Melting and dehydration within mantle plumes and the formation of sub-parallel volcanic trends at intra-plate hotspots
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One of the defining characteristics of plume-fed hotspots is the formation of a linear chain of age-progressive volcanoes [Wilson, 1963; Morgan, 1972; Courtillot et al, 2003]. The most prominent example of this is the Hawaii-Emperor Seamount Chain, a 6000-km long age-progressive chain of volcanoes that stretch from the present-day island of Hawaii to the Aleutian Trench [van Ark and Lin, 2004; Sharp and Clague, 2006] However, recent volcanism at Hawaii does not form a simple linear trend, but rather is organized into two physically distinct sub-parallel chains, known as the Loa and Kea trends [Jackson, 1972]. Furthermore, recent studies have revealed that volcanism at several other hotspots, including the Samoa [Workman et al., 2004], Marquesas [Chauvel et al., 2009; Huang et al., 2011], and Society [Payne et al., submitted] hotspots are similarly organized into sub-parallel trends. Hieronymus and Bercovici  developed a model in which lithospheric flexure in response to loading and combined with a change in plate motion, could generate sub-parallel trends of discrete volcanoes at plume-fed hotspots. Here, we develop an alternative mechanism for the formation of dual-chain volcanism at hotspots in which melting and dehydration of upwelling material within the plume conduit creates a buoyant, highly viscous plug of residuum that extends downwards from the base of the lithosphere above the plume conduit, causing the flow to bifurcate [Hall and Kincaid, 2003]. We report on a series of 3-D numerical experiments using CitcomCU in which an upwelling plume impinges on the base of an overriding oceanic plate. These experiments employ a diffusion creep rheology that includes the effect of water content on viscosity. Melting and dehydration are modeled using a Lagrangian particle method. This study analyzes the effect of dehydration on a plume-lithosphere setting, displaying results for both end members. Our results demonstrate the formation of the proposed viscous plug is plausible within a range of parameter space relevant to the Earth. The presence of this plug inhibits upwelling directly above the plume conduit, diverting plume flow to the edges of the plug and effectively bifurcating magma production in the mantle in the process.
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