鐵的重同位素正從地核漏出
By Becky Oskin
研究人員分析鐵在地球內部的行為模式之後，認為鐵可能正從地核熔化的部分漏出。
地核液態鐵的部分和岩石組成的地函之間的邊界大約位在地表下方2900公里處。在此交界，溫度從較高的地核過渡至較低的地函時下降了一千多度。
新研究提出鐵的重同位素會往低溫的地方遷移，也就是地函；於此同時輕同位素則會循環回地核(同一種元素的同位素擁有的中子數量不同，使它們的質量有輕微差異)。此效應會讓地核的物質滲透到地函最底部，使得此處富含鐵的重同位素。
加州大學戴維斯分校的地質學名譽教授暨丹麥奧胡斯大學地球系統岩石學的教授Charles
Lesher是主要作者。他說：「如果這項理論正確無誤，就能讓我們更加了解地核和地函之間的交互作用。」
滲漏的地核
Lesher表示在解讀地函深處的震波影像，以及模擬熱能和化學物質如何在地球深處和表面之間傳遞時，了解核幔邊界進行的物理作用是相當重要的。
Lesher和同事在高溫高壓的環境下進行實驗，藉此分析鐵同位素在不同溫度區域之間的移動過程。Lesher表示他們的發現可以解釋鐵的重同位素在地函岩石內的含量為什麼會比球粒隕石，也就是太陽系早期遺留下來的原始物質裡面還多。
他說：「如果沒錯的話，這項結果代表數十億年來鐵一直從地核滲漏到地函之中。」
研究團隊的電腦模擬結果顯示地核物質甚至可以混入高溫、上湧的地函柱而運送到地表。海裡的某些熱點，像是薩摩亞和夏威夷噴發出的岩漿富含鐵的重同位素，Lesher和研究團隊認為這就是地核的鐵漏出來的跡象。
研究4月6日發表在期刊《自然—地球科學》(Nature
Geoscience)。
研究經費來自美國國家科學基金會、丹麥國家研究基金會的Niels
Bohr地球科學教授基金、加拿大自然科學與工程研究委員會。
Heavy iron isotopes
leaking from Earth’s core
Earth’s molten core may be leaking iron,
according to researchers who analyzed how iron behaves inside our planet.
The boundary between the liquid iron core and the
rocky mantle is located some 1,800 miles (2,900 km) below Earth’s surface. At
this transition, the temperature drops by more than a thousand degrees from the
hotter core to the cooler mantle.
The new study suggests heavier iron isotopes migrate
toward lower temperatures — and into the mantle — while lighter iron isotopes
circulate back down into the core. (Isotopes of the same element have different
numbers of neutrons, giving them slightly different masses.) This effect could
cause core material infiltrating the lowermost mantle to be enriched in heavy
iron isotopes.
“If correct, this stands to improve our understanding
of core–mantle interaction,” said Charles Lesher, lead author, professor
emeritus of geology at UC Davis and professor of earth system petrology at
Aarhus University in Denmark.
Leaky at the core
Understanding the physical processes operating at the
core–mantle boundary is important for interpreting seismic images of the deep
mantle, as well as modeling the extent of chemical and thermal transfer between
the deep Earth and surface of our planet, Lesher said.
Lesher and his colleagues analyzed how iron isotopes
move between areas of different temperatures during experiments conducted under
high temperature and pressure. Their findings can explain why there are more
heavy iron isotopes in mantle rocks than in chondrite meteorites, the
primordial material from the early solar system, Lesher said.
“If true, the results suggest iron from the core has
been leaking into the mantle for billions of years,” he said.
Computer simulations performed by the research team
show this core material can even reach the surface, mixed with and transported
by hot, upwelling mantle plumes. Some lavas erupted at oceanic hot spots such
as Samoa and Hawaii are enriched in heavy iron isotopes, which Lesher and the
team propose could be a signature of a leaky core.
The study was published April 6 in the journal Nature Geoscience.
This research was funded by the National Science
Foundation, the Niels Bohr Professorship in Geoscience from the Danish National
Research Foundation, and The Natural Sciences and Engineering Research Council
of Canada.
原始論文：Charles E. Lesher, Juliane Dannberg, Gry H.
Barfod, Neil R. Bennett, Justin J. G. Glessner, Daniel J. Lacks, James M.
Brenan. Iron
isotope fractionation at the core–mantle boundary by thermodiffusion. Nature Geoscience, 2020; DOI: 10.1038/s41561-020-0560-y
引用自：University of California - Davis. "Heavy
iron isotopes leaking from Earth's core."
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