在一團混亂中分離的地核和地函
根據卡內基研究院和史密森尼學會的科學家團隊發表在《自然》(Nature)的新研究，從地函內部湧升至火山熱點的岩石熱柱所含有的證據顯示，地球形成時的那段歲月可能比我們之前認為的還要混亂許多。
科學家已經清楚知道地球是由環繞幼年太陽的物質加積而成。當地球最後成長至足夠大小時密度較高的鐵沉入地球內部而形成地核的原型，剩下富含矽酸鹽的地函漂在上方。
但卡內基研究院的Yingwei
Fei，以及同時隸屬於卡內基研究院和史密森尼學會的Colin Jackson領導的團隊進行的新研究，卻主張地函和地核的分離過程並非很有秩序地進行。
「我們的發現指出雖然地核是從地函分離出來，但地函本身從來沒有完整混合過。」Jackson解釋，「這令人感到十分驚訝。在地球的成長過程中，經歷了早期太陽系的其他天體重重撞擊之後緊接著形成了地核。這些撞擊事件跟之後形成月球的超大撞擊事件頗為相似。過去，科學家大都認為這些撞擊事件產生的巨大能量可以徹底翻攪地函，使得地函成分被攪拌得相當均一。」
讓團隊得出他們假設的關鍵證據來自於夏威夷之類的火山熱點，他們在此找到了獨特且古老的鎢和氙同位素訊號。雖然一般相信地函柱是來自於地函最深的區域，但這些特殊的同位素訊號起源為何仍有很大的爭議。研究團隊認為答案就在氙的母元素――碘――在非常高的壓力之下的化學性質。
同位素是同一元素質子數相同但中子數不同的版本。元素的放射性同位素是不穩定的，比方說碘-129。為了變成穩定狀態，碘-129會衰變成氙-129。因此，在地函柱樣品中的氙同位素訊號和碘在地核跟地函分離時的行為有直接關聯。
Jackson、Fei和共同作者――卡內基研究院的Neil Bennett和Zhixue Du，以及史密森尼學會的Elizabeth Cottrell，利用鑽石高壓砧來重現地球的地核從地函分離時的極端環境，來測量碘在當時如何分散至金屬地核和矽酸鹽質地函。他們也證實了如果地函還在成長時最深處就已經分離出原型地核，則地函中這些區塊擁有的化學性質可以解釋現今觀察到鎢和氙同位素呈現的特殊訊號，代表直到今日這些區塊和地函其他區域始終沒有完全混和。
據Bennett所言：「我們發現碘具有一種重要性質，它在相當高的溫度和壓力之下會開始溶入地核當中。在此極端環境下，碘和鉿(一種會放射性衰變成氙和鎢的元素)對形成地核的金屬會呈現出相反的喜好。這種行為造成的特殊同位素訊號跟我們現今在熱點看到的如出一轍。」
團隊的計算結果也預測鎢和氙的同位素訊號應該跟地函內部密度較高的區塊有關。
「就像是做餅乾的麵糰中加入的巧克力脆片，地函中這些密度較高的區塊非常難以攪拌開來。對於它們蘊含的古老鎢和氙同位素訊號來說，或許這是它們可以保存至今日的重要因素之一。」Jackson解釋。
「讓人更加興奮的是有越來越多地球物理證據顯示，緊鄰在地核上方的地函事實上有些密度較高的區域，它們被稱為超低速帶(ultralow velocity
zones)或是大型低剪力波速群(large low shear velocity provinces)。而我們的研究成果和這些觀察可以緊密結合。」Fei補充，「我們在此研究中發展的方法也讓我們擁有新的機會能夠直接研究在地球深處發生的作用。」
本研究由國家科學基金會、卡內基科學研究院以及史密森尼學會資助。
Earth's core and mantle separated in a disorderly fashion
Plumes of hot rock surging upward from the Earth's mantle at
volcanic hotspots contain evidence that the Earth's formative years may have
been even more chaotic than previously thought, according to new work from a
team of Carnegie and Smithsonian scientists published in Nature.
It is well understood that
Earth formed from the accretion of matter surrounding the young Sun. Eventually
the planet grew to such a size that denser iron metal sank inward, to form the
beginnings of the Earth's core, leaving the silicate-rich mantle floating
above.
But new work from a team
led by Carnegie's Yingwei Fei and Carnegie and the Smithsonian's Colin Jackson
argues that this mantle and core separation was not such an orderly process.
"Our findings suggest
that as the core was extracted from the mantle, the mantle never fully
mixed," Jackson explained. "This is surprising because core formation
happened in the immediate wake of large impacts from other early Solar System
objects that Earth experienced during its growth, similar to the giant impact
event that later formed the Moon. Before now, it was widely thought that these
very energetic impacts would have completely stirred the mantle, mixing all of
its components into a uniform state."
The smoking gun that led
the team to their hypothesis comes from unique and ancient tungsten and xenon
isotopic signatures found at volcanic hotspots, such as Hawaii. Although it was
believed that these plumes originated from the mantle's deepest regions, the
origin of these unique isotopic signatures has been debated. The team believes that
the answer lies in the chemical behavior of iodine, the parent element of
xenon, at very high pressure.
Isotopes are versions of
elements with the same number of protons, but different numbers of neutrons.
Radioactive isotope of elements, such as iodine-129, are unstable. To gain
stability, iodine-129 decays into xenon-129. Therefore, the xenon isotopic signatures
in plume mantle samples are directly related to iodine's behavior during the
period of core-mantle separation.
Using diamond anvil cells
to recreate the extreme conditions under which Earth's core separated from its
mantle, Jackson, Fei, and their colleagues -- Carnegie's Neil Bennett and
Zhixue Du and Smithsonian's Elizabeth Cottrell -- determined how iodine was
partitioning between metallic core and silicate mantle. They also demonstrated
that if the nascent core separated from the deepest regions of the mantle while
it was still growing, then these pockets of the mantle would possess the
chemistry needed to explain the unique tungsten and xenon isotopic signatures,
provided these pockets remained unmixed with the rest of the mantle all the way
up through the present day.
According to Bennett:
"The key behavior we identified was that iodine starts to dissolve into
the core under very high pressures and temperatures. At these extreme
conditions, iodine and hafnium, which decay radioactively to xenon and
tungsten, display opposing preferences for core-forming metal. This behavior
would lead to the same unique isotopic signatures now associated with
hotspots."
Calculations from the team
also predict that the tungsten and xenon isotopic signatures should be
associated with dense pockets of the mantle.
"Like chocolate chips
in cookie batter, these dense pockets of the mantle would be very difficult
stir back in, and this may be a crucial aspect to the retention of their
ancient tungsten and xenon isotopic signatures to the modern day," Jackson
explained.
"Even more exciting is
that there is increasing geophysical evidence that there actually are dense
regions of mantle, resting just above the core -- called ultralow velocity
zones and large low shear velocity provinces. This work ties together these
observations," Fei added. "The methodology developed here also opens
new opportunities for directly studying the deep Earth processes."
This work was supported by
the National Science Foundation, the Carnegie Institution for Science, and the
Smithsonian Institution.
原始論文：Colin R. M.
Jackson, Neil R. Bennett, Zhixue Du, Elizabeth Cottrell & Yingwei
Fei. Early episodes of high-pressure core formation preserved in plume
mantle. Nature, 2018 DOI: 10.1038/nature25446
引用自：Carnegie
Institution for Science. "Earth's core and mantle separated in a
disorderly fashion."
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