原文網址：https://carnegiescience.edu/news/modern-volcanism-tied-events-occurring-soon-after-earth%E2%80%99s-birth
於近代發生的火山活動跟地球誕生之後不久的事件有所關聯
印度洋上的留尼旺島是由火山熱點形成。根據卡內基科學研究所的Bradley
Peters、Richard
Carlson、Mary
Horan，以及斯克里普斯海洋研究所的James Day發表在《自然》(Nature)的新成果，此熱點的高溫岩漿柱是從地下深處成分異常原始的地方湧升而成。
留尼旺島標示出一處曾在6600萬年前噴發的熱點當今的所在位置。該次噴發產生的洪流玄武岩形成了覆蓋印度大部分地區的德干高原，而恐龍的滅絕它可能也參與了一部分。相較於地表多數火山，一般認為洪流玄武岩和其他熱點產生的岩漿係源自於地球深部的不同位置。因此研究這些物質或許可以幫助科學家更加瞭解我們居住的地球是如何演變。
地球形成過程中的高熱使得當時的地球處於大規模的熔融狀態，造成地球分離成兩層――密度高的金屬往內沉至核心位置形成地核，留下浮於上方富含矽酸鹽的地函。
在地球接下來45億年的演變過程中，地函深部上湧、熔化，然後再次因為密度而分離，形成地殼並在整個過程中改變地球內部的化學成分。今日，地殼正沿著太平洋的邊界沉回地球內部，此種現象會讓地函緩緩流動而攪動這些物質，使得地殼特有的化學成分重新回到地球深處。
但是，地函並非全都如此過程所述的經過充分混和。有些年代較老的區塊至今仍然存在――就像未經充分攪拌的蛋糕麵糊中出現的麵粉團塊一樣。分析留尼旺島火山岩的化學成分，顯示它們的來源不同於地函今日其他混和良好的地方。
研究團隊利用新的同位素數據，顯示產生留尼旺島岩漿的區域是獨立於範圍較廣、充分混和的地函之外。這些獨立存在的區塊形成的時間早於地球歷史的最初十分之一。
同位素是一群質子數量相同，但是中子數量卻不同的元素。有時原子核含有的中子數量會讓同位素呈現不穩定的狀態。為了變成穩定狀態，這類同位素會經由放射性衰變的過程來釋放出帶有能量的粒子。同位素的質子和中子數量在過程中會有所變化，使它們轉變成另一種元素。這項新研究利用此種作用留下的線索來得到這些特殊地函區塊的年代以及歷史。
釤-146就是一種不穩定的(即「放射性」)同位素，它會衰變成釹-142，半衰期僅有1.03億年。雖然地球剛形成時帶有釤-146，但它很快地就會在地球的幼年期消失殆盡。這意謂釹-142可以詳細紀錄地球最早期的歷史，卻不會記下釤-146通通轉變成釹-142之後的地球歷史。在地球45億年的歷史當中，只有最初5億年在地函發生的成分變化，才會造成釹-142跟其他釹同位素含量的比例出現差異。
分析留尼旺島火山岩中釹-142跟釹-144的比例，配合在實驗室進行的模擬研究，指出地函的混和作用儘管已經進行數十億年之久，留尼旺島地函柱的岩漿可能是來自於地函一處保存良好的區塊，曾經歷地函最初發生的大規模熔融作用使成分有所改變。
研究團隊的發現也有助於科學家解釋正好位在地核和地核邊界的高密度區域起源為何。這類區域稱為大型低剪力波速群(large low shear velocity provinces, LLSVP)與超低速帶(ultralow velocity
zones ,ULVZ)，當地震波經過深部地函的這些區域時波速通常會下降。它們也許是地球早期的熔融事件殘留下來的痕跡。
主要作者Peters表示：「這些熱點地函柱中保留下來的地函分化事件，可以讓我們更加瞭解早期地球的地球化學作用，並且解釋那些位在地函深處的高密度區域產生的謎樣地震波訊號。」
Modern volcanism tied to
events occurring soon after earth’s birth
Plumes of hot magma from the volcanic
hotspot that formed Réunion Island in the Indian Ocean rise from an unusually
primitive source deep beneath the Earth’s surface, according to new work in Nature from
Carnegie’s Bradley Peters, Richard Carlson, and Mary Horan along with James Day
of the Scripps Institution of Oceanography.
Réunion
marks the present-day location of the hotspot that 66 million years ago erupted
the Deccan Traps flood basalts, which cover most of India and may have
contributed to the extinction of the dinosaurs. Flood basalts and other hotspot
lavas are thought to originate from different portions of Earth’s deep interior
than most volcanoes at Earth’s surface and studying this material may help
scientists understand our home planet’s evolution.
The
heat from Earth’s formation process caused extensive melting of the planet,
leading Earth to separate into two layers when the denser iron metal sank
inward toward the center, creating the core and leaving the silicate-rich
mantle floating above.
Over
the subsequent 4.5 billion years of Earth’s evolution, deep portions of the
mantle would rise upwards, melt, and then separate once again by density,
creating Earth’s crust and changing the chemical composition of Earth’s
interior in the process. As crust sinks back into Earth’s interior—a
phenomenon that’s occurring today along the boundary of the Pacific Ocean—the
slow motion of Earth’s mantle works to stir these materials, along with their
distinct chemistry, back into the deep Earth.
But
not all of the mantle is as well-blended as this process would indicate. Some
older patches still exist—like powdery pockets in a poorly mixed bowl of cake
batter. Analysis of the chemical compositions of Réunion Island volcanic rocks
indicate that their source material is different from other, better-mixed parts
of the modern mantle.
Using
new isotope data, the research team revealed that Réunion lavas originate from
regions of the mantle that were isolated from the broader, well-blended mantle.
These isolated pockets were formed within the first ten percent of Earth’s
history.
Isotopes
are elements that have the same number of protons, but a different number of
neutrons. Sometimes, the number of neutrons present in the nucleus make an
isotope unstable; to gain stability, the isotope will release energetic
particles in the process of radioactive decay. This process alters its number
of protons and neutrons and transforms it into a different element. This new
study harnesses this process to provide a fingerprint for the age and history
of distinct mantle pockets.
Samarium-146
is one such unstable, or radioactive, isotope with a half-life of only 103
million years. It decays to the isotope neodymium-142. Although samarium-146
was present when Earth formed, it became extinct very early in Earth’s infancy,
meaning neodymium-142 provides a good record of Earth’s earliest history, but
no record of the Earth from the period after all the samarium-146 transformed
into neodymium-142. Differences in the abundances of neodymium-142 in
comparison to other isotopes of neodymium could only have been generated by changes
in the chemical composition of the mantle that occurred in the first 500
million years of Earth’s 4.5 billion-year history.
The
ratio of neodymium-142 to neodymium-144 in Réunion volcanic rocks, together
with the results of lab-based mimicry and modeling studies, indicate that
despite billions of years of mantle mixing, Réunion plume magma likely
originates from a preserved pocket of the mantle that experienced a
compositional change caused by large-scale melting of the Earth’s earliest
mantle.
The
team’s findings could also help explain the origin of dense regions right at
the boundary of the core and mantle called large low shear velocity provinces
(LLSVPs) and ultralow velocity zones (ULVZs), reflecting the unusually slow
speed of seismic waves as they travel through these regions of the deep mantle.
Such regions may be relics of early melting events.
“The
mantle differentiation event preserved in these hotspot plumes can both teach
us about early Earth geochemical processes and explain the mysterious seismic
signatures created by these dense deep-mantle zones,” said lead author Peters.
原始論文：Bradley J.
Peters, Richard W. Carlson, James M. D. Day, Mary F. Horan. Hadean
silicate differentiation preserved by anomalous 142Nd/144Nd ratios in the
Réunion hotspot source. Nature, 2018; 555 (7694): 89 DOI: 10.1038/nature25754
引用自：Carnegie Institution for Science. "Modern volcanism tied to events
occurring soon after Earth's birth: Primordial mantle pockets preserved under
Réunion Island for billions of years."
