13:45 〜 14:00
[SIT21-01] Mantle plumes: effect rather than cause of lithospheric breakup
★招待講演
キーワード：Mantle plume, rift, de-compressive melting, thermal expansion
In 1971, Morgan suggested a concept of mantle plumes. The model is widely but not unequivocally accepted as the cause for flood basalt provinces. However, scientists familiar with volcanic activities or flood basalts, both time and regions, puzzled over how the mantle plume concept accorded with the observations. Many researchers argue that lithosphere breakup and associated volcanic or flood basaltic outbreaks and hotspots are controlled, top-down, by shallow processes, rather than by rising mantle plumes. Anderson has formalized this opposing view as the Plate paradigm. This paradigm includes concepts related to crack propagation, internal plate deformation, volcanic activities, recycled subducted slabs, and lithospheric breakup. Although both Plume hypothesis and Plate hypothesis relate to the thermal effect, few studies have been found to attempt to explain how the initial thermal instability forms or why the source of magma for volcanisms and LIPs might be maintained. In Plume hypothesis, they always assumed that such thermal instabilities are formed mostly at the core-mantle boundary and that the plume conduits remain for millions of years, as implied by the persistence of hotspots. White and McKenzie have developed a detailed formulation of the more passive and uniformitarian rifting model, providing an different explanation for the presence of huge basalt accumulations along rifted continental margins. However, they placed little emphasis on how mantle plumes begin and reach the base of the lithosphere.
In this paper, I combine the failure dynamic studies with inferences drawn from a three-dimensional modeling of surface cracking under thermal expansion induced extension and the results of geological observations to consider the dynamics of the putative link between mantle plume, flood basalts, and lithospheric breakup. I will show a numerical modeling result of a surface failure pattern occurred on a spherical shell loaded from inside with an internal pressure in a displacement control manner, resembling the behavior of thermal expansion, from which I derive an intuitive physical model of the process of surface cracking as a self-organized phenomena. It is shown that deep mantle plume is not required as the prerequisite for such a process. A new hypothesis for no-root mantle plume, starting from the top of the asthenosphere in a top-down pattern is then proposed, which implies that the mantle plumes should then be regarded as the effect rather than the cause of lithospheric breakup.
Based on the model, a LIP event can be considered as a positive feedback loop of a process that the response to its change amplifies the change. During a LIP event, heat accumulation in the mantle may cause continental uplift in the ways of thermal expansion and volume increase during the phase change. The lithospheric uplift may trigger rift in global scale with a pattern of polygonal fractures. This shallow-based lithospheric process can locally release stresses, thus promoting local de-compressive melting. Extra volume increase of the magma (during phase change from solid to liquid) may serve as the driving force for eruption, which in turn should result in new cracking and associated sudden pressure drop. The process will become unstable, providing the coupling between pressure and temperature within the mantle satisfy certain conditions for phase change. No deep mantle plume is needed to for such a LIP. This mechanism furthers our understanding of global cooling events: the gradual accumulation of heat within the earth may result in large igneous provinces, which may cause abrupt loss of heat during large volcano eruptions or huge flood basalts. This abrupt loss of Earth’s heat makes the warming cycle to an end and a new start of the cooling cycle initiates, with glaciations as the extreme results.
In this paper, I combine the failure dynamic studies with inferences drawn from a three-dimensional modeling of surface cracking under thermal expansion induced extension and the results of geological observations to consider the dynamics of the putative link between mantle plume, flood basalts, and lithospheric breakup. I will show a numerical modeling result of a surface failure pattern occurred on a spherical shell loaded from inside with an internal pressure in a displacement control manner, resembling the behavior of thermal expansion, from which I derive an intuitive physical model of the process of surface cracking as a self-organized phenomena. It is shown that deep mantle plume is not required as the prerequisite for such a process. A new hypothesis for no-root mantle plume, starting from the top of the asthenosphere in a top-down pattern is then proposed, which implies that the mantle plumes should then be regarded as the effect rather than the cause of lithospheric breakup.
Based on the model, a LIP event can be considered as a positive feedback loop of a process that the response to its change amplifies the change. During a LIP event, heat accumulation in the mantle may cause continental uplift in the ways of thermal expansion and volume increase during the phase change. The lithospheric uplift may trigger rift in global scale with a pattern of polygonal fractures. This shallow-based lithospheric process can locally release stresses, thus promoting local de-compressive melting. Extra volume increase of the magma (during phase change from solid to liquid) may serve as the driving force for eruption, which in turn should result in new cracking and associated sudden pressure drop. The process will become unstable, providing the coupling between pressure and temperature within the mantle satisfy certain conditions for phase change. No deep mantle plume is needed to for such a LIP. This mechanism furthers our understanding of global cooling events: the gradual accumulation of heat within the earth may result in large igneous provinces, which may cause abrupt loss of heat during large volcano eruptions or huge flood basalts. This abrupt loss of Earth’s heat makes the warming cycle to an end and a new start of the cooling cycle initiates, with glaciations as the extreme results.