Melt Segregation and Depletion During Ascent of Buoyant Diapirs in Subduction Zones

Abstract

Cold, low‐density diapirs arising from hydrated mantle and/or subducted sediments on the top of subducting slabs have been invoked to transport key chemical signatures to the source region of arc magmas. However, to date there have been few quantitative models to constrain melting in such diapirs. Here we use a two‐phase Darcy‐Stokes‐energy model to investigate thermal evolution, melting, and depletion in a buoyant sediment diapir ascending through the mantle wedge. Using a simplified 2‐D circular geometry, we investigate diapir evolution in three scenarios with increasing complexity. In the first two scenarios we consider instantaneous heating of a diapir by thermal diffusion with and without the effect of the latent heat of melting. Then, these simplified calculations are compared to numerical simulations that include melting, melt segregation, and the influence of depletion on the sediment solidus along pressure‐temperature‐time (P‐T‐t) paths appropriate for ascent through the mantle wedge. The high boundary temperature induces a rim of high porosity, into which new melts are focused and then migrate upward. The rim thus acts like an annulus melt channel, while the effect of depletion buffers additional melt production. Solid matrix flow combined with recrystallization of melt pooled near the top of the diapir can result in large gradients in depletion across the diapir. These large depletion gradients can either be preserved if the diapir leaks melt during ascent, or rehomogenized in a sealed diapir. Overall our numerical simulations predict less melt production than the simplified thermal diffusion calculations. Specifically, we show that diapirs whose ascent paths favor melting beneath the volcanic arc will undergo no more than ~40–50% total melting.

On the Evolution and Fate of Sediment Diapirs in Subduction Zones

“…Similarly, the interval above the solidus where phengite remains stable is a function of bulk sediment composition and available fluid, but generally can extend to ∼900°C at 3 GPa (Mann & Schmidt, 2015). If this wet‐melting stage is near fractional, a refractory residue will be produced if phengite breaks down, limiting further melting (Zhang et al., 2020).…”

“…For quartz‐dominated sediment layers, k is approximately 0.5 based on non‐Newtonian scaling analyses (Miller & Behn, 2012). Then, assuming that diapirs nucleate from a volume with horizontal dimensions λ and height h , the radius r can be estimated as (Zhang et al., 2020). Given this scaling relationship, expected sediment diapir radii range from ∼0.5 to 4.0 km, which we take as the range of diapir radii examined in our models.…”

“…Further, sediment diapirs will be significantly more fusible compared to the surrounding mantle, which will generate sharp permeability gradients at the diapir margin. These sharp gradients may limit the efficiency of melt extraction from the diapir (Zhang et al., 2020). Finally, it is not clear that melt extraction will be favored given the modeled rapidly ascending diapirs (see below).…”

Weak orogenic lithosphere guides the pattern of plume-triggered supercontinent break-up

“…(3). Temperature T is calculate by adding a pre-factor (1 þ L C p ΔT 0 ) 98,99 before the first term of the left-hand side in Eq. (3), where L is the latent heat (assume to be 640 kJ/kg 43,100 ) and ΔT′ is the difference between solidus and liquidus temperatures.…”

Cold Plumes Initiated by Rayleigh‐Taylor Instabilities in Subduction Zones, and Their Characteristic Volcanic Distributions: The Role of Slab Dip

“…A number of previous studies have shown that partially molten zones formed by dehydration melting can be 2 to 20 km thick, depending upon the thermal structure of the subduction zone, and the depth and the degree of dehydration melting above the dipping slab (Gerya & Yuen, 2003;Grove et al, 2006Grove et al, , 2009Marsh, 1979). In some cases, they may incorporate materials derived from serpentinized subduction channel and subducted crustal sediments, as reported from the recycled sediment signatures in arc volcanoes (Marschall & Schumacher, 2012;Zhang et al, 2020).…”