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Nature:中科院高压科学中心毛河光研究组揭示地球氧循环过程

摘要 : 2016年6月8日,国际学术权威刊物自然出版集团《Nature》杂志在线发表了中科院北京高压科学研究中心毛河光领导的团队题为“FeO2 and FeOOH under deep lower-mantle conditions and Earth’s oxygen–hydrogen cycles ”的研究论文。

 2016年6月8日,国际学术权威刊物自然出版集团《Nature》杂志在线发表了中科院北京高压科学研究中心毛河光领导的团队题为“FeO2 and FeOOH under deep lower-mantle conditions and Earth’s oxygen–hydrogen cycles ”的研究论文。

我们在地球生存,离不开空气中的氧气。其实在地球45亿年漫长的历史中,前一半的地质纪录显示空气中是没有氧的。24亿年前的一次“大氧化事件”之后,空气中的氧含量才突然提升到今日的水平,才有生物的存在、繁荣和演进。但长久以来,科学家认为地球有个铁核,表示内部极度缺氧。大氧化事件不可能从内部取得来源。氧是如何产生的?一直是科学界未解的难题。

北京高压科学研究中心(简称高科)毛河光领导的团队,发现在地球1800公里之下的深处极端高温高压环境下,能造成含氧极高的过氧化铁FeO2。地面上司空见惯的沼铁矿FeOOH(也就是铁锈),到了深部就会变成过氧化铁,并产生氢。过氧化铁富集在地幔和地核的边界,能解释核幔边界观察到的一些地震波异常现象。其含氧的突然释放,也可能造成大氧化事件。

研究组利用上海同步辐射光源BL15U1线站和美国APS光源的线站,观察到赤铁矿与氧反应形成稳定的二价铁氧化物过氧化铁FeO2,在92GPa,2050K下,沼铁矿FeOOH分解产生FeO2并释放氢气。当沼铁矿随板块运动俯冲进入1800公里的深下地幔,将形成高密度过氧化铁沉到核幔边界,并释放氢,向上迁移。板块运动不断地提供沼铁矿,使得下地幔形成巨大的氧库同时维持氢循环。在核幔边界D”层探测到的地震异常信号,可能与大量的FeO2层有关;而在板块运动或喷发中FeO2矿物分解将释放氧气。

毛河光指出:这个发现表明地幔可能有更丰富的氧库存,而沼铁矿的分解又将氢循环联系起来,地球内部水循环提供了氧循环的路径;24亿年前氧含量的急剧提高这一地质事件的原因可能是FeO2层氧库向大气中释放形成;需开展关于过氧化铁物理、化学、矿物学性质的研究,进一步验证以上推断。

下地幔H-O-Fe循环示意图

原文链接:

FeO2 and FeOOH under deep lower-mantle conditions and Earth’s oxygen–hydrogen cycles

原文摘要:

The distribution, accumulation and circulation of oxygen and hydrogen in Earth’s interior dictate the geochemical evolution of the hydrosphere, atmosphere and biosphere1. The oxygen-rich atmosphere and iron-rich core represent two end-members of the oxygen–iron (O–Fe) system, overlapping with the entire pressure–temperature–composition range of the planet. The extreme pressure and temperature conditions of the deep interior alter the oxidation states, spin states and phase stabilities of iron oxides, creating new stoichiometries, such as Fe4O5 (ref. 5) and Fe5O6. Such interactions between O and Fe dictate Earth’s formation, the separation of the core and mantle, and the evolution of the atmosphere. Iron, in its multiple oxidation states, controls the oxygen fugacity and oxygen budget, with hydrogen having a key role in the reaction of Fe and O (causing iron to rust in humid air). Here we use first-principles calculations and experiments to identify a highly stable, pyrite-structured iron oxide (FeO2) at 76 gigapascals and 1,800 kelvin that holds an excessive amount of oxygen. We show that the mineral goethite, FeOOH, which exists ubiquitously as ‘rust’ and is concentrated in bog iron ore, decomposes under the deep lower-mantle conditions to form FeO2 and release H2. The reaction could cause accumulation of the heavy FeO2-bearing patches in the deep lower mantle, upward migration of hydrogen, and separation of the oxygen and hydrogen cycles. This process provides an alternative interpretation for the origin of seismic and geochemical anomalies in the deep lower mantle, as well as a sporadic O2 source for the Great Oxidation Event over two billion years ago that created the present oxygen-rich atmosphere.

来源: Nature 浏览次数:1

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