Another Nail in Which Coffin?
Another Nail in Which Coffin?
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DOI：
10.1126/science.315.5808.39c
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发表时间：
2007-01
期刊：
影响因子：
56.9
通讯作者：
A. Hofmann;S. Hart
中科院分区：
文献类型：
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作者：
A. Hofmann;S. Hart
最近的一份报告“火山作用对板块挠曲的反应”由N。平野等人（9月8日，第1426页）关于远离板块边界的小体积火山和M.麦克纳特：“羽毛棺材上的另一颗钉子？”（9月8日，第1394页），其中投在正在进行的羽争论的背景下，这一贡献，启发了我们的评论。Hirano等人开始声称“地球上的火山活动已知发生在三种构造环境中：分散的板块边界.、会聚的板块边界.和热点”，接着是“如果没有热点的存在，就不会有新的火山活动”。他们继续表明，板块挠曲可以引起断裂和小体积的火山活动，这不太可能与任何“地幔柱”有关。麦克纳特的观点的要旨是，这构成了对热点火山是由从地幔深处上升的热物质（羽流）的射流引起的假设的持续斗争的胜利（“因此，地球科学家们正在意识到，地幔羽流的概念可能并不好”）。事实上，小体积的板内火山活动并不是什么新闻。关于三种类型的板内火山活动，通常是碱性火山活动，有着丰富的既定知识，但不能直接由深部地幔柱造成：㈠数以万计的小型火山海山（1、2）; ㈡被称为喀麦隆线的1 600公里长的海洋和大陆火山链，该线没有可检测到的喷发时间进程，长期以来一直被讨论为“热线”，而不是地幔柱（3）;和（iii）与大陆（和海洋）裂谷有关的火山活动，与板块边界无关。发现另一条并非由热点或羽流造成的小型碱性海山线既不新鲜，也不令人惊讶。地球化学的基本规则告诉我们，碱性玄武岩中发现的高度富集的不相容微量元素需要低程度的熔融;这种熔融在厚的海洋岩石圈地区并不奇怪。火山活动本身是由地幔柱还是岩石圈断裂引发的是另一个问题。裂缝能够而且确实引发了火山活动，这一观察结果并不能成为反对深地幔柱的论据，正如通过地震层析成像（4）证实地幔柱一样，也不能成为反对与火山活动有关的火山活动的论据。
A recent Report “Volcanism in response to plate flexure” by N. Hirano et al.(8 Sept., p. 1426) on small-volume volcanoes located far from plate boundaries and the related Perspective by M. McNutt “Another nail in the plume coffin?”(8 Sept., p. 1394), which casts this contribution in the context of the ongoing plume controversy, have inspired us to comment. Hirano et al. begin with the claim that “[v] olcanism on Earth is known to occur in three tectonic settings: divergent plate boundaries…, convergent plate boundaries…, and hot spots,” followed by “Without the presence of a hot spot, new volcanism is not anticipated.” They go on to show that plate flexure can induce fracturing and small-volume volcanism, which is unlikely to be related to any “mantle plume.” The tenor of McNutt's Perspective is that this constitutes a victory in the ongoing battle against the hypothesis that volcanoes at hot spots are caused by jets of hot material (plumes) rising from the deep mantle (“It is thus with much kicking, dragging, and screaming that geoscientists are being brought to the realization that all might not be well with the concept of mantle plumes”). Actually, small-volume, within-plate volcanism isn't exactly news. There is a wealth of well-established knowledge about three types of within-plate, often alkalic volcanism that cannot directly be caused by deep mantle plumes:(i) tens of thousands of small volcanic seamounts (1, 2);(ii) the 1600-km-long chain of oceanic and continental volcanoes known as the Cameroon Line, which shows no detectable time progression of eruptions and has long been discussed as a “hot line,” not a plume (3); and (iii) volcanism associated with continental (and oceanic) rifts not related to plate boundaries. The discovery of another line of small alkalic seamounts not caused by a hot spot or a plume is neither new nor surprising. And the suggestion that this constitutes a “nail in the plume coffin” merely obfuscates the plume debate.The basic rules of geochemistry tell us that high enrichments of incompatible trace elements found in alkali basalts require low degrees of melting; such melts are hardly a surprise in regions of thick oceanic lithosphere. Whether the volcanism itself is triggered by a mantle plume or a fracture in the lithosphere is a separate question. The observation that fractures can and do trigger volcanism is not an argument against deepmantle plumes, any more than confirmation of mantle plumes, for example, through seismic tomography (4), could be an argument against fracture-related volcanism.