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    Melt segregation and depletion during ascent of buoyant diapirs in subduction zones

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    Date
    2020-01-31
    Author
    Zhang, Nan  Concept link
    Behn, Mark D.  Concept link
    Parmentier, E. Marc  Concept link
    Kincaid, Christopher  Concept link
    Metadata
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    Citable URI
    https://hdl.handle.net/1912/25871
    As published
    https://doi.org/10.1029/2019JB018203
    DOI
    10.1029/2019JB018203
    Keyword
     sedimentary diapirs; subduction wedge; melt migration 
    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.
    Description
    Author Posting. © American Geophysical Union, 2020. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research-Solid Earth 125(2), (2020): e2019JB018203, doi:10.1029/2019JB018203.
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    • Geology and Geophysics (G&G)
    Suggested Citation
    Zhang, N., Behn, M. D., Parmentier, E. M., & Kincaid, C. (2020). Melt segregation and depletion during ascent of buoyant diapirs in subduction zones. Journal of Geophysical Research-Solid Earth, 125(2), e2019JB018203.
     

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