Earth's Rocks Contain a Hidden Ocean's Worth of Water

WASHINGTON — If you want to find Earth's vast reservoirs of water, you may have to look beyond the obvious places like the oceans and polar ice caps.

Scientists say massive amounts of water appear to exist deep beneath the planet's surface, trapped in a rocky layer of the mantle at depths between 250 and 410 miles (410 to 660 kilometers).

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Image: Ringwoodite — Fragments of a blue-colored mineral called ringwoodite can be synthesized in the laboratory. The mineral can include a significant amount of water in its crystal structure, deep below Earth's surface.

But do not expect to quench your thirst down there. The water is not liquid — or any other familiar form like ice or vapor. It is locked inside the molecular structure of minerals called ringwoodite and wadsleyite in mantle rock that possesses the remarkable ability to absorb water like a sponge.

"It may equal or perhaps be larger than the amount of water in the oceans," Northwestern University geophysicist Steve Jacobsen said Friday in a telephone interview. "It alters our thoughts about the composition of the Earth."

"It's no longer liquid water that we're talking about at these great depths. The weight of hundreds of kilometers of rock and very high temperatures above 1,000 degrees Celsius (1,832 Fahrenheit) break down water into its components. And it's not accessible. It's not a resource in any way," Jacobsen added.

Jacobsen said water is taken down into the mantle with minerals during the process known as plate tectonics — the slow, inexorable movement of the colossal rock slabs that make up the Earth's surface.

When the minerals containing this water reach certain depths, they break down in a process called dehydration and release the water to form magmas. Such "dehydration melting" is common in the shallow mantle and forms the source for magmas in many volcanoes.

In a study published in the journal Science, the researchers present evidence that this is also occurring much deeper in the mantle, in a region called the "transition zone" between Earth's upper and lower mantle.

The study combined lab experiments involving synthetic ringwoodite being exposed to conditions simulating the heat and pressure of the transition zone, and observations of events in this zone, based on seismic data from a network of more than 2,000 seismometers across the United States.

A team led by Jacobsen and University of New Mexico seismologist Brandon Schmandt identified deep pockets of magma, a likely signature of the presence of water at those depths.

— Will Dunham, Reuters

科學家新發現：地球內部潛藏巨大海洋

編譯中心 2014年06月13日 18:243577 點擊數

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地表下方數百公里深處可能有一個「隱藏的海洋」，而且正是地表海洋水量的來源。（取自網路）

美國科學家12日說，地表下方數百公里深處，可能有一個「隱藏的海洋」，水量相當於地表海洋總水量的3倍。此一發現也許有助於解釋地球上海洋的水從何而來：來自地球內部。

新墨西哥大學和西北大學的研究人員在《科學》（Science）雜誌上的報告說，這一「隱藏的海洋」位於地球內部410公里至660公里深處、上下地幔（mantle）之間的過渡帶（transition zone），其水分並不是一般熟悉的液態、氣態或固態，而是以水分子的形式存在於一種名為「林伍德石」（ringwoodite）的岩石中。

研究人員利用遍佈全美的2000多個地震儀分析了500多次地震的地震波。這些地震波會穿透包括地核在內的地球內部，由於水會降低地震波傳播的速度，研究人員可以據此分析地震波穿透的是什麼類型的岩石。

結果顯示，就在美國地下660公里深處，岩石發生部分熔融，且從地震波傳播速度減緩來看，這是可能有水存在的信號。

與此同時，研究人員在實驗室中合成上下地幔過渡帶中存在的林伍德石，當模擬地下660公里深處的高溫高壓環境時，林伍德石發生部分熔融，就像出汗一樣釋放出水分子。林伍德石由於具有特殊的晶體結構，會像海綿一樣吸引水分子。

西北大學地球物理學家雅各布森（Steve Jacobsen）說：「我想我們最終找到了整個地球水循環的證據，這或許有助於解釋地球地表大量液態水的存在，幾十年來，科學家一直在尋找這一缺失的深層水。」

地球上水的來源有多種說法，一些人認為是彗星或隕石撞擊地球帶來，也有人認為是從早期地球的內部慢慢滲透出來。前一種理論較為盛行，但新發現為後者提供了新的證據。

今年3月，加拿大亞伯達大學研究人員在英國《自然》（Nature）雜誌報告說，他們首次發現了來自上下地幔過渡帶的一塊林伍德石，其含水量為1.5%，從而證明有關過渡區含有大量水的理論是正確的。