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

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."