Growth and mixing dynamics of mantle wedge plumes

Abstract

Recent work suggests that hydrated partially molten thermal-chemical plumes that originate from subducted slab as a consequence of Rayleigh-Taylor instability are responsible for the heterogeneous composition of the mantle wedge. We use a two-dimensional ultrahighresolution numerical simulation involving 10 × 10 9 active markers to anticipate the detailed evolution of the internal structure of natural plumes beneath volcanic arcs in intraoceanic subduction settings. The plumes consist of partially molten hydrated peridotite, dry solid mantle, and subducted oceanic crust, which may compose as much as 12% of the plume. As plumes grow and mature these materials mix chaotically, resulting in attenuation and duplication of the original layering on scales of 1-1000 m. Comparison of numerical results with geological observations from the Horoman ultramafi c complex in Japan suggests that mixing and differentiation processes related to development of partially molten plumes above slabs may be responsible for the strongly layered lithologically mixed (marble cake) structure of astheno spheric mantle wedges.

A community benchmark for subduction zone modeling

“…This causes an (integrable) singularity in pressure but as discussed below leads to some numerical difficulties when the Stokes flow is solved numerically. In nature the transition from fault contact at the seismogenic zone to the full viscous coupling between slab and mantle wedge is finite and can be approximated by improved and more detailed rheological descriptions and freely evolving slabs (Sobolev and Babeyko, 2005;Gorczyk et al, 2007;Gerya et al, 2008).…”

“…The modification could have impacts on thermal structure that are significantly larger than differences between simulation results from different codes. These factors include the dynamical effects of mineralogical phase changes including the likely serpentization of the wedge corner (Hyndman and Peacock, 2003;Gorczyk et al, 2007), melting and melt transport (Katz et al, 2007;Cagnioncle et al, 2007), and threedimensional effects (Kneller et al, 2007;Behn et al, 2007).…”

Testing the effects of basic numerical implementations of water migration on models of subduction dynamics

“…But water released from the subducting slab decreases the pressure and temperature at which melting occurs, thus enhancing mantle wedge melting and causing volcanism. Mantle material that is hydrated by water released from the slab may also form weak, positively buoyant "wet plumes" that rise upwards and efficiently hydrate the mantle wedge (Billen and Gurnis, 2001;Billen, 2008;Gorczyk et al, 2007;Richard and Iwamori, 2010). These fluids can then cause a more vigorous flow in the mantle wedge.…”

“…The effective migration path of water is therefore no longer purely vertical, but can include a horizontal component. This method has been implemented as an imposed vertical velocity added to the velocity of the solid-phase flow (Gorczyk et al, 2007) or as a dehydration front with an imposed horizontal and vertical velocity (Gerya et al, 2002).…”

“…III Free water migrates as a Darcy flow, following the density gradient between the solid phase and the fluid phase in the mantle wedge (Cagnioncle et al, 2007 Studies that use the above water migration schemes show differences in the spatial distribution of hydrated material in the mantle wedge as subduction evolves (Arcay et al, 2005;Gerya et al, 2002;Gorczyk et al, 2007;Cagnioncle et al, 2007). However, as the numerical setup of the subduction models also differs between these studies, it is difficult to evaluate the possible effects of the numerical implementation of water migration.…”