From Snowball to Phaneorozic Earth
From Snowball to Phaneorozic Earth
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DOI：
10.2747/0020-6814.47.8.775
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发表时间：
2005-08
影响因子：
2.6
通讯作者：
Shigenori Maruyama;J. Liou
中科院分区：
文献类型：
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作者：
Shigenori Maruyama;J. Liou
所谓的晚元古代雪球地球在中生代开始时消失了：中生代地球的表面被海洋和富含氧气的大气层所包围，地球气候温暖。我们认为，这一巨大的变化是由于海水回流到地幔开始在750马，作为一个结果，地球的冷却和开始的主要冰期。MORB + H2O和橄榄岩+ H2O系统中矿物的相关系表明，古老、寒冷的俯冲洋板块会通过超高磷含水硅酸盐将地表水输送到410-660 km深处的地幔过渡带。该区域可储存丰富的H2O，因为橄榄石组合物的含水β和γ相可含有高达2至3wt%的水。对世界各地区域变质带的古俯冲带的岩石计算的地热记录了750 Ma时岩石圈内和岩石圈下的这种暖-冷转变。地球内部沿着消耗板块边界的失控冷却开始于750 Ma，通过Wadati-Benioff平面上方的一条狭窄通道，或直接通过水合板状橄榄岩的下降，将水化锋面从70 km深处传播到410-660 km。由此产生的地幔楔和邻近地区的水化作用导致海平面在750-600 Ma的时间间隔内迅速下降，并出现大的陆块。这一变化是由于楔状地幔中角闪石橄榄岩的含水量从0.5wt%增加到叶长橄榄岩的6.5wt%，以及爆米花效应推高大陆边缘。据估计，自那时以来，海水流失的总量使海平面下降了约600米。海水进入地幔后，地幔物质的熔融温度和粘度急剧降低，这两个因素都重新激活了板块和地幔柱的构造过程。750 Ma以来，地表火山活动释放出地幔CO2，引起温室效应，使雪球地球的全球冰川融化。巨大大陆块的出现使地表环境多样化;沉积物的侵蚀和沉积变得像今天的地球一样重要。沉积盆地和增生复合体的广泛形成沿着消耗板块边界，通过埋藏在这些沉积物中的有机物，大大增加了大气中的自由氧，这对大型多细胞生物的进化至关重要。
The so-called Late Proterozoic snowball Earth disappeared at the beginning of Phanerozoic time: the surface of the Phanerozoic Earth was enveloped by an ocean and oxygen-rich atmosphere, and the planet had a warm climate. We propose that this dramatic change resulted from the return flow of seawater into the mantle beginning at 750 Ma, as a consequence of the cooling of the Earth and start of the main glacial epoch. Phase relations for minerals in the system MORB + H2O and peridotite + H2O suggest that old, cold subducting oceanic slabs would transport surface water through ultrahigh-P hydrous silicates to the mantle transition zone at depths of 410-660 km. This zone could store abundant H2O, inasmuch as hydrous β and γ phases of the olivine composition may contain up to 2 to 3 wt% water. Geotherms computed for rocks of ancient subduction zones from regional metamorphic belts around the world record such a warm-to-cold transition in and below the lithosphere at 750 Ma. The runaway cooling of the Earth's interior along consuming plate boundaries started at 750 Ma by propagation of a hydration front into the mantle from 70 km depth to 410-660 km through a narrow channel above the Wadati-Benioff plane, or directly by the descent of hydrated slab peridotites. Resultant hydration of the mantle wedge and adjacent regions caused a rapid sea-level drop during the interval of 750-600 Ma, and large landmasses appeared. This change was due to the increase of water content from 0.5 for amphibole peridotite to 6.5 wt% for antigorite peridotite in the wedge mantle, and to the popcorn effect to push up continental margins. The total seawater lost since then is estimated to have lowered sea level about 600 m. Introduction of seawater into the mantle drastically lowered the melting temperature and viscosity of mantle materials, both of which reactivated the plate and plume tectonic processes. Surface volcanism released mantle CO2, causing the greenhouse effect to melt global glaciers of the snowball Earth since about 750 Ma. The emergence of huge continental landmasses diversified surface environments; erosion and deposition of sediments became significant as in the present-day Earth. Extensive formation of sedimentary basins and accretionary complexes along consuming plate boundaries drastically increased free oxygen in the atmosphere by the burial of organic matter in those sediments, which was critical to the evolution of large multicellular organisms.