Mechanism of Emplacement of Flood Basalt

[706396]科洪玄武岩是地球表面第二重要的基性火成岩物质堆积，长期以来，人们一直认为它们的喷发速度比现在的中央玄武岩火山喷发速度高出三到四个数量级。这个想法依赖于一个假设，即在没有明显中断的情况下放置单个流。最近有人提出，这种流动可能是内生的，也就是说，在一个多阶段的过程中，熔岩被注入已经凝固的流动的内部，并顶起顶部表面。如果内生增长是一种常见的就位机制，那么它将降低巨大的喷发速度和相关的大量熔体的快速生产或储存，而这些都是这些省份特征的非常大的单个熔岩流的单阶段就位所必需的。我们的试点项目是通过确定哥伦比亚河洪水玄武岩的Birkett流的磁性结构来测试内生生长的假设，对于内部囊泡层的现场观察已经表明了多级就位的可能性。磁性结构分析可以探测到流动过程中发生更强烈剪切的区域：对于多级就位，流动内部应存在两个或更多剪切边界。如果囊泡层与剪切边界重合，与初始凝固后熔岩的注入相一致，则将为更大规模地研究大陆洪水玄武岩内生生长的作用奠定基础。

9706396 Coe Flood basalts are the second most significant accumulation of mafic igneous material on the surface of the Earth, and their rate of eruption has long been considered to be three to four orders of magnitude greater than occurs at present-day central basaltic volcanoes. This idea depends on the assumption that a single flow is emplaced without significant interruption. Recently it has been proposed that such flows may have grown endogenously, that is, in a multistage process by which lava is injected into the interior of an already solidified flow and jacks up the top surface. If endogenous growth is a common mechanism of emplacement, it would reduce the enormous eruption rates and associated rapid production or storage of vast quantities of melt that are required for single-stage emplacement of very large individual lava flows characteristic of these provinces. Our pilot project is to test the hypothesis of endogenous growth by determining the magnetic fabric of the Birkett flow of the Columbia River Flood Basalts, for which field observations of interior vesicle layers have already suggested the possibility of multistage emplacement. Magnetic fabric analysis can detect zones where more intense shear occurred during flow: for multistage emplacement, two or more shear boundaries should be present inside the flow. If the vesicular layers coincide with shear boundaries, consistent with the injection of lava after initial solidification, the stage will be set for a larger investigation of the role of endogenous growth in continental flood basalts.