Volcanic rocks hold clues to
Earth's interior
火山岩握有地球深部活動的資訊
The journey for volcanic rocks found on
many volcanic islands began deep within Earth.
Brought to Earth's surface in eruptions of deep volcanic material, these rocks hold clues as to what is going on deep beneath Earth's surface.
火山岩自地球深處啟程，歷經漫漫長路，自火山深部噴發後才能在許多海島上為我們所見。也因此它們帶有地表之下十分深處正在進行何種活動的資訊。
Studies of rocks found on certain volcanic islands, known as ocean island basalts, revealed that although these erupted rocks originate from Earth's interior, they are not the same chemically.
科學家詳細研究某些火山島嶼上這種稱作洋島玄武岩(ocean island basalt)的岩石後，發現雖然這些岩石都是源自地球深處，然而它們的化學性質卻不盡相同。
According to a group of current and former researchers at Arizona State University, the key to unlocking this complex, geochemical puzzle rests in a model of mantle dynamics consisting of plumes -- upwelling's of abnormally hot rock within Earth's mantle -- that originate in the lower mantle and physically interact with chemically distinct piles of material.
由一群現任與曾經任職於亞利桑那州立大學的研究人員組成的團隊，表示解開這道複雜地球化學謎題的關鍵在於有考量到地函柱的地函動力學模型之中。地函柱是在地函中因為異常高熱而上湧的岩石，它們源自下部地函，會跟地函中化學性質不同的區域產生物理交互作用。
The team revealed that this theoretical model of material transport can easily produce the chemical variability observed at hotspot volcanoes (such as Hawaii) around the world.
研究團隊顯示我們在世界各地不同熱點火山(hotspot volcano，如夏威夷)觀察到的岩石化學性質變化，可以利用此理論模型架構出的地函物質輸送模式來輕易產生。
"This model provides a platform for understanding links between the physics and chemistry that formed our modern world as well as habitable planets elsewhere," says Curtis Williams, lead author of the study whose results are published in the Nov. 24 issue of the journal Nature Communications.
「這個模型提供的平台不只能讓我們了解物理化學作用彼此之間的鏈結如何形塑我們居住的世界，它同樣也能適用於某處適合我們生存的星球。」第一作者Curtis Williams說，他們的研究成果刊登於11/24的《自然通訊》（Nature Communications）。
Basalts collected from ocean islands such as Hawaii and those collected from mid-ocean ridges (that erupt at spreading centers deep below oceans) may look similar to the naked eye; however, in detail their trace elements and isotopic compositions can be quite distinct. These differences provide valuable insight into the chemical structure and temporal evolution of Earth's interior.
從夏威夷這類的海洋島嶼和中洋脊(在海床因板塊張裂而噴發的火山)取得的玄武岩，雖然由肉眼看來兩者之間十分相似，但仔細分析便會發現它們的稀有元素含量和同位素組成之間有天壤地別的差異。這些差異提供了一扇珍貴的窗口，讓我們有機會一窺地球內部不同化學成分的空間分布，以及它們如何隨著時間演變。
"In particular, it means that Earth's mantle -- the hot rock below Earth's crust but above the planet's iron core -- is compositionally heterogeneous. Understanding when and where these heterogeneities are formed and how they are transported through the mantle directly relates to the initial composition of Earth and how it has evolved to its current, habitable state," said Williams, a postdoc at UC Davis.
「尤其重要的是，這代表地球地函(位於地殼之下，鐵質地核之上的熾熱岩石)的成分是不均勻的。瞭解這種不均勻性質是何時何地發展出來，以及它如何散播到地函各處，跟我們想要得知地球最初成分為何，和地球如何演化成目前的適居狀態這些問題之間有著直接關係。」加州大學戴維斯分校的博士後研究員Williams說。
While a graduate student in ASU's School of Earth and Space Exploration, Williams and faculty members Allen McNamara and Ed Garnero conceived a study to further understand how chemical complexities that exist deep inside Earth are transported to the surface and erupt as intraplate volcanism (such as that which formed the Hawaiian islands). Along with fellow graduate student Mingming Li and Professional Research Associate Matthijs van Soest, the researchers depict a model Earth, where in its interior resides distinct reservoirs of mantle material that may have formed during the earliest stages of Earth's evolution.
還是亞利桑那州立大學地球與太空探索學院的研究生時，Williams與系所中的同仁Allen McNamara和Ed Garnero合作，深入探討地球深處的化學複雜性是如何傳送至表面，並以板塊內火山活動(intraplate volcanism，夏威夷火山即為一例)的形式噴發。在同樣是研究生的Mingming Li和專業副研究員Matthijs van Soest的協助下，研究人員建構出一個地球模型，其內部有數個地方在地球演化最初期時便已經擁有性質特異的地函物質。
Employing such reservoirs into their models is supported by geophysical observations of two, continent-sized regions -- one below the Pacific Ocean and one below parts of the Atlantic Ocean and Africa -- sitting atop the core-mantle boundary.
將這種儲庫納入模型當中是有地球物理觀測數據支持的。目前觀測結果顯示在太平洋下方，以及在部分大西洋至非洲下方，分別有兩個陸塊般大小的不均勻帶覆於核幔邊界之上。
"In the last several years, we have witnessed a sharpening of the focus knob on seismic imaging of Earth's deep interior. We have learned that the two large anomalous structures at the base of the mantle behave as if they are compositionally distinct. That is, we are talking about different stuff compared to the surrounding mantle. These represent the largest internal anomalies in Earth of unknown chemistry and origin," said Garnero.
「在最近這幾年，我們目睹到地球內部地震成像的清晰程度正不斷提高。我們體認到地函底部有兩處巨大的異常構造，從它們的行為看起來它們是由截然不同的成分組成。精確來說，我們討論的便是與周遭地函性質大異其趣的那些東西。這些區塊代表著地函之中我們仍未知曉其化學性質與起源的最大異常帶。」Garnero說。
These chemically distinct regions also underlie a majority of hotspot volcanism, via hot mantle plumes from the top of the piles to Earth's surface, suggesting a potential link between these ancient, chemically distinct regions and the chemistry of hotspot volcanism.
在大型熱點火山下方也常常會有這些化學異常帶，它們可能是從深部異常帶的頂端藉由高熱地函柱而來到地表。這顯示在古老化學異常帶與熱點火山化學性質之間可能具有某種潛在關聯。
To test the validity of their model, Williams and coauthors compare their predictions of the variability of the ratios of helium isotopes (helium-3 and helium-4) in plumes to that observed in ocean island basalts.
為了測試他們模型的可信度，Williams和共同作者比較模型預估的地函柱氦同位素比(氦-3和氦-4)變化值，以及從洋島玄武岩中得到的觀測值。
3He is a so-called primordial isotope found in Earth's mantle. It was created before Earth was formed and is thought to have become entrapped within Earth during planetary formation. Today, it is not being added to Earth's inventory at a significant rate, unlike 4He, which accumulates over time.
氦-3是一種可以從地函中發現的原生同位素(primordial isotope)。這種同位素是在地球誕生之前就已經形成，一般認為它們會在行星形成過程當中被困在地球當中。在今日，他們進入地球的速率非常緩慢，而非像氦-4一樣會隨著時間逐漸累積。
Williams explained: "The ratio of helium-3 to helium-4 in mid-ocean ridge basalts are globally characterized by a narrow range of small values and are thought to sample a relatively homogenous upper mantle. On the other hand, ocean island basalts display a much wider range, from small to very large, providing evidence that they are derived from different source regions and are thought to sample the lower mantle either partially or in its entirety."
Williams解釋：「在全球各地中洋脊玄武岩中的氦-3對氦-4比例都集中於非常窄的一段範圍之內，而我們認為它們代表了相對性質較為均一的上部地函的樣品。另一方面，洋島玄武岩的氦同位素比卻呈現出大範圍的變動，值可以從很小到很大。這證明了它們的來源並不相同，我們認為它們可能有部分或著整體都是來自下部地函。」
The variability of 3He to 4He in ocean island basalts is not only observed between different hotspots, but temporally within the different-aged lavas of a single hotspot track.
洋島玄武岩中的氦-3對氦-4變化不只可在不同熱點產生的玄武岩之間觀察到，就算是在同一熱點造成的火山序列中，不同年代的熔岩之間也會有此差異。
"The reservoirs and dynamics associated with this variability had remained unclear and was the primary motivation behind the study presented here," said Williams.
「這種多樣性之所以產生的來源與動力學仍未明朗，而這也是我們呈現在此的研究背後的主要動機。」Williams說。
引用自：Arizona State University.
"Volcanic rocks hold clues to Earth's interior." ScienceDaily.
ScienceDaily, 24 November 2015.
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