原文網址：https://science.ubc.ca/news/researchers-uncover-source-rocks-first-real-continents
地球科學家揭露了陸地形成謎團中缺失的環節，他們修改後的起源故事不需要板塊構造運動開始運作，或是其他外來因素來解釋陸地的形成。反之，上周發表於《自然通訊》(Nature Communications)的發現，只需要在地球歷史最初數億年形成的海底高原，內部發生的地質內營力就可以解釋陸地的形成。
三種花崗岩類：英雲閃長岩(tonalite)、奧長花崗岩(trondhjemite)和花崗閃長岩(granodiorite)，合稱TTG。
在釐清太古宙(距今40到25億年前)的陸地如何形成時，一個主要的障礙是要先找出地球早期地殼的建造材料。在此時期形成的「新型」太古宙地殼是由一套極為特別的岩石組成，包含了三種花崗岩類：英雲閃長岩(tonalite)、奧長花崗岩(trondhjemite)和花崗閃長岩(granodiorite)，合稱TTG。
想要瞭解TTG的原料是什麼以及它們是由哪種岩漿形成相當困難，因為從最初的熔融到最後的結晶過程當中牽涉到非常大量的地質作用。先前的研究將重點放在TTG的稀有元素組成，希望可以從中找到線索來得知形成TTG的岩漿及其來源。
「我們追蹤了一組特定的稀有元素，它們不會受到蝕變作用影響，因此可以把岩漿的原始訊號保存下來，指示這些新型TTG地殼的岩漿來源，」Matthijs
Smit博士表示。他是英屬哥倫比亞大學地球、海洋與大氣科學系的副教授與加拿大客席研究員。「這些元素讓我們可以回顧TTG岩漿一路以來經歷的化學變化，並且追溯熔融的組成至它們的初始狀態與來源——最有可能來自於某種輝長岩。」
「有趣的是，許多人家中的廚房檯面就是由這類岩石的其中一種做成的，」Smit博士表示。「從某種程度上來說，許多人正在用形成我們現代陸地的岩石種類，來準備他們的晚餐。」
今日的陸地有一部份仍是太古宙形成的TTG。比方說，北美洲西部的科迪勒拉山脈與東部的格林維爾、阿帕拉契山脈之間的加拿大內陸地區，大部分就是由TTG組成。安大略省、魁北克省、曼尼托巴省、薩斯喀徹溫省、西北地區、努納武特地區多數地方是由太古宙的地殼碎片組成，成分主要為TTG以及比TTG稍晚形成、演化程度更高的花崗岩類。
「全部這些岩石——特別是它們一起出現的現象——都可以用我們提出的模型來解釋，」Smit博士表示。「我們的模型並不複雜，它指出TTG以及通常跟TTG有關、年代稍晚的岩石，是由可能類似於海底高原的地殼前身，經過緩慢的埋藏、增厚與融化作用而形成。該過程勢必會讓大陸地殼發展出來，因為在不斷越埋越深的同時，基部的岩石只能走向融化一途。這麼做的結果是把TTG製造出來——它們的組成配方已經證明是讓陸地存活並成長的首選。」
英屬哥倫比亞大學研究人員的發現顯示地殼內部的獨立機制就可以製造TTG，這駁斥了長久以來認為太古宙的TTG是在地球的第一個隱沒帶內形成，因此可以標記板塊構造運動起始的理論。
「一直以來都有這種『先有雞還是先有蛋』的問題——是先有板塊構造運動，還是形成新型大陸地殼的TTG岩漿活動，」Smit博士說。「我們指出兩者之間其實可能沒有直接關聯。辨認出TTG來自哪一種岩石使我們得以跳脫上述問題，而且也排除了在解釋第一塊真正的大陸如何形成時，還要加入其他機制，像是隕石撞擊的需求。」
這篇由Smit博士和他的英屬哥倫比亞大學團隊進行的研究，資料來源為至今分析過的所有TTG樣品，其為過去30多年研究人員仔細檢視世界各地出露的太古宙穩定地塊碎片的結果。這些資料使得Smit和團隊可以過濾掉地區性的異常值以及分析過程造成的問題，而得出岩石成分的真實變化趨勢。研究運用的大量資料，現在也可以從德國哥廷根大學營運的地球化學開源資料庫「海洋與陸地岩石地球化學」(Geochemistry
of Rocks of the Oceans and Continents)取得。
Researchers uncover source rocks of
the first real continents
Geoscientists have uncovered a missing
link in the enigmatic story of how the continents developed—a revised origin
story that doesn’t require the start of plate tectonics or any external factor
to explain their formation. Instead, the findings published last week in Nature Communications, rely solely on
internal geological forces that occurred within oceanic plateaus that formed
during the first few hundred million years of Earth’s history.
A major hurdle in understanding how the continents
formed during the Archean Eon (four to 2.5 billion years ago) has been
identifying the building blocks of Earth’s early crust. Much of the “new” Archean
crust formed during this period comprised a very distinct association of three
types of granitoid rocks—tonalite, trondhjemite and granodiorite (TTG).
Understanding what went into making TTGs and the
magmas they formed from has been difficult, because so many geological
processes occurred between their initial melting and ultimate crystallization.
Earlier researchers focused on the trace element composition of these rocks,
hoping to find clues about TTG magmas and their source.
“We tracked a specific set of trace elements that
aren’t affected by alteration and pristinely preserve signatures from the
original magma that made new TTG crust,” said Dr. Matthijs Smit, associate
professor and Canada Research Chair at the University of British Columbia’s
(UBC) Department of Earth, Ocean and Atmospheric Sciences. “These elements
allowed us to look back through the chemical changes that TTG magmas go through
and trace the melt compositions back to their initial state and source—most
likely a sort of gabbro.”
“Funnily enough, many people have varieties of this
type of rock as a kitchen countertop,” Dr. Smit says. “In a way, many people
are preparing their dinner on the type of rock that was responsible for making
our modern continents.”
The Archean TTG crust is still part of the continents
today. For instance, in North America they make up much of the Canadian
Interior between the Cordillera mountain belt in the west and the Grenville and
Appalachian mountain belts in the east. The majority of Ontario, Quebec,
Manitoba, Saskatchewan, Northwest Territories and Nunavut is made up of Archean
crustal fragments that are dominated by TTGs and their slightly younger and
more evolved granite counterparts.
“All of these rocks—and especially their
combination—can be explained by the model we present,” said Dr. Smit. “Ours is
a simple model in which TTGs, as well as the younger rocks that TTGs are
typically associated with, resulted from the slow burial, thickening and
melting of precursor crust that likely resembled oceanic plateaus. The
continental crust was destined to develop the way it did, because it kept
getting buried further and the rocks at its base had no choice but to melt. In
doing so, they made the TTGs that proved a winning recipe for continental
survival and growth.”
The UBC researchers’ discovery of a stand-alone
“intra-crustal” mechanism to make TTGs dispels the long-standing theory that
Archean TTGs are formed in Earth’s first subduction zones and mark the start of
plate tectonics.
“There’s always been a ‘chicken-and-egg’ question of
which came first—the start of plate tectonics or TTG magmatism to make new
continental crust,” says Dr. Smit. “We show that these things may actually not
be directly related. The recognition of the type of source rock makes this leap
possible and also takes away the need to have other mechanisms, such as
meteorite impact, explain the growth of the first real continents.”
The study by Dr. Smit and his UBC-based team used
data from all the TTG samples ever analyzed—samples from Archean cratonic
fragments exposed worldwide scrutinized by researchers over the past 30 years.
This allowed Dr. Smit and his team to filter out local anomalies and analytical
issues, and get at the actual trends in composition that the rocks capture. The
study used a huge volume of data, now available in the open-source Geochemistry
of Rocks of the Oceans and Continents geochemical data repository hosted by the
Georg-August-Universität, Göttingen.
原始論文：Matthijs A.
Smit, Kira A. Musiyachenko, Jeroen Goumans. Archaean continental crust
formed from mafic cumulates. Nature Communications, 2024; 15
(1) DOI: 10.1038/s41467-024-44849-4
引用自：University of British Columbia. "Source
rocks of the first real continents."
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