發現地函中的地磁來源
研究人員證實赤鐵礦(一種氧化鐵)在地函深處仍有磁性
包圍地球的巨大磁場可以保護地球不受太空來的帶電粒子與輻射傷害，許多動物還能利用地磁來辨認方位。由於地磁隨時都在變動，使得地質學家需要持續監控地磁。長久以來眾所皆知的地磁來源為深達地球內部6000公里的地核；另外一個來源則是地殼，也就是我們腳下踩的土地。而涵蓋地下35至2900公里的地函，大部分都被認為是「毫無地磁活動」。不過，德國、法國、丹麥、美國組成的國際研究團隊，最近證實了一種氧化鐵――赤鐵礦，即使到了地函的深度依然可以保留磁性。這種現象發生於相對低溫的隱沒板塊內部，在西太平洋之下特別容易發現。
地球內部與實驗裝置的示意圖。藍色虛線代表了包圍地球的磁場。研究人員把可以在地函內部找到的赤鐵礦樣品(一種氧化鐵)放在兩顆鑽石之間(如圖片右方)，施予極大的壓力並加熱，藉此模擬地函內部的極端環境。觀察結果顯示在此條件下，赤鐵礦仍然具有磁性。圖片來源：Timofey
Fedotenko
研究第一作者，德國明斯特大學的礦物物理學家Ilya
Kupenko博士表示：「這讓我們對地函以及西太平洋下方具有強力磁場的地區有全新的認識，並為地磁觀測的結果帶來嶄新觀點。」舉例來說，這項新發現可以運用在往後地球觀測到的磁場異常，或者是其他星球的觀測結果，像是火星。火星的發電機已經停擺許久，因此不像地球可以從核心散發並建構出強力的磁場。但有了這項發現之後，或許火星的地函值得進行更加仔細的調查。研究發表於期刊《自然》(Nature)。
研究背景與方法
在地球深部的金屬核心之中，液態鐵合金可以引發電流。而在地球最外層的地殼，岩石則帶有磁性。但到了地球較深的區域，一般認為此處極大的壓力和高溫會讓岩石喪失磁性。
研究人員更加仔細地探究最有可能讓地函具備磁性的物質：氧化鐵。這是因為氧化鐵的臨界溫度(critical
temperature)比較高，也就是需要比較高的溫度才會完全喪失磁性。地函的氧化鐵存在於埋到此處的隱沒板塊內部，它們經由板塊構造運動造成的隱沒作用而來。隱沒板塊可以到達地球內部410至660公里，介於上部地函與下部地函之間的過渡帶。但之前從來沒有人可以在過渡帶極端的壓力和溫度條件下，成功測量出氧化鐵的磁性。
這群科學家結合了兩種實驗方法。首先，他們利用稱為鑽石鉆的儀器，把極其微小的赤鐵礦樣品放在兩顆鑽石之間用力擠壓，同時以雷射加熱，讓壓力達到90
GPa(1 GPa=10億帕斯卡)、溫度超過1000°C
(1,300 K)。接著他們將鑽石鉆結合穆斯堡爾譜儀(Mössbauer
spectroscopy)，運用同步輻射偵測樣品的磁狀態。這部分的研究在法國格勒諾布爾的歐洲同步輻射裝置(ESRF)進行，藉此他們得以偵測氧化鐵的磁序(magnetic
order)變化。
結果令人驚訝。赤鐵礦在高達925°C
(1,200 K)的溫度下依然保有磁性，而西太平洋下方位在過渡帶的隱沒板塊，普遍來說就處於該溫度。明斯特大學礦物研究所的Carmen
Sanchez-Valle教授表示：「因此，我們可以得出地函絕對不像迄今假設的是『毫無地磁活動』。」她接著補充：「這項發現或許也能為觀測整個地球磁場的結果提出合理解釋。」
跟地球磁場和磁極飄移研究的關聯
透過衛星觀測和研究岩石，研究人員可以觀察到整個地球，乃至於局部地區的磁場強度變化。這裡先提供個背景知識：地球的磁極跟地理上的南北極是不同概念，而磁極是會一直移動的。磁極移動的結果是南北極每過一段時間就會交換位置，在近代地球歷史中大概每20萬至30萬年就會發生一次。磁極最近一次反轉發生在距今78萬年前，而數十年來科學家的報告指出地球磁極的移動速度正在加快。如果磁極翻轉勢必會對現代人類文明造成深遠影響。但是控制磁極移動和翻轉的因素，以及磁極會沿著什麼樣的路徑反轉，我們都還沒有完全理解。
觀察結果指出磁極翻轉的其中一條路徑經過了西太平洋，也就是研究人員提出的地函電磁性來源，因此令他們相當注意。他們提出了這個可能性：在太平洋利用衛星輔以岩石紀錄觀察到的磁場變化，並非代表磁極移動路徑從地表觀測到的結果，而是西太平洋下方的地函存在著尚未發現、含有赤鐵礦的岩石，散發出電磁場的訊號。
共同作者，德國拜羅伊特大學巴伐利亞實驗地球化學和地球物理研究所的Leonid
Dubrovinsky教授表示：「現在我們知道地函內部存在著磁有序材料，因此未來分析地球磁場和磁極移動的結果時，都應該將它們的影響考慮進去。」
Magnetism discovered in the Earth’s
mantle
Researchers show that the iron oxide
hematite remains magnetic deep within the Earth’s mantle
The huge magnetic field which surrounds
the Earth, protecting it from radiation and charged particles from space – and
which many animals even use for orientation purposes – is changing constantly,
which is why geoscientists keep it constantly under surveillance. The old
well-known sources of the Earth’s magnetic field are the Earth’s core – down to
6,000 kilometres deep down inside the Earth – and the Earth’s crust: in other
words, the ground we stand on. The Earth’s mantle, on the other hand,
stretching from 35 to 2,900 kilometres below the Earth’s surface, has so far
largely been regarded as “magnetically dead”. An international team of
researchers from Germany, France, Denmark and the USA has now demonstrated that
a form of iron oxide, hematite, can retain its magnetic properties even deep
down in the Earth’s mantle. This occurs in relatively cold tectonic plates,
called slabs, which are found especially beneath the western Pacific Ocean.
“This new knowledge about the Earth’s mantle and the
strongly magnetic region in the western Pacific could throw new light on any
observations of the Earth’s magnetic field,” says mineral physicist and first
author Dr. Ilya Kupenko from the University of Münster (Germany). The new
findings could, for example, be relevant for any future observations of the
magnetic anomalies on the Earth and on other planets such as Mars. This is
because Mars has no longer a dynamo and thus no source enabling a strong
magnetic field originating from the core to be built up such as that on Earth.
It might, therefore, now be worth taking a more detailed look on its mantle.
The study has been published in the “Nature”
journal.
Background and
methods used:
Deep in the metallic core of the Earth, it is liquid
iron alloy that triggers electrical flows. In the outermost crust of the Earth,
rocks cause magnetic signal. In the deeper regions of the Earth's interior,
however, it was believed that the rocks lose their magnetic properties due to
the very high temperatures and pressures.
The researchers now took a closer look at the main
potential sources for magnetism in the Earth’s mantle: iron oxides, which have
a high critical temperature – i.e. the temperature above which material is no
longer magnetic. In the Earth’s mantle, iron oxides occur in slabs that are
buried from the Earth’s crust further into the mantle, as a result of tectonic
shifts, a process called subduction. They can reach a depth within the Earth’s
interior of between 410 and 660 kilometres – the so-called transition zone
between the upper and the lower mantle of the Earth. Previously, however, no
one had succeeded in measuring the magnetic properties of the iron oxides at
the extreme conditions of pressure and temperature found in this region.
Now the scientists combined two methods. Using a
so-called diamond anvil cell, they squeezed micrometric-sized samples of iron
oxide hematite between two diamonds, and heated them with lasers to reach
pressures of up to 90 gigapascal and temperatures of over 1,000 °C (1,300 K).
The researchers combined this method with so-called Mössbauer spectroscopy to
probe the magnetic state of the samples by means of synchrotron radiation. This
part of the study was carried out at the ESRF synchrotron facility in Grenoble,
France, and this made it possible to observe the changes of the magnetic order
in iron oxide.
The surprising result was that the hematite remained
magnetic up to a temperature of around 925 °C (1,200 K) – the temperature
prevailing in the subducted slabs beneath the western part of Pacific Ocean at
the Earth’s transition zone depth. “As a result, we are able to demonstrate
that the Earth’s mantle is not nearly as magnetically ‘dead’ as has so far been
assumed,” says Prof. Carmen Sanchez-Valle from the Institute of Mineralogy at
Münster University. “These findings might justify other conclusions relating to
the Earth’s entire magnetic field,” she adds.
Relevance for
investigations of the Earth's magnetic field and the movement of the poles
By using satellites and studying rocks, researchers
observe the Earth’s magnetic field, as well as the local and regional changes
in magnetic strength. Background: The geomagnetic poles of the Earth – not to
be confused with the geographic poles – are constantly moving. As a result of
this movement they have actually changed positions with each other every
200,000 to 300,000 years in the recent history of the Earth. The last poles
flip happened 780,000 years ago, and last decades scientists report
acceleration in the movement of the Earth magnetic poles. Flip of magnetic
poles would have profound effect on modern human civilisation. Factors which
control movements and flip of the magnetic poles, as well as directions they
follow during overturn are not understood yet.
One of the poles’ routes observed during the flips
runs over the western Pacific, corresponding very noticeably to the proposed
electromagnetic sources in the Earth’s mantle. The researchers are therefore
considering the possibility that the magnetic fields observed in the Pacific
with the aid of rock records do not represent the migration route of the poles
measured on the Earth's surface, but originate from the hitherto unknown
electromagnetic source of hematite-containing rocks in the Earth's mantle
beneath the West Pacific.
“What we now know – that there are magnetically
ordered materials down there in the Earth’s mantle – should be taken into
account in any future analysis of the Earth’s magnetic field and of the
movement of the poles,” says co-author Prof. Leonid Dubrovinsky at the Bavarian
Research Institute of Experimental Geochemistry and Geophysics at Bayreuth
University.
原始論文：I. Kupenko, G.
Aprilis, D. M. Vasiukov, C. McCammon, S. Chariton, V. Cerantola, I. Kantor, A.
I. Chumakov, R. Rüffer, L. Dubrovinsky, C. Sanchez-Valle. Magnetism in
cold subducting slabs at mantle transition zone depths. Nature,
2019; 570 (7759): 102 DOI: 10.1038/s41586-019-1254-8
引用自：University of Münster. "New findings on
Earth's magnetic field."
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