Lindsey Valich
鋯石晶體與岩漿為地球數十億年前的板塊構造活動提供了新的資訊。
板塊構造作用透過融化與混和不同岩石來創造出具有特定化學組成的岩漿。羅徹斯特大學的地質學家利用這類化學證據，解出關於40多億年前地球的板塊構造活動資訊。(Getty Images photo)
地球是個活躍且無時無刻都在變化的星球。從高山海洋的形成到火山噴發，地球的表面總是處在變動的狀態。造成這些變化的關鍵為板塊構造作用的強烈力量，也就是地殼板塊的移動過程。這道重要的作用塑造了地球當今的地貌，在未來也會持續發揮它的影響。
但是早期地球的板塊構造活動是什麼樣子？甚至在認知中生命形成的時候它就已經開始作用了嗎？
「當代地球擁有活躍的構造作用這份特質，是今日生命得已存在的理由之一，」Wriju
Chowdhury表示。他是羅徹斯特大學地球與環境科學系的副教授Dustin
Trail實驗室裡的博士後助理研究員。「探討早期地球的地球動力學與岩石多樣性可以揭露地球的生命最初是如何出現。」
以Chowdhury為第一作者，發表在《自然通訊》上的論文概述了羅徹斯特大學的研究人員如何利用微小的鋯石晶體，解出關於早期地球的岩漿與板塊構造運動的訊息。這份研究提出的化學證據指出板塊構造作用發生的時間，很有可能比我們認為地球生命最初形成的時候，也就是42億年前還要更早。此發現也能幫助我們尋找其他星球上的生命。
板塊構造作用驅動了地殼的形成與滅亡
Trail表示當代地球具有板塊構造作用「極為重要」，因為「它是地殼形成與滅亡的主要機制。」
地球是已知的星球中，唯一上部地殼會移動並經歷生死循環的。這道過程可以把重要的元素，像是鐵和鎂從地球內部運到地球表面，並且調控地球的水循環和碳循環。然而對地質學家來說更重要的是，板塊構造作用把岩石融化並加以混和之後，會依據「銷毀」的岩石種類與地點，創造出具有特定化學組成的岩漿。因此，岩漿的化學組成便能透露出創造它的構造作用類型。
微小的時光膠囊：古代的礦物晶體
Chowdhury和同事利用鋯石來進行他們的研究，這種岩石中的小型晶體就像微型時光膠囊一樣。鋯石晶體在形成的時候會把微量化學元素封存進去，它們能透露結晶出鋯石的母岩漿的化學組成是什麼。接著研究人員定年鋯石並結合關於岩漿的資訊，加以反推就能重建鋯石形成的時候，早期地球的物理與化學環境，進而推測當時的板塊構造作用型態。在此研究中，鋯石的年代大約是在38到42億年之間。
Chowdhury表示大多數研究人員推測跟早期地球有關的資訊時，是利用鋯石建立的機率模型來呈現不同的構造作用情境。Chowdhury和同事則更進一步，他們描述的對象不只是鋯石，還有母岩漿。
羅徹斯特大學的研究人員Wriju Chowdhury和Dustin Trail利用微小的鋯石晶體得出了關於早期地球的資訊。這些年代有幾十億年的晶體只有零點幾微米長(圖中比例尺的單位為微米，1微米=0.000001公尺)。研究人員運用「陰極射線發光」技術，從陰極對樣品發射電子而得到這幅影像。晶體上的孔洞是研究人員用雷射挖除部分樣品所留下的，他們從中可以得到樣品的化學資訊。照片來源：史密森尼學會
「母岩漿的訊息更加直觀且可信，因為它更接近來源，即實際的構造作用循環，」Chowdhury表示。「我們的研究描述了鋯石中矽和氧的同位素含量以及母岩漿的稀有元素含量，之前從來沒有人把它們結合起來並一同呈現。」
Chowdhury、Trail和同事發現早期地球岩漿的化學組成，類似於當今在構造活躍的板塊邊界所產生的岩漿，像是喀斯喀特山脈、阿留申島鏈、日本地區與安地斯山脈。
「這暗示了構造作用從遠古時期一直延續到當代，」Trail表示。「也就是說，我們的研究顯示出地球在數十億年前的運作方式，有可能跟現在相差無幾。」
星球宜居的關鍵特徵
研究並未判定板塊構造作用開始的時候生命是否已經存在——「目前還沒有精準得出生命和構造作用開始的時間，」Chowdhury表示，指出地質學界對於這兩點仍有分歧。但是這篇新數據提出的化學證據暗示板塊構造作用可能早在42億年前就已經在進行了。
他接著說，無論如何，板塊構造作用都是地球現在適合生命居住的關鍵因素之一，而且在尋找其他星球是否具有宜居的環境時也是很重要的條件。
「如果一顆星球具有某些動態作用的話，那麼生命誕生的機率就會增加許多，」他說。
Was plate
tectonics occurring when life first formed on Earth?
Zircon crystals and magmas reveal new
information about plate tectonic activity on Earth billions of years ago.
Earth is a dynamic and constantly changing planet.
From the formation of mountains and oceans to the eruption of volcanoes, the
surface of our planet is in a constant state of flux. At the heart of these
changes lies the powerful force of plate tectonics—the movements of Earth’s
crustal plates. This fundamental process has shaped the current topography of
our planet and continues to play a role in its future.
But what was plate tectonic activity like during
early Earth? And was the process even occurring during the time when life is
thought to have formed?
“The dynamic tectonic nature of the modern Earth is
one of the reasons why life exists today,” says Wriju Chowdhury, a research
associate in the lab of Dustin Trail, an associate professor of earth and
environmental sciences at the University of Rochester. “Exploring the geodynamics
and the lithological diversity of the early Earth could lead to revelations of
how life first began on our planet.”
Chowdhury is the first author of a paper published in
Nature Communications that outlines
how Rochester researchers used small zircon crystals to unlock information
about magmas and plate tectonic activity in early Earth. The research provides
chemical evidence that plate tectonics was most likely occurring more than 4.2
billion years ago when life is thought to have first formed on our planet. This
finding could prove beneficial in the search for life on other planets.
Plate tectonics
power the creation and destruction of Earth’s crust
Plate tectonics on modern Earth is “extremely
important,” Trail says, because it is “the dominant mechanism for the creation
and destruction of Earth’s crust.”
Earth is the only known planet that has a mobile
upper crust that is cyclically destroyed and created. The process delivers
critical elements, such as iron and magnesium, from the interior of the earth
to its surface and controls Earth’s water and carbon cycles. But, more
importantly to geologists, plate tectonics melts and mixes rocks to create
magmas with specific chemical makeups, depending on the rocks involved and the
location where the “destruction” occurred. The chemical makeup of magma can
therefore indicate the style of tectonics that created it.
Ancient crystals
as tiny time capsules
Chowdhury and his colleagues conducted their research
using zircons—tiny crystals in rocks that are like small time capsules. The
zircons contain trace amounts of chemical elements, locked into the crystals at
the time when the crystals were formed. The researchers date the zircons and then
work backward, with zircons revealing information about the chemical makeup of
the parent magmas from which the zircons crystallized. Researchers then use
information about the magmas to reconstruct the physical and chemical
environment—and to infer plate tectonic styles—of the early Earth, during the
time when the zircons formed. In this case, the zircons were around 3.8 to 4.2
billion years old.
According to Chowdhury, most researchers infer
information about early Earth using zircons to create probabilistic models to
present different tectonic scenarios. Chowdhury and his colleagues went a step
further to describe not only the zircons but also the parent magmas.
“Parent magmas are much more direct and reliable
because they are closer to the source—the actual tectonic style,” Chowdhury
says. “Our study describes the silicon and oxygen isotopic content of the
zircons and the trace element content of the parental magmas, which has not
been combined and presented before.”
Chowdhury, Trail, and their colleagues found chemical
similarities between early Earth magmas and modern magmas created at
tectonically active plate boundaries such as the Cascade and Aleutian Island
chains or areas in Japan and the Andes Mountains.
“This suggests tectonic continuity from the ancient
to modern times,” Trail says. “That is, our study shows the earth, billions of
years ago, might have operated similarly as it does today.”
A key characteristic
of a habitable planet
The researchers did not determine whether life
existed when plate tectonics began—“neither life nor tectonics have an accurate
start date yet,” Chowdhury says, noting that the geology community is divided
on these points. But the new data provides chemical evidence suggesting that
plate tectonics could have been occurring more than 4.2 billion years ago.
Whatever the case, he continues, plate tectonics is a
key reason why Earth currently has a temperate living environment—and could be
an important factor in the search for habitable living environments on other
planets.
“The chances for life to originate increase manifold
if there is some planetary dynamism,” he says.
原始論文：Wriju
Chowdhury, Dustin Trail, Martha Miller, Paul Savage. Eoarchean and
Hadean melts reveal arc-like trace element and isotopic signatures. Nature
Communications, 2023; 14 (1) DOI:
引用自：University of Rochester. "Was plate
tectonics occurring when life first formed on Earth?
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