting the stage for the origin of complex life,’ Dr Elis Hoffmann of Freie
Universität Berlin added.
The geochemical fingerprint of the early Earth can
also be compared with findings about other planets obtained during space
missions. For example, data from Mars missions and studies of Martian
meteorites show that Mars still has a very old surface due to the lack of plate
tectonics, and that its composition may correspond to that of the young Earth.
原始論文：Jonas Tusch,
J. Elis Hoffmann, Eric Hasenstab, Mario Fischer-Gödde, Chris S. Marien, Allan
H. Wilson, Carsten Münker. Long-term preservation of Hadean protocrust
in Earth’s mantle. Proceedings of the National Academy of Sciences,
2022; 119 (18) DOI: 10.1073/pnas.2120241119
引用自：University of Cologne. "Sampling the deep
graveyard of Earth's earliest crust."
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