在地球深處相遇的水和鐵或許孕育了生命誕生的條件
根據最近一組國際團隊刊登在《國家科學評論》(National
Science Review)的研究，地核和地函邊界儲有大量氧的鐵，或許對地球歷史上的事件具有重大影響，像是超大陸分裂、地球大氣組成的劇烈改變、以及生命的誕生。
這組團隊包括了美國卡內基科學研究院、史丹佛大學、芝加哥大學，以及中國北京高壓科學研究中心的科學家。他們詳細探討在地球核幔邊界的極端溫度壓力下水和鐵的化學性質。
當板塊構造運動將含水礦物拖曳至地球深處而接觸鐵質地核，此處的極端環境會讓鐵抓住水分子中的氧原子，並將氫原子釋放出來。之後氫會往地表逸出，但氧則會困在二氧化鐵――這種只能在如此極端的高溫高壓下存在的化合物的結晶構造中。
結合理論計算結果以及重現核幔邊界環境的實驗室試驗，團隊確定利用鑽石高壓砧將材料暴露在介於平常大氣壓力950至100萬倍的壓力，以及超過1900℃的溫度下可以形成二氧化鐵。
主要作者Ho-kwang
“Dave” Mao表示：「板塊構造運動會將隱沒板塊拖進地球內部深處。基於我們對隱沒板塊化學組成的認識，我們認為每年會有3億噸的水被帶到地球深處跟地核的鐵接觸，並產生大量二氧化鐵。」
這類極度富含氧的固態岩石可能會年復一年地持續堆積在地核上方，逐漸成長為陸塊大小的巨大岩體。如果有地質事件加熱這些二氧化鐵岩體，就可能會引發大規模的噴發事件，瞬間將大量氧氣釋放到地球表面。
作者提出的假說認為這類氧氣爆發事件可以將巨量氧氣釋放到地球大氣當中――多到足以使大氧化事件(Great
Oxygenation Event)發生。發生於25億年前的大氧化事件使得地球大氣充滿氧氣，而讓我們所知以氧氣維生的生物就此崛起。
「這項新發現水在高溫高壓下的裂解反應，對地球內部深處到大氣的地球化學皆有影響。」Mao表示。「(它讓)我們需要重新審視過往發表的許多理論。」
When water met iron deep inside the
earth, it might have created conditions for life
Reservoirs
of oxygen-rich iron between the Earth’s core and mantle could have played a
major role in Earth’s history, including the breakup of supercontinents,
drastic changes in Earth’s atmospheric makeup, and the creation of life,
according to recent work from an international research team published in National Science Review.
The team—which includes scientists from Carnegie,
Stanford University, the Center for High Pressure Science and Technology
Advanced Research in China, and the University of Chicago—probed the chemistry
of iron and water under the extreme temperatures and pressures of the Earth’s
core-mantle boundary.
When the action of plate tectonics draws
water-containing minerals down deep enough to meet the Earth’s iron core, the
extreme conditions cause the iron to grab oxygen atoms from the water molecules
and set the hydrogen atoms free. The hydrogen escapes to the surface, but the
oxygen gets trapped into crystalline iron dioxide, which can only exist under
such intense pressures and temperatures.
Using theoretical calculations as well as laboratory
experiments to recreate the environment of the core-mantle boundary, the team
determined that iron dioxide can be created using a laser-heated diamond anvil
cell to put materials under between about 950 and 1 million times normal
atmospheric pressure and more than 3,500 degrees Fahrenheit.
“Based on our knowledge of the chemical makeup of the
slabs that are drawn into the Earth’s deep interior by plate tectonics, we
think 300 million tons of water could be carried down to meet iron in the core
and generate massive iron dioxide rocks each year,” said lead author Ho-kwang
“Dave” Mao.
These extremely oxygen-rich solid rocks may
accumulate steadily year-by-year above the core, growing into gigantic,
continent-like sizes. A geological event that heated up these iron dioxide
rocks could cause a massive eruption, suddenly releasing a great deal of oxygen
to the surface.
The authors hypothesize that such an oxygen explosion
could put a tremendous amount of the gas into the Earth’s atmosphere—enough to
cause the so-called Great Oxygenation Event, which occurred about 2.5 billion
years ago and created our oxygen-rich atmosphere, conditions that kickstarted
the rise oxygen-dependent life as we know it.
“This newly
discovered high-temperature and intense-pressure water-splitting reaction affects
geochemistry from the deep interior to the atmosphere” said Mao. “Many previous
theories need to be re-examined now.
原始論文：Ho-Kwang
Mao, Qingyang Hu, Liuxiang Yang, Jin Liu, Duck Young Kim, Yue Meng, Li Zhang,
Vitali B. Prakapenka, Wenge Yang, Wendy L. Mao. When water meets iron at Earth's core–mantle boundary. National Science Review, 2017; DOI:
10.1093/nsr/nwx109
引用自：Carnegie
Institution for Science. " When water met iron deep inside the earth, it
might have created conditions for life."
沒有留言:
張貼留言