岩石樣品指出水是形成地殼的關鍵要素

原文網址：www.sciencedaily.com/releases/2017/05/170501131717.htm

岩石樣品指出水是形成地殼的關鍵要素

中洋脊是製造新生地殼的地點。由德克薩斯大學奧斯汀分校的傑克遜地球科學院領導的科學家，在分析了於地表下方超過10英里之處形成的岩石的冷卻速率之後，發現在中洋脊，水可以深深滲透至地殼和上部地函內部。長久以來對於地函生成的岩漿如何冷卻而形成下部地殼一直有所爭議，此發現使其中一方獲得了新的證據。

這項研究由傑克遜地球科學院的博士後研究員Nick Dygert領導，並刊登於五月份《行星與地球科學通訊》(Earth and Planetary Science Letters)的印刷版。合作人員包括哥倫比亞大學的Peter Keleme和布朗大學的Yan Liang。

地球的地函是分隔地殼與地核的半固體層。Dygert表示雖然科學家已經明瞭在中洋脊擴張帶，從地函上湧的岩漿會形成新生地殼，但是關於此作用的詳細過程仍有許多問題尚待解答。

「科學界對於海洋地殼的形成過程仍有爭議。」Dygert表示，「而不同模型所需的冷卻方式有極大差異。」

為了更加瞭解岩漿變成地殼岩石時處在何種環境，Dygert和他的同伴檢視了數億年前曾是地函一部份，如今卻是阿曼一處峽谷中一部份的岩石樣品。

「這相當於實際走入地球內部20公里以下。」Kelemen說，「科學家藉此可以取得海床下方深處無法拿來研究的岩石。」

團隊使用的方法名為「地質溫度計」(geothermometers)，此技術為利用岩石樣本中的礦物成分來計算岩石曾處的溫度以及冷卻歷史。地質溫度計有助於科學家確立岩漿和岩石冷卻時經歷的溫度，並進一步得知冷卻發生的速率有多快。研究中用到了由Liang發展出來的新式地質溫度計，其記錄了岩石冷卻之前達到的最高溫度。

「傳統地質溫度計告訴你的通常是岩石冷卻時的溫度，而非形成時的溫度。」Dygert表示，「這種溫度計是相當優秀的新工具，因為它可以讓我們看到火成岩冷卻歷史中，之前不為所見的一部份。」

岩石紀錄的溫度顯示下部地殼和最上層的地函幾乎是瞬間冷卻下來――Dygert表示就像是把炙熱的平底鍋噗通一聲丟到裝滿水的水槽――深部地函則比較偏向逐步冷卻。溫度變化指出在中洋脊的擴張中心下方，水可以循環至地殼和地函最上部，而地函深部的熱能則是藉由跟上方較冷的岩石接觸來緩慢散出。

關於地殼形成的主要理論目前分成兩派。在層狀岩床(Sheeted Sill)假說中，循環滲入的海水會冷卻在下部地殼不同深度的許多小型岩漿體，並同步冷卻上部地函。在輝長岩冰河(Gabbro Glacier)假說中，岩漿則是在從主要岩漿庫流出的過程中逐步散失熱量。

Dygert說地質溫度計紀錄的溫度變化符合層狀岩床假說的冷卻過程。

「由於層狀岩床假說中，地殼不同深度的結晶作用幾乎是在同一時間發生，因此需要效率極佳的冷卻機制。」Dygert表示，「而我們發現的事物強烈指出熱液循環作用在地殼各處都進行得相當有效率。」

Dygert說要瞭解地球的地質史，核心問題之一便是要解開地殼是如何形成，但此答案同樣也對地球的未來有所啟發。有些科學家提議的對抗氣候變遷方法是將二氧化碳跟水混和後，注入地函的岩石之中。但Dygert指出已經暴露在海水中的地函岩石或許不會跟二氧化碳迅速反應，這會使碳捕捉作用減緩下來。Dygert表示新的研究認為中洋脊之下的海水循環僅在地殼區段中能有效進行，因此海洋地殼之下的龐大地函區間可以作為困住二氧化碳的高效場所。

資助此研究的機構包括傑克遜地球科學院、國家科學基金會、艾爾弗·斯隆基金會和國際陸地鑽探計畫。

Rock samples indicate water is key ingredient for crust formation

By examining the cooling rate of rocks that formed more than 10 miles beneath the Earth's surface, scientists led by The University of Texas at Austin Jackson School of Geosciences have found that water probably penetrates deep into the crust and upper mantle at mid-ocean spreading zones, the places where new crust is made. The finding adds evidence to one side of a long-standing debate on how magma from the Earth's mantle cools to form the lower layers of crust.

The research was led by Nick Dygert, a postdoctoral fellow in the Jackson School's Department of Geological Sciences, and was published in May in the print edition of Earth and Planetary Science Letters in May. Collaborators include Peter Kelemen of Colombia University and Yan Liang of Brown University.

The Earth's mantle is a semi-solid layer that separates the planet's crust from the core. Dygert said that while it's well known that magma upwelling from the mantle at mid-ocean spreading zones creates new crust, there are many questions on how the process works.

"There's a debate in the scientific community how oceanic crust forms," Dygert said. "And the different models have very different requirements for cooling regimes."

To learn more about the conditions under which magma turns into crustal rock, Dygert and his collaborators examined rock samples that were part of the Earth's mantle a hundred million years ago, but are now part of a canyon in Oman.

"One can effectively walk down 20 kilometers in the Earth's interior," said Kelemen. "This allows scientists to access rocks that formed far below the seafloor which are not available for study."

The team used "geothermometers" -- the name of a technique that uses mineral compositions inside rock samples to calculate temperatures and reveal the cooling history of the rock. Geothermometers help scientists determine the temperatures experienced by magmas and rocks as they cool, and infer how rapidly the cooling occurred. The study included use of a new geothermometer developed by Liang, which records the maximum temperature a rock attained before it cooled.

"Traditional geothermometers usually give you a cooling temperature rather than a formation temperature for the rock," Dygert said. "This thermometer is a neat new tool because it allows us to look at a part of the cooling history that was inaccessible for igneous rocks previously."

The temperatures recorded in the rocks show that the lower crust and uppermost mantle cooled and solidified almost instantly, Dygert said -- like a "hot frying pan being plopped in a sink of water" -- while the deeper mantle cooled more gradually. The temperature change is indicative of water circulating through the crust and uppermost mantle beneath mid-ocean spreading centers, and the heat from deeper portions of the mantle being dissipated through contact with the cooler upper rocks.

Currently, there are two primary theories for crust formation. In the Sheeted Sill hypothesis, circulating seawater cools many small magma deposits at different depths in the lower crust, which would simultaneously cool the upper mantle. In the Gabbro Glacier hypothesis, magma gradually loses heat as it flows away from a central magma chamber.

Dygert said that temperatures recorded by the geothermometers matched with the Sheeted Sill cooling process.

"The Sheeted Sill model requires a very efficient mechanism for cooling because crystallization is happening at all different depths within the crust at the same time," Dygert said. "And what we were able to find strongly implies that hydrothermal circulation was very efficient throughout the crustal section."

Uncovering how crust forms is at the heart of understanding the geological history of our planet, Dygert said, but the results could also have implications for our planet's future. Some scientists have proposed mixing carbon dioxide (CO₂) with water and injecting it into mantle rock as a means to fight climate change. The CO₂ reacts with minerals in the mantle, which safely locks up the carbon up in their crystal structures. However, Dygert notes that mantle rock that has already been exposed to seawater may not react as readily with CO₂, which would slow the carbon capture process. Dygert said that the new results suggest that circulation of water beneath mid-ocean ridges is effectively limited to the crustal section, and that enormous sections of the mantle could be available beneath the oceanic crust to efficiently trap CO₂.

The research was supported by the Jackson School of Geosciences, the National Science Foundation, the Alfred P. Sloan Foundation, and an International Continental Drilling Program grant.

原始論文：Nick Dygert, Peter B. Kelemen, Yan Liang. Spatial variations in cooling rate in the mantle section of the Samail ophiolite in Oman: Implications for formation of lithosphere at mid-ocean ridges. Earth and Planetary Science Letters, 2017; 465: 134 DOI: 10.1016/j.epsl.2017.02.038

引用自：University of Texas at Austin. "Earth science: Rock samples indicate water is key ingredient for crust formation." ScienceDaily. ScienceDaily, 1 May 2017.