火山弧是由混雜的岩石於深處熔融而成

原文網址：www.sciencedaily.com/releases/2017/04/170407143316.htm

火山弧是由混雜的岩石於深處熔融而成

研究改變了我們對於隱沒帶內部發生作用的理解

在海洋底部，巨大的板塊彼此之間發生碰撞及摩擦，使得其中一方潛入至另一方之下。這種稱為隱沒的強力擠壓作用是火山弧的成因。火山弧是地球某些最強烈地質事件的發源處，像是劇烈的火山噴發以及超級地震。

刊登在期刊《科學前緣》(Science Advances)的新研究挑戰了我們對於火山弧熔岩形成方式的理解，或許對地震和火山爆發災害的研究有所啟發。

伍茲霍爾海洋研究所(WHOI)的研究人員領導的新研究發現了一種前所未知的作用，跟一種十分混亂的變質岩――稱作「混雜岩」(mélange)有關。混雜岩是在隱沒過程中，受到板塊跟地函邊界的強烈壓力而形成。

到目前為止，科學家長久以來認為熔岩的形成過程一開始是隱沒板塊釋出的水分，跟熔化的沉積物一同滲入地函。它們進入地函之後就會互相混合並引發更多熔融作用，最後噴發到地表。

「我們的研究清楚顯示現行的液體/沉積物熔融模型並不正確。」WHOI的地質學家，同時也是本論文的主要作者Sune Nielsen表示，「這項結論十分重要，因為最近20年對於隱沒帶的地球物理和地球化學資料，幾乎都是根據此模型來詮釋它們的意義。」

反之，Nielsen和他的同事發現在跟地函混和之前，混雜岩實際上已經在隱沒板塊的頂部先行參與作用了。

Nielsen說：「這是第一次有研究顯示混雜岩的熔融作用，才是隱沒板塊跟地函之間發生交互作用的主要驅動力。」

這項區別十分重要，因為科學家利用分析同位素和稀有元素來測定火山弧熔岩的成分，並藉此更加瞭解隱沒帶的重要事件發生區域。稀有元素混和、融化以及重新分布的發生時間和位置會使得同位素記號的比例產生重大差異。

此研究奠基於一篇之前出版的論文，作者為德國法蘭克福歌德大學的教授Horst Marschall，他是Nielsen的同事和此研究的共同作者。根據對混雜岩露頭進行的野外觀察，Marschall注意到稱作貫入體(diapir)――由混雜岩物質形成的低密度液滴，可能會從隱沒板塊的表面緩緩上升，而將混和均勻的物質帶到火山弧之下的地函。

「混雜岩—灌入體模型的想法來自於電腦模型和在世界各地進行的詳細野外工作。這些地方的岩石形成於地底深處隱沒板塊跟地函的交界，之後由板塊構造運動的力量帶到地表。」Marschall表示，「我們討論此模型到現在至少已經有5年之久，但許多科學家認為混雜岩在產生岩漿的過程中無足輕重。他們將此模型貶抑為『空想地質』。」

在他們的新研究中，Nielsen和Marschall拿兩種模型產生的混和比例對照至之前出版的論文中，全球八個代表性火山弧的化學和同位素組成數據。這八個火山弧分別是：馬里亞納群島、東加、小安地列斯群島、阿留申群島、琉球、斯科細亞、千島群島和巽他。

「我們的大規模分析顯示混雜岩模型跟文獻中世界各地火山弧的數據幾乎是完美地吻合，但現行的沉積物熔融/液體模型產生的混合曲線，在圖上卻跟實際數據相差甚遠。」

有很多理由可以說明為何瞭解在隱沒帶發生的作用相當重要。隱沒帶時常被稱作地球的引擎，此處為老舊海洋板塊中含有的二氧化碳和水重新循環回地球深處的主要地區，因而在長期氣候的調控和地球熱平衡的演化上具有重要地位。

雖然這些複雜作用的發生尺度是以數十至數千公里為範圍，數月至數億年為時間來計算，但卻能產生過程僅僅數秒的毀滅性大地震與致命海嘯。

「地球上的地震和火山災害有很大一部份都跟隱沒帶有關，而有些隱沒帶即位於有數億人居住的區域附近，像是印尼。」Nielsen表示，「要瞭解地震發生的原因和地點，就必須知道在那下方到底存在著什麼樣的物質，以及發生的作用為何。」

研究團隊表示此研究的發現提醒科學家需要重新審視前人發表的數據，並重新考量跟隱沒帶作用有關的概念。由於混雜岩過往處於被忽略的地位，因此對它們的物理性質以及融化時的溫度壓力範圍所知甚少。未來把這些參數定量化的研究勢必能讓我們更加瞭解混雜岩在隱沒帶扮演的角色，以及它在地震形成和隱沒帶的火山作用中發揮了何種影響。

Volcanic arcs form by deep melting of rock mixtures

Study changes our understanding of processes inside subduction zones

Beneath the ocean, massive tectonic plates collide and grind against one another, which drives one below the other. This powerful collision, called subduction, is responsible for forming volcanic arcs that are home to some of Earth's most dramatic geological events, such as explosive volcanic eruptions and mega earthquakes.

A new study published in the journal Science Advances changes our understanding of how volcanic arc lavas are formed, and may have implications for the study of earthquakes and the risks of volcanic eruption.

Researchers led by the Woods Hole Oceanographic Institution (WHOI) have discovered a previously unknown process involving the melting of intensely-mixed metamorphic rocks -- known as mélange rocks -- that form through high stress during subduction at the slab-mantle boundary.

Until now, it was long-thought that lava formation began with a combination of fluids from a subducted tectonic plate, or slab, and melted sediments that would then percolate into the mantle. Once in the mantle, they would mix and trigger more melting, and eventually erupt at the surface.

"Our study clearly shows that the prevailing fluid/sediment melt model cannot be correct," says Sune Nielsen, a WHOI geologist and lead author of the paper. "This is significant because nearly all interpretations of geochemical and geophysical data on subduction zones for the past two decades are based on that model."

Instead, what Nielsen and his colleague found was that mélange is actually already present at the top of the slab before mixing with the mantle takes place.

"This study shows -- for the first time -- that mélange melting is the main driver of how the slab and mantle interact," says Nielsen.

This is an important distinction because scientists use measurements of isotope and trace elements to determine compositions of arc lavas and better understand this critical region of subduction zones. When and where the mixing, melting, and redistribution of trace elements occurs generates vastly different isotopic signature ratios.

The study builds on a previous paper by Nielsen's colleague and co-author Horst Marschall of Goethe University in Frankfurt, Germany. Based on field observations of mélange outcrops, Marschall noted that blobs of low-density mélange material, called diapirs, might rise slowly from the surface of the subducting slab and carry the well-mixed materials into the mantle beneath arc volcanoes.

"The mélange-diapir model was inspired by computer models and by detailed field work in various parts of the world where rocks that come from the deep slab-mantle interface have been brought to the surface by tectonic forces," Marschall says. "We have been discussing the model for at least five years now, but many scientists thought the mélange rocks played no role in the generation of magmas. They dismissed the model as 'geo-fantasy.'"

In their new work, Nielsen and Marschall compared mixing ratios from both models with chemical and isotopic data from published studies of eight globally representative volcanic arcs: Marianas, Tonga, Lesser Antilles, Aleutians, Ryukyu, Scotia, Kurile, and Sunda.

"Our broad-scale analysis shows that the mélange mixing model fits the literature data almost perfectly in every arc worldwide, while the prevailing sediment melt/fluid mixing lines plot far from the actual data," Nielsen says.

Understanding the processes that occur at subduction zones is important for many reasons. Often referred to as the planet's engine, subduction zones are the main areas where water and carbon dioxide contained within old seafloor are recycled back into the deep Earth, playing critical roles in the control of long-term climate and the evolution of the planet's heat budget.

These complex processes occur on scales of tens to thousands of kilometers over months to hundreds of millions of years, but can generate catastrophic earthquakes and deadly tsunamis that can occur in seconds.

"A large fraction of Earth's volcanic and earthquake hazards are associated with subduction zones, and some of those zones are located near where hundreds of millions of people live, such as in Indonesia," Nielsen says. "Understanding the reasons for why and where earthquakes occur, depends on knowing or understanding what type of material is actually present down there and what processes take place."

The research team says the study's findings call for a reevaluation of previously published data and a revision of concepts relating to subduction zone processes. Because mélange rocks have largely been ignored, there is almost nothing known about their physical properties or the range of temperatures and pressures they melt at. Future studies to quantify these parameters stand to provide even greater insight into the role of mélange in subduction zones and the control it exerts over earthquake generation and subduction zone volcanism.

原始論文：Sune G. Nielsen, Horst R. Marschall. Geochemical evidence for mélange melting in global arcs. Science Advances, 2017; 3 (4): e1602402 DOI: 10.1126/sciadv.1602402

引用自：Woods Hole Oceanographic Institution. "Volcanic arcs form by deep melting of rock mixtures: Study changes our understanding of processes inside subduction zones." ScienceDaily. ScienceDaily, 7 April 2017.