科學家運用歐洲同步輻射裝置(ESRF)的ID21光束線，研究古代岩漿裡的鋯石晶體所含有的磷灰石包裹體，結果揭曉了關於大氧化事件的重要訊息。這項成果發表於《自然—地球科學》(Nature Geosciences)。
用ESRF的ID21光束線測量鋯石內部的磷灰石包裹體的硫物種。大氧化事件前後，硫的能譜從還原硫(S2-)轉變成氧化硫(S6+)。作者主張原因為被空氣置換過的沉積物滲入地函，使得岩漿的氧化還原狀態有所改變。圖片來源：ESRF
距今24億年前左右，發生了地球歷史上一起至關重要的事件：大氧化事件。這段期間大氣層累積了相當大量的氧氣。飆升的氧氣不僅使大氣成分出現了巨大的變化，也改變了地球的化學性質。隨著氧濃度上升，更複雜的多細胞生命形式得以發展並徹底改變了地球的生態系，使得該事件成為地球歷史上的轉捩點。
板塊構造運動是種相當有效率的機制，讓元素可以在地表、大氣和地函之間循環交換。當高山跟水與空氣發生反應而受到風化侵蝕作用，就會崩解成沉積物。這些沉積物有部分可以經由隱沒作用(一個板塊下潛到另一個板塊之下的過程)回到地函。在隱沒帶上方由地函形成的岩漿提供了獨一無二的機會，讓我們探討藉著隱沒的沉積物跟地函物質發生的同化作用(assimilation)，大氣如何影響地函的成分，進而瞭解這種有趣的地質關係。
長久以來，科學家一直在試著釐清地函和大氣之間的交互作用。光是討論現今的地球這項任務就已經複雜到難以達成，對於大氣和板塊構造運動快速變化的早期地球來說更是困難無比。最近蒙彼利埃大學和樸次茅斯大學領導的團隊發現了一種方法來克服其中的阻礙：研究來自隱沒帶的鋯石所含有的磷灰石包裹體。
「2017年，一篇關於磷灰石的論文發現在還原條件，也就是很少或者幾乎沒有自由氧可供發生化學反應的條件下成長出來的磷灰石，其中的硫會呈現出一種非常特殊的訊號。然而，如果磷灰石是在氧化條件下結晶出來，其中的硫看起來就會相當不一樣。這意味著磷灰石可以當作一種氧化還原條件的代用指標，」Hugo
Moreira表示。他是蒙彼利埃大學在CNRS的博士後研究員，也是本論文的第一作者。
Moreira和同僚決定探討遠古隱沒帶形成的岩漿，從中結晶出來的鋯石顆粒所含有的磷灰石(一種磷酸鹽礦物)包裹體。方法是在ESRF的ID21光束線，運用X光吸收近緣結構原理來測量這些磷灰石中不同價數的硫物種。由於團隊之前從未用過ESRF，因此由該單位的科學家來指導實驗就相當重要。然而，當時的時機卻不太好，「這是我們在COVID-19疫情期間以遠端模式進行的第一個實驗——不過這也是一個機會，讓我們可以演練之後要進行的遠端實驗，」負責管理ID21光束線的科學家Marine
Cotte解釋。Moreira補充：「由於各種後勤支援的問題加上我們從未用過同步光子設施，因此過程十分繁複，但最後我們還是得到了想要的的結果。這都得歸功於在非常複雜的情形下，ESRF的人員仍然展現出他們的專業知識以及職業素養。」
不同種類的硫與磷灰石結合的程度基本上取決於岩漿的氧逸度(fugacity)，因此這項特質在推測岩漿系統演化過程中的氧化還原狀態是相當理想的工具。「我們利用的是鋯石裡的磷灰石包裹體而非岩石基質裡的磷灰石，這是最關鍵的一點。因為極為堅固的鋯石晶體可以保護包裹體，使其原始成分能夠保存下來，」Moreira解釋。
實驗結果顯示從岩漿結晶出來的鋯石裡的磷灰石包裹體，在大氧化事件之前含有的硫相對來說較為還原，而在大氧化事件之後則較為氧化。分析鋯石顯示事件前後的岩漿有相似的來源，不過較晚的樣品則有更多來自沉積物的成分。總而言之，結果清楚指出氧氣逐漸增多的大氣影響了沉積物，沉積物又改變了地函並且讓岩漿的逸度朝向更加氧化的狀態。
「我們的研究顯示在界定重要的岩漿參數時，運用同步輻射X光源來探討磷灰石包裹體是相當有力的工具，」Moreira總結。
團隊的下一步是研究地球歷史上其他關鍵時期結晶出來的岩漿，比方說開始於8億5000萬年前的新元古代氧化事件，以及第一道氧氣的跡象是出現在太古宙的什麼時間。
New research reveals Earth's ancient
'breath'
Scientists have unveiled important
information on The Great Oxidation Event by studying apatite inclusions in
zircon crystals from old magmas at the ESRF’s ID21 beamline. The results are
published in Nature Geosciences.
Around 2.4 billion years ago, a pivotal moment in
Earth's history took place: The Great Oxidation Event. During this period, a
significant amount of oxygen accumulated in the atmosphere. This surge in
oxygen production led to a dramatic shift in the composition of the atmosphere,
altering the chemistry of the planet. The event marked a turning point as
oxygen levels rose, enabling the development of more complex multicellular life
forms and fundamentally reshaping Earth's ecosystems.
Plate tectonics are an effective mechanism for the
cycling and interchange of elements among Earth's surface, atmosphere and
mantle. As mountains undergo weathering and erosion through interactions with
water and the atmosphere, they break down into sediments. These sediments are
then partially returned to the mantle through subduction processes (one
tectonic plate sinking beneath another). The formation of magmas in the mantle
above subduction zones provides a unique opportunity to explore how the
atmosphere could have impacted the mantle by assimilating materials from subducted
sediments, offering insights into this intriguing geological relationship.
Scientists have long tried to study the interaction
between atmosphere and the Earth’s mantle. The mission is already complicated
to accomplish in the modern Earth, and even more so in the early Earth, when
the atmosphere and plate tectonics were changing at rapid rates. Now, a team
led by the University of Montpellier and University of Portsmouth have found a
way to overcome obstacles by studying apatite inclusions in zircon from
subduction zones.
“In 2017, a paper on the mineral apatite unveiled
that when it grows at reduced conditions, meaning there is little or no free
oxygen for chemical reactions, its sulphur would show a very specific
signature. However, if it crystallised in oxidised conditions, the sulphur
inside the apatite would look very different. This means that apatite is a
proxy for redox conditions,” explains Hugo Moreira, a CNRS postdoctoral
researcher at the University of Montpellier and first author of the paper.
Moreira and colleagues decided to explore inclusions
of phosphate-mineral apatite in zircon grains that are crystallised in magmas
formed in an ancient subduction zone, and measured their sulphur valence
speciation using X-ray absorption near-edge structure (XANES) at the ESRF’s
ID21 beamline. The team had never used the ESRF before, so guidance in the
experiment from the ESRF scientists was crucial. However, the timing was not
the best: “The experiment was the first one we carried out in the middle of the
COVID-19 pandemic, in remote mode, but it served as a testbed for the future
experiments we did in remote,” explains Marine Cotte, scientist in charge of
the beamline. Moreira adds: “It was complicated because of all the logistical issues,
and because we’d never used a synchrotron before, but the results were exactly
what we needed, and this is thanks to the expertise and professionalism of the
staff, in very complex circumstances.”
Sulphur incorporation and speciation in apatite is
intrinsically dependent on the oxygen fugacity of the magma and therefore ideal
for assessing the oxidation state during the evolution of magmatic systems.
“Using apatite inclusions in zircons rather than apatite from the rock matrix
was paramount, as the inclusions had been shielded by the extremely robust
zircon crystals, preserving their original composition,” explains Moreira.
The experiment results show that apatite inclusions
in zircons from magmas that crystallised prior to the Great Oxidation Event have
a relatively reduced sulphur redox state, whereas after the Great Oxidation
Event they are more oxidised (Figure 1). The analysis of the zircon shows that
these magmas shared a similar source and that the younger samples had
incorporated a sediment component. Overall, the clear implication is that
sediments affected by an increasingly oxidised atmosphere modified the mantle
and shifted the fugacity of magmas towards more oxidised conditions.
“Our study shows that investigating apatite
inclusions in zircon using synchrotron X-rays is a powerful tool to constrain a
critical magma parameter,” concludes Moreira.
The next step for the team is to study other magmas
that crystallised in key periods of Earth’s history, such as the Neoproterozoic
Oxidation Event (beginning 850 million years ago) and when the first signs of
oxygen emerged in the Archaean period.
原始論文：Hugo Moreira,
Craig Storey, Emilie Bruand, James Darling, Mike Fowler, Marine Cotte, Edgar E.
Villalobos-Portillo, Fleurice Parat, Luís Seixas, Pascal Philippot, Bruno
Dhuime. Sub-arc mantle fugacity shifted by sediment recycling across
the Great Oxidation Event. Nature Geoscience, 2023; DOI: 10.1038/s41561-023-01258-4
引用自：European Synchrotron Radiation Facility.
"New research reveals Earth's ancient 'breath'."
沒有留言:
張貼留言