島弧火山會釋放出地殼與地函的混合物

原始網址：www.sciencedaily.com/releases/2016/06/160613153408.htm

Arc volcano releases mix of material from Earth's mantle and crust

島弧火山會釋放出地殼與地函的混合物

Volcanoes are an explosive and mysterious process by which molten rock from Earth's interior escapes back into the atmosphere. Why the volcano erupts -- and where it draws its lava from -- could help trace the lifecycle of materials that make up our planet.

火山噴發這種極具爆炸性且神秘的作用會讓地球內部的熔岩重見天日。瞭解火山為何噴發，以及岩漿來自何處，有助於我們追蹤地球組成物質的循環過程。

New University of Washington research shows that a common type of volcano is not just spewing molten rock from the mantle, but contains elements that suggest something more complicated is drawing material out of the descending plate of Earth's crust.

華盛頓大學的新研究顯示一種常見火山類型噴發出的熔岩不只來自地函，它含有的元素指出還有某些更加複雜的作用從隱沒板塊的地殼中抽取出別的物質。

Geologists have long believed that solidified volcanic lava, or basalt, originates in the mantle, the molten rock just below the crust. But the new study uses detailed chemical analysis to find that the basalt's magnesium -- a shiny gray element that makes up about 40 percent of the mantle but is rare in the crust -- does not look like that of the mantle, and shows a surprisingly large contribution from the crust. The paper was published the week of June 13 in the Proceedings of the National Academy of Sciences.

玄武岩是一種由火山岩漿凝固而成的岩石，地質學家長久以來認為其來源為地殼正下方的地函產生的熔岩。但此篇新研究利用詳盡的化學分析發現玄武岩的鎂含量跟地函並不相似，反之，他們驚訝地發現鎂大部分是由地殼提供。鎂是一種亮灰色的金屬，占地函組成約40%，但在地殼當中卻十分稀少。此篇論文刊登於6月13日這周的《美國國家科學院院刊》(Proceedings of the National Academy of Sciences)。

"Although the volcanic basalt was produced from the mantle, its magnesium signature is very similar to the crustal material," said lead author Fang-Zhen Teng, a UW associate professor of Earth and space sciences. "The ocean-floor basalts are uniform in the type of magnesium they contain, and other geologists agree that on a global scale the mantle is uniform," he said. "But now we found one type of the mantle is not."

「雖然火山玄武岩的來源為地函，然而它的鎂含量呈現出來的訊號卻跟地殼物質十分雷同。」第一作者，華盛頓大學地球與太空科學的副教授 Fang-Zhen Teng說。「洋底玄武岩的鎂含量在類型上相當一致，因此其他地質學家同意以全球尺度來看，地函成分也是相當均一的。」他說。「但現在我們找到一種地函其鎂含量的類型並不相同。」

The study used rock samples from an inactive volcano on the Caribbean island of Martinique, a region where an ocean plate is slowly plunging, or subducting, beneath a continental plate. This situation creates an arc volcano, a common type of volcano that includes those along the Pacific Ocean's "Ring of Fire."

研究使用的岩石樣品來自於加勒比海天堂島上的休火山，該區域的海洋板塊緩緩俯衝(隱沒)至大陸板塊之下。此種環境會創造出島弧火山這種常見的火山類型，沿著太平洋「火環」分布的火山多隸屬之。

Researchers chose to study a volcano in the Caribbean partly because the Amazon River carries so much sediment from the rainforest to the seabed. One reason scientists want to pin down the makeup of volcanic material is to learn how much of the carbon-rich sediment from the surface gets carried deep in the Earth, and how much gets scraped off from the descending plate and reemerges into the planet's atmosphere.

研究人員會選擇研究加勒比海的火山有部分是因為亞馬遜河會攜帶大量雨林來的沉積物至海床上。科學家想定出火山物質成分的其中一個原因，便是想要得知有多少富含碳的的沉積物會從地表被帶至地球深處，又有多少會從隱沒板塊中脫離而重回地球大氣層。

Analyzing the weight of magnesium atoms in the erupted basalt shows that they came not from the mantle, nor from the organic sediment scraped off during the slide, but directly from the descending oceanic crust. Yet the volcanic basalt lacks other components of the crust.

分析噴發行成的玄武岩中鎂原子的重量顯示鎂並非來自地函，也不是隱沒過程中剝落的有機沉積物，而是直接來自下沉的海洋地殼。但火山玄武岩中卻缺乏其他地殼應有的成分。

"The majority of the other ingredients are still like the mantle; the only difference is the magnesium. The question is: Why?" Teng said.

「其他成分大體上還是跟地函十分相似，唯一有差異的就只有鎂。問題便是：為什麼會有這種現象?」Teng說。

The authors hypothesize that at great depths, magnesium-rich water is squeezed from the rock that makes up Earth's crust. As the fluid travels, the surrounding rock acts like a Brita filter that picks up the magnesium, transferring magnesium particles from the crust to the mantle just below the subduction zone.

作者假設在地球相當深處富含鎂的水分會從組成地殼的岩石內部擠壓出來。隨著這些水分在岩石當中行進，周圍的岩石會像Brita濾水器般將鎂萃取出來，造成隱沒帶下方的鎂原子從地殼轉移到地函當中。

"This is what we think is very exciting," Teng said. "Most people think you add either crustal or mantle materials as a solid. Here we think the magnesium was added by a fluid."

「我們的想法相當振奮人心。」Teng說。「大多數的人會想像地殼和地函的物質是以固體形式彼此參和。但我們在這裡卻認為鎂是以液體的形式混和至另一方。」

Fluids seem to play a role in seismic activity at subduction zones, Teng said, and having more clues to how those fluids travel deep in the Earth could help better understand processes such as volcanism and deep earthquakes.

Teng說液體在隱沒帶的地震活動中可能是相當重要的要素，瞭解液體如何在地球深處流動的更多資訊，有助於讓我們愈加理解火山噴發以及深層地震的形成過程。

He and co-author Yan Hu, a UW doctoral student in Earth and space sciences, plan to do follow-up studies on basalt rocks from the Cascade Mountains and other arc volcanoes to analyze their magnesium composition and see if this effect is widespread.

他和共同作者，華盛頓大學地球與太空科學的博士生Yan Hu，計畫在之後的研究當中探討喀斯開山脈及其他島弧火山的玄武岩，分析這些岩石的鎂含量以觀察這種效應是否十分普遍。

The other co-author is Catherine Chauvel at the University of Grenoble in France. The research was funded by the U.S. National Science Foundation and the French National Research Agency.

此篇研究的另一位共同作者是法國科諾伯勒第一大學的Catherine Chauvel，並由美國國家科學基金會和法國國家研究總署資助。

引用自：University of Washington. "Arc volcano releases mix of material from Earth's mantle and crust." ScienceDaily. ScienceDaily, 13 June 2016.