地球深處還有另一片海洋----中壢英文補習班－金學堂英語的部落格

【聯合報╱By HENRY FOUNTAIN╱陳世欽譯】

Another Ocean, Deep Inside the Earth

An analysis of seismic waves passing through the deep earth appears to confirm what laboratory experiments have suggested was possible: that an ocean of water is tied up in the mantle, 640 kilometers below the surface. The water is not liquid, but rather bound in minerals that exist at the extreme pressures found at such depths.

The finding suggests that processes that occur in the shallower mantle and that cause volcanoes and related activity at the surface are also occurring farther down.

“It’s a new view of the structure of this part of the earth,” said Brandon Schmandt, a geophysicist at the University of New Mexico and an author of a recent paper in the journal Science describing the research.

The work also adds credence to the idea that the earth’s water accumulated in the interior during the planet’s formation, rather than arriving later through the bombardment of icy comets. In this view, water bound up in minerals in the mantle, the 2,900-kilometer-thick layer between the thin crust and the hot metallic core, degassed over time and reached the surface.

The scientists studied a part of the mantle called the transition zone, from about 480 kilometers to 640 kilometers deep. The ability of this zone to contain water — and apparently to retain a lot of it — “may have something to do with stabilizing or buffering the size of the oceans,” said Steven D. Jacobsen, a mineralogist at Northwestern University in Illinois, another author of the paper. “It may be fortunate that the earth’s interior can act like a sponge.”

Dr. Jacobsen synthesizes deepearth minerals by replicating the extreme pressures that exist hundreds of kilometers below the surface. Over the years when he has made a blue mineral called ringwoodite, which is ubiquitous in the transition zone, it has formed with water. But that was just lab work, Dr. Jacobsen said, adding, “We haven’t known whether it could happen down there.”

Dr. Schmandt analyzed seismic data from the USArray project, in which 400 mobile seismometers have been deployed to create high-resolution images of the mantle. The analysis showed signs of melting in the transition zone, in areas where convection was causing the mantle to flow downward.

Melting of the mantle occurs close to the surface, creating the magma that is responsible for volcanic hot spots around the world. The process is called dehydration melting, because as parts of the mantle slide deeper at places where the earth’s tectonic plates meet, the increasing pressure causes minerals in the mantle to release their water, lowering the melting temperature. But finding evidence of melting much farther down, as Dr.

Schmandt did, was difficult to explain “unless you invoke water,” Dr. Jacobsen said.

Their work suggests that what happens deep in the mantle is similar to what happens close to the surface, the researchers said. “You end up having a mushy rock,” Dr. Jacobsen said.

The work is all part of trying to understand how the dynamics of the deep earth affect what happens at the surface.

“We’re trying to connect the rock cycles — i.e., plate tectonics — with water cycles,” Dr. Jacobsen said. “The more we look, the deeper it goes.”

中譯

科學家分析通過地球深處的地震波，似乎證實了實驗顯示的一種可能性：地表以下640公里處的地幔鎖住一大片海洋。

這些水不是液態，而是鎖在如此深度所產生極端壓力下的礦物當中。

這項發現意味，發生在較淺層地幔，在地表引發火山與相關活動的過程也會在更深處發生。

新墨西哥大學地球物理學家席曼特最近在「科學」期刊發表報告說明這項研究。他說：「這是對地球這個部位結構的新見解。」

這項研究也使以下看法更具可信性：地球的水是地球形成期間在其內部累積而得，而非冰質彗星後來撞擊地球帶來的。依此理論，經過相當時間之後，鎖在地幔礦物中的水脫除氣體，來到地表。地幔是介於薄質地殼與含金屬滾熱地球核心之間的2900公里厚地層。

科學家研究深度介於480公里與640公里之間名為過渡區域的地幔。共同撰寫研究報告的伊利諾州西北大學礦物學家雅各森表示，這個區域具備含水與保水的能力，而且顯然是極大量的水，這可能與「穩定或緩衝大面積海洋有關。地球內部有如一塊海綿。這可能是我們幸運之處」。

雅各森複製地表以下數百公里處的極端壓力，進而合成地球深處的礦物。在他製出過渡區隨處可見的藍色礦物尖晶橄欖石的多年間，水亦同時形成。不過他表示這只是實驗結果，「還不知道會不會也在地球深處發生」。

席曼特分析來自地震台陣計畫的地震資料。該計畫部署400個移動式地震儀，以呈現高解析度的地幔影像。分析顯示在對流作用導致地幔向下流動的過渡區，有融化徵象。

地幔融化發生在接近地表的區域，產生的岩漿與全球各地的滾熱火山景象有關。這種過程名為脫水融化，因為隨著地幔滑向地球板塊相會所在的地球深處，壓力不斷增加導致地幔當中的礦物釋出水分，進而降低融化的溫度。