CO2 windows from mantle to atmosphere: Models on ultrahigh-temperature metamorphism and speculations on the link with melting of snowball Earth
CO2 windows from mantle to atmosphere: Models on ultrahigh-temperature metamorphism and speculations on the link with melting of snowball Earth
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DOI：
10.1016/j.gr.2007.11.001
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发表时间：
2008-08
影响因子：
6.1
通讯作者：
M. Santosh;S. Omori
中科院分区：
文献类型：
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作者：
M. Santosh;S. Omori
我们试图将地球上主要雪球地球事件的融化阶段与极端地壳变质作用和超高温麻粒岩相岩形成的过程联系起来。虽然UHT麻粒岩的干矿物组合可能是由不同的机制造成的，但富含二氧化碳的流体参与产生诊断性UHT组合的直接证据已经记录在几个地块中常见的纯二氧化碳流体包裹体中。在这里，我们使用现代类似物评估了UHT岩石产生的构造环境，并表明伸展构造-碰撞后伸展和裂谷-起着至关重要的作用。为了推测碳酸化构造圈的co2释放、超高温变质作用和主要地球过程之间的联系，我们提出了一些重要问题，如：(a)次大陆地幔即构造圈是如何碳酸化的；(b)构造圈如何及何时脱气；(c)元古代造山带与现今造山带的区别。地球作为宜居行星的命运可能是由冥古宙时期通过选择性地将二氧化碳清除到地幔中形成海洋的基本过程的逆转，太古宙地幔楔的碳酸化作用，以及晚元古代以来通过分散变质作用和水渗透的碳酸化地幔的脱碳作用决定的。沿消耗板块边界的亚固相脱碳释放出的大量二氧化碳可能是造成温室效应的因素之一，从而引发了雪球地球的消冰。基于对全球元古代超大陆组合重建中碳酸化次大陆地幔分布的评价，以及它们与在极端条件下经历了co2辅助干变质作用的地壳域的联系，我们推测UHT岩石可能代表了co2从地幔转移到中地壳并最终转移到大气的窗口。
We attempt here to correlate the melting phase of major snowball Earth events in the planet with the processes associated with extreme crustal metamorphism and formation of ultrahigh-temperature (UHT) granulite facies rocks. While the dry mineral assemblages that characterize UHT granulites can result from different mechanisms, the direct evidence for the involvement of CO2-rich fluids in generating diagnostic UHT assemblages has been recorded from the common occurrence of pure CO2fluid inclusions in several terranes. Here we evaluate the tectonic settings under which UHT rocks are generated using modern analogues and show that divergent tectonics—both post-collisional extension and rifting—play a crucial role. In an attempt to speculate the link among CO2liberation from the carbonated tectosphere, UHT metamorphism and major earth processes, we address some of the important issues such as: (a) how the subcontinental mantle i.e., the tectosphere, had become carbonated; (b) how and when the tectosphere degassed; and (c) what is the difference between Proterozoic orogens and those of the present day. The fate of the Earth as a habitable planet was possibly dictated by a reversal of the fundamental process of formation of oceans through the selective removal of CO2into mantle in the Hadean times, carbonation of the Archean mantle wedge, and subsequent decarbonation of the carbonated mantle through divergent metamorphism and water infiltration since the Late Proterozoic. The abundant CO2liberated by subsolidus decarbonation along consuming plate boundaries was probably one of the factors that contributed to the greenhouse effect thereby triggering the deglaciation of snowball Earth. Based on an evaluation of the distribution of carbonated subcontinental mantle in global reconstructions of the Proterozoic supercontinent assembly, and their link with crustal domains that have undergone CO2-aided dry metamorphism at extreme conditions, we speculate that the UHT rocks might represent windows for the transfer of CO2from the mantle into the mid crust and ultimately to the atmosphere.