原文網址：http://www.geologypage.com/2017/05/continental-crust-thanks-right-mixture.html
適當的混和方式才能調出大陸地殼
現今大陸地殼的形成可以在板塊構造運動的框架中來研究；然而，在30至40億年前沒有板塊運動的太古代時，並不清楚當時的大陸地殼是如何形成。
在期刊《自然》(Nature)中，由蘇黎世聯邦理工學院地球物理所的高級研究員Antoine Rozel主導的地球物理學家團隊，呈現出的電腦模擬結果可能會對這項科學議題加入更多討論。他們利用模型首度重建出早期大陸地殼的起源，這在目前為止被認為是特別難模擬的。
金星或者木衛一？
研究人員為了建立他們的電腦模型，他們從兩種地外行星形成地殼，但截然不同的方式中擷取靈感。其中一種方法假設太古代的地殼物質僅由火山活動堆積而成，就像研究人員認為在木衛一之上發生的作用一樣。反之，另一種方法則推測可能如同金星的例子中，新生地殼是在地殼內部由保有餘熱的岩漿聚積而成。
ETH研究人員的模擬結果無法確認這兩種極端情況曾經發生，因為兩者產生的大陸地殼成分，都與根據野外觀察得來的應有成分不符。
嚴格界定溫度壓力範圍
「最初的大陸地殼只能在相對嚴格定義的溫度壓力條件下形成。在前述兩種極端狀況下，都無法存在此種條件。」Rozel解釋，「如果新生地殼僅由火山形成，也就是岩漿直接在地球表面冷卻，地殼的溫度就會太低。相反來說，如果地殼是由另一種方式形成，它的溫度就會比應有的還高。」
相較而言，理想情況是形成地殼的作用混和了兩種機制，最佳比例中大約有30%的新生地殼是由火山作用形成。它所形成的地殼成分，比方來說，會跟格陵蘭西岸找到的岩石很類似。
二維的全球模型
然而，研究人員為了要能實際運算他們的模型，必須得做出一些妥協。雖然他們的模型涵蓋了全球，但只能模擬二維情況。Rozel表示：「如果我們要做出高空間解析度的三維模型，那我們大概得在超級電腦上運算十年才能完成。」
在他們的模型中，研究人員考慮了許多變量，像是溫度、壓力、岩石的水含量和黏滯度，並且將這些參數以不同數值代入，總計模擬次數多達將近100次。
Continental crust thanks to the right mixture
For the first
time, ETH scientists have successfully recreated the formation of continental
crust in the Archean using a computer simulation. The model helps scientists to
better understand processes that took place three to four billion years ago.
The
present-day formation of continental crust can be investigated in the framework
of plate tectonics; however, it is unclear how continental crust could have
formed in the Archean, a period three to four billion years ago, when there was
no plate tectonics.
In the
journal Nature, geophysicists led by
Antoine Rozel, a senior assistant at the Institute of Geophysics at ETH Zurich,
have now presented a computer model that is likely to add fuel to the
scientific debate. With their model, they were able to recreate the origination
of earlier continental crust for the first time, something which until now had
proven particularly challenging.
Venus or Io?
For their
computer model, the researchers took inspiration from two opposing explanatory
approaches. One approach postulates that the Archean crustal material was built
up through volcanic activity alone, as it has been suggested to happen on
Jupiter’s moon Io. The other approach, by contrast, assumes that new crust was
formed by the accumulation of magma remaining warm in the crust might be the
case on Venus.
The ETH
researchers’ simulations could not confirm either extreme position, since
neither of the two approaches produces a continental crust that is composed as
it should be based on field observations.
Temperature and pressure narrowly
defined
“The rocks of
the original continental crust could only form under relatively narrowly
defined temperature and pressure conditions. In both extremes, these conditions
do not exist,” explains Rozel. “If a new crust is formed solely by volcanoes,
whereby the magma cools immediately on the Earth’s surface, the crust would be
too cold. Conversely, the crust in the other approach would be hotter than it
should be.”
By contrast,
the ideal situation is when the crust is created through a mixture of the two
mechanisms, preferably when around 30 percent of the new crust is formed by
volcanism. This results in a rock composition similar to what can be found on
the west coast of Greenland, for example.
Two-dimensional and global
For the
researchers to calculate their model, however, they had to make some
compromises. Although their model is global, it is only two-dimensional. “If we
had wanted a high regional resolution and a three-dimensional model, we would
have had to run the calculations on a supercomputer for ten years,” says Rozel.
In their
model, the researchers considered various quantities, such as temperature,
pressure, water content of the rock and its viscosity, and simulated the
processes up to 100 times to test the parameters with various values.
原始論文：A. B. Rozel et
al. Continental crust formation on early Earth controlled by intrusive
magmatism, Nature (2017). DOI:
10.1038/nature22042
沒有留言:
張貼留言