原文網址：https://phys.org/news/2023-09-plate-tectonics-billion-years-life.html
by Hannah Bird ,
Phys.org
地球表層形成陸地的部分，或稱作地殼，最初大約是在40億年前形成，成分為25-50公里厚的玄武岩(一種火山岩)。科學家原先認為當時是由一塊完整的岩石圈地殼將整個地球包覆起來；相較之下我們今日所見各自分離的板塊，一般相信還要再過10億年才開始形成。然而，現在有越來越多科學家對此假說抱持質疑的態度。
史太古代的大陸地殼可能的形成機制：(a)重力沉降(重力造成密度較高的地殼沉入地函)；(b) 大陸地殼在聚合性板塊邊界層層堆疊，但密度仍低於下方的地函。(c)在聚合性板塊邊界發生的淺層隱沒作用。此研究支持模型b與c。圖片來源：Hastie et al., 2023與Nutman, 2023。
大陸地殼的形成機制多少還是有未解之處，不過科學界目前認為動力應該是板塊構造運動，此作用係指地球表面的主要板塊數十億年來在全球各處的運動過程，它也形成了我們今日所見的各個陸地以及千變萬化的地形。
其中一個理論著重於板塊開始聚合的時間，該作用通常會讓一個板塊隱沒到另一方之下，造成部分熔融而影響岩漿成分。另一個理論則探討發生在板塊內部的機制(小於50公里深的地方)，它雖然跟板塊邊界完全無關卻也能造成部分熔融。
海底高原是一片表面平坦、邊緣陡降的廣大高地，可以代表在史太古代(距今36到40億年前)最初形成的原始玄武岩地殼。最近發表在《自然—地球科學》(Nature
Geoscience)的新論文，描述了科學家對於類似海底高原的岩石所進行的實驗成果。
愛丁堡大學的Alan
Hastie博士和同僚從西南太平洋的翁通爪哇海底高原，取得了海底高原玄武岩的原始樣品之後，在高溫高壓的條件下進行了熔融實驗。
結果揭露了壓力不到1.4
GPa(至多地下50公里深)
的環境中無法形成大陸地殼，意味著這類岩漿要在聚合性隱沒帶才能形成。因此他們提出在距今40億年前就已經存在板塊構造運動，即便只是非常原始的形式。
這項知識極具意義，因為板塊構造運動造成了侵蝕、沉積、山脈形成、火山活動……等作用，它們在大陸地殼形成的過程中扮演了各式各樣的腳色。研究團隊提出火山活動釋放的氣體，尤其是一氧化碳和甲烷或許協助了地球生命的誕生，因為它們可以做為生命素材分子的來源，使得第一個微生物誕生。
除了地球以外，在體積較小的火星與金星也能發現富含矽的大陸地殼。因此這項結果也能讓我們對於在遼闊的太陽系當中，板塊構造運動所具有的功能有更多瞭解。
Hastie博士和同僚探討了地函位溫(potential
mantle temperature)達到1500至1650°C的情況下，幾種礦物在不同壓力之下的穩定性(1.2至1.4
GPa，大致相當於地下40-50公里深)，藉此找出它們在什麼位置會開始轉變。研究的重點礦物為(1)石榴子石：已知在壓力>1GPa，大致比地下30公里還要深的地方為穩定；(2)斜長石：在壓力大概到1.8GPa，約60公里深的地方變得不穩定；(3)金紅石：在0.7到1.6GPa，約25到55公里深的地方為穩定；(4)角閃石：控制了脫水造成的熔融反應。
實驗結果發現石榴子石和金紅石要到1.4GPa(約45到50公里深)才會穩定下來，比前人研究的結果還高。不過團隊認為原因是他們用的海洋地殼初始鎂含量就比較高，為的是符合估計中始太古代的基性(富含鎂鐵質)地殼成分。
他們也進行了逆向實驗：先在較高的壓力下(2GPa)培養出石榴子石晶體，再降低它們受到的壓力(1.4GPa)，結果發現這些晶體會開始分解。他們隨後發現石榴子石穩定的壓力大概是1.6GPa(大於50至55公里深)，比之前認為在1GPa就會穩定的壓力還要高，代表石榴子石的形成深度也得更深。總結來說，隱沒是更適合用來解釋這些反應的機制。
模擬也得出早期地球的岩漿在上升過程中，經過地殼時會出現結晶分化(fractional
crystallization)。該現象是指晶體從液態岩漿分離出來之後，剩餘的岩漿便會缺少用來形成這些晶體的元素，因此隨著越來越多晶體形成，岩漿成分也會逐漸改變。
研究團隊藉此確認角閃石的結晶作用為部分熔融發生的主要動力，原因為這種含水礦物或許可以透過翻轉與埋藏作用而進到地殼內部。這也符合已知的始太古代火山岩，像是英雲閃長岩(tonalite)和石英奧長岩(trondhjemite)表現出來的訊號。
伊蘇阿綠岩帶和加拿大的太古宙奴隸古陸塊(Slave
Craton)被認為是兩個殘留至今的古代隱沒帶之上的聚合性板塊交界。在這種地方，變質基性岩(變質玄武岩和相關岩石)岩漿會和隱沒地殼融化所產生的液體混和，製造出富含矽的新型岩漿——這開啟了陸地毀滅與重生的循環，塑造了我們今日所見的世界。
Plate tectonics 4
billion years ago may have helped initiate life on Earth
The Earth's oldest surface layer forming
continents, termed its crust, is approximately 4 billion years old and is
comprised of 25–50km-thick volcanic rocks known as basalts. Originally,
scientists thought that one complete lithospheric crust covered the entire
planet, compared to the individual plates we see today which were believed to
have only begun formation 1 billion years later. However, attitudes towards
this hypothesis are being challenged.
The formation mechanism of this continental crust is
somewhat enigmatic, with academics now suggesting it may have been driven by
plate tectonics, the movement of Earth's major surface plates across the globe
over billions of years, forming the landmasses and topographic features which
we see today.
One theory focuses on when the plates converge, often
causing one to subduct beneath the other, resulting in partial melting to
change magma composition, while another studies mechanisms occurring within the
crust itself (at less than 50km depth) that are entirely separate from plate
boundaries but also cause partial melting.
New research published in Nature Geoscience reports experimental work on an analog for
oceanic plateaus, large flat elevations with steep edges, that are
representative of this early basaltic crust which initially formed in the
Eoarchean (3.6–4 billion years ago).
Dr. Alan Hastie, based at the University of
Edinburgh, and colleagues subjected primitive oceanic plateau basalts from the
southwestern Pacific Ontong Java Plateau to high-pressure-temperature melting
experiments.
This revealed continental crust could not form at
pressures <1.4 GigaPascals (GPa) occurring up to 50km depth, therefore
indicating such magmas formed during convergent subduction zones. Consequently,
they suggest plate tectonics, even if only a primitive form, existed 4 billion
years ago.
This knowledge is powerful as plate tectonics are
responsible for erosion, deposition, mountain formation and volcanic activity,
which play various roles in the formation of continental crust. The research
team suggest that gases released from volcanism, especially carbon monoxide and
methane, may have helped the initiation of life on Earth by being a source of
prebiotic molecules leading to the first microbial organisms.
Beyond Earth, the silica-rich continental crust here
has also been found in smaller volumes on Mars and Venus, offering insight into
the role of plate tectonic in the wider solar system.
Dr. Hastie and colleagues investigated the stability
of a number of minerals at varying pressures (1.2–1.4GPa, equivalent to
~40–50km depth) to determine at which point they transformed, with potential
mantle temperatures reaching 1,500–1,650°C. Key minerals for the study were
garnet (which is known to be stable at pressures >1GPa, equating to ~30km
depth) and plagioclase feldspar (stable up to ~1.8GPa, ~60km depth), rutile
(stable at 0.7-1.6GPa, ~25–55km depth) and amphibole (controls dehydration
melting reactions).
The experimental results found that garnet and rutile
were not stabilized at <1.4GPa (~45–50km depth), which was higher than
previous studies had found, but the team attribute to their starting oceanic crust
having a higher magnesium content more in line with the expected composition of
Eoarchean mafic (iron and magnesium-rich) crust.
They also ran a reverse experiment in which they grew
garnet crystals at higher pressure (2GPa) before subjecting it to the lower
pressure of 1.4GPa and found that the garnet crystals began to break down.
Subsequently, they found that a pressure of ~1.6GPa (>50–55km depth) was
stable for garnet, increasing the previously held belief of stability at 1GPa,
and therefore increasing depth of formation. Consequently, subduction is the
more suitable mechanism to explain this response.
Modeling also suggests that early magmas were
subjected to fractional crystallization as they rose through the crust, whereby
crystals separated from the liquid magma, leaving the remaining magma pool
depleted in certain elements used in the initial crystals so the composition
continually changes as more crystals form.
Through this, the research team identified amphibole
crystallization as a major driver in partial melting, due to it being a hydrous
mineral that may have been incorporated into the crust by overturning and
burial. This matches signatures of known Eoarchean volcanic rocks, such as
tonalites and trondhjemites.
The Isua Greenstone Belt, Greenland, and Archaean
Slave Craton, Canada, are thought to be two remnants of convergent plate
margins above ancient subduction zones. In such areas, metabasic (metamorphosed
basaltic and allied rocks) magmas would have mixed with fluids from the melting
subducting crust to produce new silica-rich magmas, the beginnings of a cycle
of continental destruction and rebirth that has shaped the world we see today.
原始論文：Alan
R. Hastie et al, Deep formation of Earth's earliest continental crust
consistent with subduction, Nature Geoscience (2023). DOI:
10.1038/s41561-023-01249-5
引用至：Allen
P. Nutman, Forming the oldest-surviving crust, Nature Geoscience (2023). DOI:
10.1038/s41561-023-01252-w
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