地球化學家對地函成份有新的測量結果
研究人員認為地球表面和地函之間的互動比過往推測的還要活躍。
這顆橄欖石含有只有幾微米大小的熔體包裹體(黑色斑點)。地球化學家將包裹體分離出來之後，運用質譜儀來分析它們的同位素組成。圖片來源：明斯特大學/Felix
Genske
地球內部的化學成分是什麼？由於我們無法鑽到地球內部超過十公里深的地方，因此我們所知的相關訊息通常來自於地球深部融化之後形成的火山岩。德國明斯特大學和荷蘭阿姆斯特丹大學的地球化學家，最近探討了構成葡萄牙亞速群島的火山岩，目標是要蒐集新的資訊以了解地球內部大約30公里至2900公里深，稱為地函的區域其成分如何演變。他們運用極為精確的分析技術，發現亞速群島下方的地函成分跟過去認為的不同――結果顯示此處大部分區域含有的不相容元素(incompatible
element)少得驚人。這類元素因為地函持續熔融而累積在地球最外層的固體地殼當中。
研究人員的結論認為在地球歷史上熔化的地函――也就是最終形成的地殼比過往認為的還多。明斯特大學的教授Andreas
Stracke是研究主持人，他說：「要讓地函和地殼的物質達到收支平衡，地表和地球內部的物質勢必得以更高的速率流通才行。」
雖然亞速群島的岩石是從地函相當深的地方上來，但是其成分出乎意料地和大部分的上部地函相當類似，因此地函整體成分可能跟現有的認知不同。「我們的結果開拓出一種全新觀點，」Andreas
Stracke表示，「現在必須重新估算地球最大的區塊成分為何，畢竟地函佔了地球體積的80%以上。」研究發表於期刊《自然―地球科學》(Nature Geoscience)。
背景與方法
這群地球化學家在研究中探討了橄欖石(一種礦物)和其中的熔體包裹體(melt
inclusion)，也就是熔岩噴出前在橄欖石結晶過程中被包覆住的岩漿。研究人員挑出這些只有幾微米大小的熔體包裹體，接著用化學方法溶解它們，分離出特定的化學元素。這些元素在一生之中以及來到地表的過程中會因為放射性衰變而有所改變。它們可能得花費數億、甚至數十億年才能從地球內部數千公里深的地方來到地表。
研究人員利用極為靈敏的質譜儀來分析岩漿的同位素組成，這種方法可以讓他們測出同一元素當中的不同原子，也就是同位素的相對含量。共同作者，明斯特大學礦物學研究所的Felix
Genske博士負責研究中的大多數分析工作。他說：「由於我們的測量方法相當有效率，使我們只要有十億分之一克的元素就能分析同位素組成。」研究人員藉由分析結果可以間接得知有關地函組成物質的資訊，結果顯示它含有的稀土元素(像是釤和釹)，以及化學性質類似的元素(像是釷和鈾)皆相當稀少。
Andreas Stracke表示：「根據別處擁有類似地球化學數據的火山岩，比如說夏威夷，地函其他部分的組成中，極度缺乏不相容元素的物質佔的比例或許也相當高。」研究人員推測不相容元素在全世界的虧損狀況，或許要由富含這類元素的地殼以更高的速率回到地函來彌補。研究人員接下來會繼續探討世上其他火山島的樣品，希望可以證明他們的現行假說。
Geochemists measure new composition of
Earth’s mantle
Researchers suspect greater dynamics than
previously assumed between the Earth’s surface and its mantle.
What is the chemical composition of the Earth’s
interior? Because it is impossible to drill more than about ten kilometres deep
into the Earth, volcanic rocks formed by melting Earth’s deep interior often
provide such information. Geochemists at the Universities of Münster (Germany)
and Amsterdam (Netherlands) have investigated the volcanic rocks that build up
the Portuguese island group of the Azores. Their goal: gather new information
about the compositional evolution of the Earth’s mantle, which is the layer
roughly between 30 and 2,900 kilometres deep inside the Earth. Using
sophisticated analytical techniques, they discovered that the composition of
the mantle below the Azores is different than previously thought –suggesting
that large parts of it contain surprisingly few so-called incompatible
elements. These are chemical elements which, as a result of the constant
melting of the Earth’s mantle, accumulate in the Earth’s crust, which is
Earth’s outermost solid layer.
The researchers conclude that, over Earth’s history,
a larger amount of Earth’s mantle has melted – and ultimately formed the
Earth’s crust – than previously thought. “To sustain the material budget
between Earth’s mantle and crust, mass fluxes between the surface and Earth’s
interior must have operated at a higher rate,” says Münster University’s Prof.
Andreas Stracke, who is heading the study.
As the material below the Azores rises from very deep
within Earth’s mantle – and is unexpectedly similar to most of its upper part –
the composition of Earth’s entire mantle may differ from current thinking. “Our
results have opened up a new perspective,” says Andreas Stracke, “because we
will now have to reassess the composition of the largest part of the Earth – after
all, Earth’s mantle accounts for over 80 percent of Earth’s volume.” The study
has been published in the journal “Nature
Geoscience”.
Background and
method:
In their study, the geochemists examined the mineral
olivine and its melt inclusions, i.e. magma encapsulated during the
crystallisation of olivine before the lavas erupted. The researchers isolated
these melt inclusions, just a few micrometers in size, dissolved them
chemically and separated certain chemical elements. These elements are altered
by radioactive decay during their lifetime and ascent from Earth's interior –
travelling over thousands of kilometres for hundreds or even thousands of
millions of years.
The researchers analysed the isotopic composition of
the melts with highly sensitive mass spectrometers. Such methods allow
measurement of the relative abundance of different atoms in an element –
so-called isotopes. “Owing to the high efficiency of our measurements, we were
able to analyse the isotopic composition of one billionth of a gram of the
element,” says co-author Dr. Felix Genske from the University of Münster’s
Institute of Mineralogy, who carried out most of the analytical work. In this
way, the researchers indirectly obtained information on the composition of the
material in the Earth’s mantle: the isotope analyses showed that it contains
far fewer rare Earth elements such as samarium and neodymium, but also of
chemically similar elements such as thorium and uranium.
“On the basis of similar geochemical data in volcanic
rocks from different regions, e.g. Hawaii, other parts of the Earth’s mantle
may also contain a higher proportion of material that is strongly depleted in
incompatible elements,” says Andreas Stracke. The researchers presume that this
global deficit may be compensated by a higher rate of recycling Earth’s
incompatible element-rich crust back into Earth’s mantle. With their continuing
studies the researchers want to confirm their working hypothesis by
investigating samples from other volcanic islands across the globe.
原始論文：Andreas Stracke, Felix Genske, Jasper Berndt,
Janne M. Koornneef. Ubiquitous
ultra-depleted domains in Earth’s mantle. Nature Geoscience,
2019; DOI: 10.1038/s41561-019-0446-z
引用自：University of Münster. "Geochemists
measure new composition of Earth's mantle.”
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