研究解釋地震波在地球深處觀測到的神祕訊號

原文網址：https://carnegiescience.edu/node/2264

研究解釋地震波在地球深處觀測到的神祕訊號

新研究探討了在地球深處的極端條件下氧和鐵的化學性質，結果或許可以解釋震測研究中一項歷時已久稱作「超低速帶」(ultralow velocity zones)的謎題。刊登於《自然》(Nature)的這項發現或許會深深影響到我們對地球地質歷史的認知，包括那些改變了地球生命發展的軌跡的事件――比方說發生在24億年前的大氧化事件。

下部地函和地核的交界位在地表以下2900公里處，坐落於附近的超低速帶(ultralow velocity zones,  UVZ)因其特殊的地震波訊號而為科學家所知。雖然這個區域的所在位置深到研究人員無法直接用肉眼觀察，但他們可以用儀器測量地震產生的地震波在此區域的傳遞情形，而得到地球內部構造如何變化的圖像；就像是醫療專業人員可以運用超音波儀器來看到我們身體的內部情況一樣。

科學家透過觀察地震波經過超低速帶時出現的減速現象，而看見超低速帶存在於沿著核幔邊界分布的某些地區之中。但是，知道超低速帶的存在並無法解釋它們的形成原理。

然而，最近研究鐵和氧在地球深處環境下的化學性質而得到的發現，對這項歷時已久的謎題提供了解答。

板塊構造活動會把某些含水礦物拖到地球深處。研究證明在極端的溫度壓力下，這些水會裂解而釋出氫，使得剩下的氧可以跟來自地核的金屬鐵結合，形成一種十分特殊的高壓礦物――過氧化鐵。

由卡內基的研究員Ho-kwang “Dave” Mao領導的研究團隊認為每年有多達3億噸的水會被帶到地球內部，並在深處產生巨大的二氧化鐵儲庫，其造成了在核幔邊界使地震波減速的超低速帶。

為了測試這個想法，研究團隊在美國阿貢國家實驗室運用雷射加熱的鑽石砧，模仿地球深處的溫度壓力條件來製造過氧化鐵樣品，接著利用精密儀器探討地震波在樣品中的傳遞情形。他們發現在平常的地函岩石中摻入40到50%的過氧化鐵所形成的混合物，地震波在其中傳遞產生的訊號跟神秘的超低速帶的訊號相同。

對研究團隊來說，此發現最令人感到興奮的其中一個層面是地球內部深處可能有儲存大量氧氣的場所，如果它會週期性地往地表釋放出氧氣就能夠劇烈改變地球早期大氣的成分。這或許可以解釋地質紀錄中大約24億年前大氣中的氧濃度為何會突然迅速上升。

「發現地球內部具有儲存氧氣的巨大場所代表的意義相當深遠。」Mao解釋。「我們現在應該好好思索偶發性的氧氣爆發事件跟地球歷史上的其他重要事件有何關聯，像是帶狀鐵礦、雪球地球、大滅絕、洪流玄武岩以及超大陸分裂。」

Mysterious deep-Earth seismic signature explained

New research on oxygen and iron chemistry under the extreme conditions found deep inside the Earth could explain a longstanding seismic mystery called ultralow velocity zones. Published in Nature, the findings could have far-reaching implications on our understanding of Earth’s geologic history, including life-altering events such as the Great Oxygenation Event, which occurred 2.4 billion years ago.

Sitting at the boundary between the lower mantle and the core, 1,800 miles beneath Earth’s surface, ultralow velocity zones (UVZ) are known to scientists because of their unusual seismic signatures. Although this region is far too deep for researchers to ever observe directly, instruments that can measure the propagation of seismic waves caused by earthquakes allow them to visualize changes in Earth’s interior structure; similar to how ultrasound measurements let medical professionals look inside of our bodies

These seismic measurements enabled scientists to visualize these ultralow velocity zones in some regions along the core-mantle boundary, by observing the slowing down of seismic waves passing through them. But knowing UVZs exist didn’t explain what caused them.

However, recent findings about iron and oxygen chemistry under deep-Earth conditions provide an answer to this longstanding mystery.

It turns out that water contained in some minerals that get pulled down into the Earth due to plate tectonic activity could, under extreme pressures and temperatures, split up—liberating hydrogen and enabling the residual oxygen to combine with iron metal from the core to create a novel high-pressure mineral, iron peroxide.

Led by Carnegie’s Ho-kwang “Dave” Mao, the research team believes that as much as 300 million tons of water could be carried down into Earth’s interior every year and generate deep, massive reservoirs of iron dioxide, which could be the source of the ultralow velocity zones that slow down seismic waves at the core-mantle boundary.

To test this idea, the team used sophisticated tools at Argonne National Laboratory to examine the propagation of seismic waves through samples of iron peroxide that were created under deep-Earth-mimicking pressure and temperature conditions employing a laser-heated diamond anvil cell. They found that a mixture of normal mantle rock with 40 to 50 percent iron peroxide had the same seismic signature as the enigmatic ultralow velocity zones.

For the research team, one of the most-exciting aspects of this finding is the potential of a reservoir of oxygen deep in the planet’s interior, which if periodically released to the Earth’s surface could significantly alter the Earth’s early atmosphere, potentially explaining the dramatic increase in atmospheric oxygen that occurred about 2.4 billion years ago according to the geologic record.

“Finding the existence of a giant internal oxygen reservoir has many far-reaching implications,” Mao explained. “Now we should reconsider the consequences of sporadic oxygen outbursts and their correlations to other major events in the Earth’s history, such as the banded-iron formation, snowball Earth, mass extinctions, flood basalts, and supercontinent rifts.”

原始論文：Jin Liu, Qingyang Hu, Duck Young Kim, Zhongqing Wu, Wenzhong Wang, Yuming Xiao, Paul Chow, Yue Meng, Vitali B. Prakapenka, Ho-Kwang Mao, Wendy L. Mao. Hydrogen-bearing iron peroxide and the origin of ultralow-velocity zones. Nature 551, 494–497 DOI: 10.1038/nature24461

引用自：Carnegie Institution for Science. “Mysterious deep-Earth seismic signature explained.”