The Formation Mechanism of Petit-Spot Volcanoes and the Nature of the Lithosphere-Asthenosphere Boundary (LAB)
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摘要: 近二十年来,在俯冲带外缘发现的斑点火山群代表了一种全新的海底岩浆活动类型.这种火山规模很小,成簇出现,年龄异常年轻,岩样以EM1型碱性玄武岩为主,孔隙度高且富含挥发性组分.有关斑点火山的岩浆起源以及与岩浆上涌相关的动力学过程目前仍然存在广泛的争论.本文系统性地介绍了斑点火山的特征,总结了前人针对其形成机制和岩浆源区研究提出的3种模型;另外,结合大洋岩石圈-软流圈边界(LAB)可能含熔体的最新研究成果,指出了斑点火山还可能与LAB的性质这一板块理论的根本科学问题密切相关;由于CO2可能是导致LAB深度处出现熔融聚集层的原因,而斑点火山岩样中富含CO2,因此斑点火山还可能是碳循环的重要组成部分.最后,本文对未来围绕斑点火山形成机制等科学问题的多学科综合研究做出展望.Abstract: The petit-spot volcanoes discovered near the subduction zone's outer rise represent a new type of seafloor magmatic activity. These volcanoes are small in scale, appear in clusters, and are extremely young. The rock samples are dominated by EM1 alkaline basalt, with high porosity and rich volatile components. The origin of magma of petit-spot volcanoes and the dynamic processes related to magma upwelling are still in debate. This paper presents the characteristics of petit-spot volcanoes and summarizes the three models proposed by previous studies on the formation mechanism and magma sources of these unique volcanoes. Based on the latest findings showing the lithosphere-asthenosphere boundary (LAB) beneath the oceanic plate may contain melts, we infer that the petit-spot volcanoes likely contain the clues about the fundamental scientific question of the nature of LAB. On the other hand, the volcanic rocks are rich in CO2, which likely is the volatiles responsible for melt generation in the asthenosphere and melt accumulation at LAB. Therefore, the petit-spot volcano may also account for an important part of the carbon cycle. Finally, this paper proposes a comprehensive multidisciplinary approach is needed to reveal the formation mechanism of such unique volcanoes.
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图 1 目前已发现的斑点火山群在全球俯冲带的分布
A, C, D. 日本海沟(Hirano et al., 2001, 2008; Fujiwara et al., 2007; Ohira et al., 2018);B. 智利海沟(Hirano et al., 2013);E. 巽他海沟(Hoernle et al., 2011; Taneja et al., 2015);F. 汤加海沟(Reinhard et al., 2019);G. 北马里亚纳海沟(Hirano et al., 2019)
Fig. 1. Global occurrence of petit-spot volcanoes
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