原文網址：https://attheu.utah.edu/uncategorized/chemical-leftovers/
By Paul
Gabrielsen
讓我們來趟前往地球深處的旅程吧，從地殼往下進入地函，一路前往地核附近。我們將利用地震波來指引方向――這些地震之後產生的迴響穿過地球內部，就像雷達一樣可以顯露其中的結構。
往下到達地核附近，地震波在某些區塊的速度會大幅減慢。猶他大學的新研究顯示這些神秘且恰如其名的「超低速帶」(ultra-low velocity zones)，意外地其實層次分明。模擬指出這些區域當中有一些可能是塑造早期地球的作用遺留下來的物質，這些沒有混和均勻的殘留物就像麵糊底部結塊的麵粉一樣。
「關於地函深處我們所知的特徵當中，超低速帶可能是最為極端的，」地質與地球物理系的副教授Michael
S. Thorne表示。「甚至從整個地球來看，它們也是最極端的特徵之一。」
這項研究發表在《自然―地球科學》(Nature
Geoscience)，經費來自美國國家科學基金會。
深入地函
讓我們複習一下地球內部的構造：我們居住在地殼上方，這是一層又硬又薄的岩石。地球的中心是由鐵鎳組成的地核，兩者之間稱為地函。地函並非是一片岩漿海，它更像是固體的岩石，但溫度相當高，並且可以載著地球表面的板塊移動。
我們怎麼知道地函與地核當中發生的事物？答案是地震波。地震發生之後，地震波就像漣漪一樣穿過地球內部。地球表面的科學家測量震波在什麼時候、經由什麼路徑到達世界各地的測站，就能反算震波是如何被地球內部的構造，像是不同密度的層位給反射與折射。這就是我們為何知道地殼、地函與地核的邊界位在什麼地方，也是我們了解它們組成的方式之一。
超低速帶坐落在地函底部、液態的金屬外核之上。在這些區域，地震波的速度最多會下降一半，而密度則會增加到三倍。
科學家原先認為這些區域是部分熔融的地函，或許還是「熱點」――冰島這類火山地區的岩漿來源。
「但是我們稱為超低速帶的那些東西大部份似乎都並非位在熱點火山的下方，」Thorne表示。「所以這不能完整的解釋。」
因此Thorne以及博士後學者Surya
Pachhai，加上澳洲國立大學、亞利桑那州立大學和卡爾加里大學的研究人員，決定要探討另一個理論：超低速帶的岩石組成可能不同於地函其他地方，而且成分可能類似於早期地球。
Thorne表示超低速帶也許是由鐵氧化物組合而成，就像我們在地表看到的鐵鏽一樣，不過它們在地函深處會有金屬般的行為。若是如此，這些緊鄰地核外圍的鐵氧化物團塊，或許會對下方不遠處產生的地球磁場有所影響。
「從超低速帶的物理性質可以得知它們的起源，」Pachhai表示。「這反過來可以提供重要的訊息來讓我們瞭解地函最下層的熱與化學狀態、演化過程及動力學。對於驅動板塊運動的地函對流來說，地函最下層是不可或缺的一環。」
地震波反向工程
為了得到清晰的圖像，研究人員對澳洲和紐西蘭之間珊瑚海下方的超低速帶進行研究。由於此處有許多地震可以提供關於核幔邊界的高解析度震波圖像，因此是個理想的地點。他們希望高解析度的觀測結果可以揭露超低速帶是由什麼組成的更多細節。
但是要穿過接近1800英哩厚的地殼和地函來得到任何物體的震波圖像並不簡單，而且也不是每次都能得到肯定的結果：較厚的低速物質反射地震波的方式有可能和較薄、但是速度更低的物質一模一樣。
因此團隊利用反向工程法。
「我們可以創造出一個地球模型，其中含有能讓波速大幅減低的物質，」Pachhai表示。「接著我們透過電腦模擬的結果，就能知道如果這是地球的實際樣貌，那麼地震波的波形會是什麼樣子。然後我們再把模擬預測出來的紀錄和我們實際擁有的紀錄互相比對。」
這種方法稱為「貝氏反演」，經過數十萬次的模擬之後，可以得到數學基礎穩健的地球內部模型，而且他們可以十分明瞭其中的不確定性，以及模型採取不同假設的優缺點。
研究人員想解答的重要問題之一是，超低速帶內部是否有結構，比方說還能再分層。根據模擬結果，答案是超低速帶很有可能具備層狀結構。這點十分重要，因為可以讓他們瞭解超低速帶是如何形成。
「就我所知，這是第一項把貝氏方法運用到如此詳細來探討超低速帶的研究，」Pachhai表示。「這也是第一項證明超低速帶內部有強烈分層的研究。」
回顧地球形成之時
超低速帶可能具有層狀結構的意義是什麼？
四十多億年前，當密度較高的鐵沉到早期地球的核心，較輕的礦物浮到地函當中的時候，可能有顆火星大小的天體撞上了幼年的地球。碰撞可能把大量碎屑拋到地球的軌道，並在之後形成了月球。這次事件也讓地球的溫度大幅升高，正如意料中兩個星球對撞會有的結果。
「結果是形成了一大片融化的物質，我們稱為岩漿海，」Pachhai表示。這片「海洋」裡有岩石、氣體與礦物晶體懸浮於岩漿當中。
這座海洋在冷卻過程中會自己進行分類，密度較高的物質往下沉而堆疊在地函底部。
經過四十億年來地函不斷的翻攪與對流，這層密度較高的物質可能被推擠成一塊一塊的，變成我們今日看到的層狀超低速帶。
「我們最主要也最驚人的發現是超低速帶並非均質，而是具有十分強烈的非均質性(結構與成分有所變化)，」Pachhai表示。「這項發現改變了我們對於超低速帶起源與動力學的看法。我們發現這種類型的超低速帶可以用地球歷史非常早期所形成的化學非均質性來解釋，而且在經過了45億年的地函對流之後仍然沒有徹底攪勻。」
尚未定調
這項研究對於某些超低速帶的起源提供了證據，然而別的證據也指出其他超低速帶另有來源，像是沉回地函的海洋板塊融化產生的物質。不過，如果至少有些超低速帶是早期地球遺留至今的物質，它們便保存了在其他地方已經消失的地球歷史片段。
「因此我們的發現可以做為一項工具，讓我們瞭解地函最初的熱與化學狀態，」Pachhai表示。「不只如此，還能瞭解它們的長期演變過程。」
Possible chemical leftovers from early
Earth sit near the core
Let’s take a journey into the depths of
the Earth, down through the crust and mantle nearly to the core. We’ll use
seismic waves to show the way, since they echo through the planet following an
earthquake and reveal its internal structure like radar waves.
Down near the core, there are zones where seismic
waves slow to a crawl. New research from the University of Utah finds that
these enigmatic and descriptively-named ultra-low velocity zones are
surprisingly layered. Modeling suggests that it’s possible some of these zones
are leftovers from the processes that shaped the early Earth—remnants of
incomplete mixing like clumps of flour in the bottom of a bowl of batter.
“Of all of the features we know about in the deep
mantle, ultra-low velocity zones represent what are probably the most extreme,”
says Michael S. Thorne, associate professor in the Department of Geology and
Geophysics. “Indeed, these are some of the most extreme features found anywhere
in the planet.”
The study is published in Nature Geoscience and is funded by the National Science Foundation.
Into the mantle
Let’s review how the interior of the Earth is
structured. We live on the crust, a thin layer of solid rock. Between the crust
and the iron-nickel core at the center of the planet is the mantle. It’s not an
ocean of lava – instead it’s more like solid rock, but hot and with an ability
to move that drives plate tectonics at the surface.
How can we have any idea what’s going on in the
mantle and the core? Seismic waves. As they ripple through the Earth after an
earthquake, scientists on the surface can measure how and when the waves arrive
at monitoring stations around the world. From those measurements, they can
back-calculate how the waves were reflected and deflected by structures within
the Earth, including layers of different densities. That’s how we know where
the boundaries are between the crust, mantle and core – and partially how we
know what they’re made of.
Ultra-low velocity zones sit at the bottom of the
mantle, atop the liquid metal outer core. In these areas, seismic waves slow by
as much as half, and density goes up by a third.
Scientists initially thought that these zones were
areas where the mantle was partially melted, and might be the source of magma
for so-called “hot spot” volcanic regions like Iceland.
“But most of the things we call ultra-low velocity
zones don’t appear to be located beneath hot spot volcanoes,” Thorne says, “so
that cannot be the whole story.”
So Thorne, postdoctoral scholar Surya Pachhai and
colleagues from the Australian National University, Arizona State University
and the University of Calgary set out to explore an alternate hypothesis: that
the ultra-low velocity zones may be regions made of different rocks than the
rest of the mantle—and that their composition may hearken back to the early
Earth.
Perhaps, Thorne says, ultra-low velocity zones could be
collections of iron oxide, which we see as rust at the surface but which can
behave as a metal in the deep mantle. If that’s the case, pockets of iron oxide
just outside the core might influence the Earth’s magnetic field which is
generated just below.
“The physical properties of ultra-low velocity zones
are linked to their origin,” Pachhai says, “which in turn provides important
information about the thermal and chemical status, evolution and dynamics of
Earth’s lowermost mantle—an essential part of mantle convection that drives
plate tectonics.”
Reverse-engineering
seismic waves
To get a clear picture, the researchers studied
ultra-low velocity zones beneath the Coral Sea, between Australia and New
Zealand. It’s an ideal location because of an abundance of earthquakes in the
area, which provide a high-resolution seismic picture of the core-mantle
boundary. The hope was that high-resolution observations could reveal more
about how ultra-low velocity zones are put together.
But getting a seismic image of something through
nearly 1800 miles of crust and mantle isn’t easy. It’s also not always
conclusive—a thick layer of low-velocity material might reflect seismic waves
the same way as a thin layer of even lower-velocity material.
So the team used a reverse-engineering approach.
“We can create a model of the Earth that includes
ultra-low wave speed reductions,” Pachhai says, “and then run a computer
simulation that tells us what the seismic waveforms would look like if that is
what the Earth actually looked like. Our next step is to compare those
predicted recordings with the recordings that we actually have.”
Over hundreds of thousands of model runs, the method,
called “Bayesian inversion,” yields a mathematically robust model of the
interior with a good understanding of the uncertainties and trade-offs of
different assumptions in the model.
One particular question the researchers wanted to answer
is whether there are internal structures, such as layers, within ultra-low
velocity zones. The answer, according to the models, is that layers are highly
likely. This is a big deal, because it shows the way to understanding how these
zones came to be.
“To our knowledge this is the first study using such
a Bayesian approach at this level of detail to investigate ultra-low velocity
zones,” Pachhai says, “and it is also the first study to demonstrate strong
layering within an ultra-low velocity zone.”
Looking back at
the origins of the planet
What does it mean that there are likely layers?
More than four billion years ago, while dense iron
was sinking to the core of the early Earth and lighter minerals were floating
up into the mantle, a planetary object about the size of Mars may have slammed
into the infant planet. The collision may have thrown debris into Earth’s orbit
that could have later formed the Moon. It also raised the temperature of the
Earth significantly—as you might expect from two planets smashing into each
other.
“As a result, a large body of molten material, known
as a magma ocean, formed,” Pachhai says. The “ocean” would have consisted of
rock, gases and crystals suspended in the magma.
The ocean would have sorted itself out as it cooled,
with dense materials sinking and layering on to the bottom of the mantle.
Over the following billions of years, as the mantle
churned and convected, the dense layer would have been pushed into small
patches, showing up as the layered ultra-low velocity zones we see today.
“So the primary and most surprising finding is that
the ultra-low velocity zones are not homogenous but contain strong
heterogeneities (structural and compositional variations) within them,” Pachhai
says. “This finding changes our view on the origin and dynamics of ultra-low velocity
zones. We found that this type of ultra-low velocity zone can be explained by
chemical heterogeneities created at the very beginning of the Earth’s history
and that they are still not well mixed after 4.5 billion years of mantle
convection.”
Not the final
word
The study provides some evidence of the origins of
some ultra-low velocity zones, although there’s also evidence to suggest
different origins for others, such as melting of ocean crust that’s sinking
back into the mantle. But if at least some ultra-low velocity zones are
leftovers from the early Earth, they preserve some of the history of the planet
that otherwise has been lost.
“Therefore, our discovery provides a tool to
understand the initial thermal and chemical status of Earth’s mantle,” Pachhai
says, “and their long-term evolution.”
原始論文：Surya Pachhai,
Mingming Li, Michael S. Thorne, Jan Dettmer, Hrvoje Tkalčić. Internal
structure of ultralow-velocity zones consistent with origin from a basal magma
ocean. Nature Geoscience, 2021; DOI: 10.1038/s41561-021-00871-5
引用自：University of Utah. "Possible chemical
leftovers from early Earth sit near the core."
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