Geodynamics of kimberlites on a cooling Earth: Clues to plate tectonic evolution and deep volatile cycles
Geodynamics of kimberlites on a cooling Earth: Clues to plate tectonic evolution and deep volatile cycles
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DOI：
10.1016/j.epsl.2017.12.013
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发表时间：
2018-02-15
影响因子：
5.3
通讯作者：
de Wit, Mike
中科院分区：
文献类型：
--
作者：
Tappe, Sebastian;Smart, Katie;de Wit, Mike
金伯利岩岩浆作用发生在各大洲的白垩纪地区。全球年龄分布表明，这种形式的地幔熔融在1.2 Ga之后，特别是在250-50 Ma之间，比2 Ga之前的早期地球历史更加突出（即，古元古代和太古代）。尽管保存偏差被认为是金伯利岩年龄分布不均匀的一个可能原因，对最新全球数据库的新处理表明，金伯利岩和相关富CO2超镁铁质岩浆作用的明显长期演化是真实的，可能与地球上地幔温度随时间降低有关。（1100-1400摄氏度）是岩石学上最可行的工艺，其可以产生具有类似于金伯利岩的富集微量元素浓度的高MgO碳酸盐硅酸盐熔体。这些条件发生在对流软流圈地幔直接下厚大陆岩石圈。在这个短暂的上地幔源区，变量CHO挥发性混合物控制橄榄岩的熔融，在没有热异常，使低度碳酸盐硅酸盐熔体可能永久存在于环境地幔温度低于1400摄氏度。然而，提取低量熔体到地球表面需要构造触发。板块运动的速度和方向的突然变化，如典型的超大陆周期的动力学，可以有效地帮助金伯利岩岩浆ascertain.Provided的CO2-和H2O-fluxed的深岩龙骨，形成较大的漂移构造板块的一部分，存在3 Ga甚至更早，金伯利岩火山活动可能是频繁的太古代。然而，我们认为，频繁的金伯利岩岩浆活动必须等待建立一个初期熔融制度下成熟的大陆，这只成为显着的后太古代的时间，可能是不久后的某个时候，世俗地幔冷却到1400摄氏度以下。大约在这个时候，金伯利岩取代科马提岩成为全球大陆盾的标志性幔源岩浆特征。250-50 Ma之间显著的中新生代“金伯利岩花”可能代表了盘古超大陆相对较冷和挥发性流动的岩浆根经历显著构造扰动的理想环境。这创造了世界上已知的金伯利岩中超过60%的氧化还原和减压相关的低程度部分熔融。不到2%的世界上已知的金伯利岩形成后，50马，罕见的“年轻”金伯利岩从非洲东部和北美西部的构造环境表明，远场应力对岩石圈的岩石圈的大陆裂谷或冷俯冲执行发挥了至关重要的作用，使金伯利岩岩浆转移到地球表面。(C)2017爱思唯尔B. V.保留所有权利。
Kimberlite magmatism has occurred in cratonic regions on every continent. The global age distribution suggests that this form of mantle melting has been more prominent after 1.2 Ga, and notably between 250-50 Ma, than during early Earth history before 2 Ga (i.e., the Paleoproterozoic and Archean). Although preservation bias has been discussed as a possible reason for the skewed kimberlite age distribution, new treatment of an updated global database suggests that the apparent secular evolution of kimberlite and related CO2-rich ultramafic magmatism is genuine and probably coupled to lowering temperatures of Earth's upper mantle through time.Incipient melting near the CO2- and H2O-bearing peridotite solidus at >200 km depth (1100-1400 degrees C) is the petrologically most feasible process that can produce high-MgO carbonated silicate melts with enriched trace element concentrations akin to kimberlites. These conditions occur within the convecting asthenospheric mantle directly beneath thick continental lithosphere. In this transient upper mantle source region, variable CHO volatile mixtures control melting of peridotite in the absence of heat anomalies so that low-degree carbonated silicate melts may be permanently present at ambient mantle temperatures below 1400 degrees C. However, extraction of low-volume melts to Earth's surface requires tectonic triggers. Abrupt changes in the speed and direction of plate motions, such as typified by the dynamics of supercontinent cycles, can be effective in the creation of lithospheric pathways aiding kimberlite magma ascent.Provided that CO2- and H2O-fluxed deep cratonic keels, which formed parts of larger drifting tectonic plates, existed by 3 Ga or even before, kimberlite volcanism could have been frequent during the Archean. However, we argue that frequent kimberlite magmatism had to await establishment of an incipient melting regime beneath the maturing continents, which only became significant after secular mantle cooling to below 1400 degrees C during post-Archean times, probably sometime shortly after 2 Ga. At around this time kimberlites replace komatiites as the hallmark mantle-derived magmatic feature of continental shields worldwide.The remarkable Mesozoic-Cenozoic 'kimberlite bloom' between 250-50 Ma may represent the ideal circumstance under which the relatively cool and volatile-fluxed cratonic roots of the Pangea supercontinent underwent significant tectonic disturbance. This created more than 60% of world's known kimberlites in a combination of redox- and decompression-related low-degree partial melting. Less than 2% of world's known kimberlites formed after 50 Ma, and the tectonic settings of rare 'young' kimberlites from eastern Africa and western North America demonstrate that far-field stresses on cratonic lithosphere enforced by either continental rifting or cold subduction play a crucial role in enabling kimberlite magma transfer to Earth's surface. (C) 2017 Elsevier B.V. All rights reserved.