Zhang Nan

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Zhang
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Nan
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  • Article
    Melt segregation and depletion during ascent of buoyant diapirs in subduction zones
    (American Geophysical Union, 2020-01-31) Zhang, Nan ; Behn, Mark D. ; Parmentier, E. Marc ; Kincaid, Christopher
    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.
  • Article
    Augmin : a protein complex required for centrosome-independent microtubule generation within the spindle
    (Rockefeller University Press, 2008-04-28) Goshima, Gohta ; Mayer, Mirjam ; Zhang, Nan ; Stuurman, Nico ; Vale, Ronald D.
    Since the discovery of γ-tubulin, attention has focused on its involvement as a microtubule nucleator at the centrosome. However, mislocalization of {gamma}-tubulin away from the centrosome does not inhibit mitotic spindle formation in Drosophila melanogaster, suggesting that a critical function for γ-tubulin might reside elsewhere. A previous RNA interference (RNAi) screen identified five genes (Dgt2–6) required for localizing γ-tubulin to spindle microtubules. We show that the Dgt proteins interact, forming a stable complex. We find that spindle microtubule generation is substantially reduced after knockdown of each Dgt protein by RNAi. Thus, the Dgt complex that we name "augmin" functions to increase microtubule number. Reduced spindle microtubule generation after augmin RNAi, particularly in the absence of functional centrosomes, has dramatic consequences on mitotic spindle formation and function, leading to reduced kinetochore fiber formation, chromosome misalignment, and spindle bipolarity defects. We also identify a functional human homologue of Dgt6. Our results suggest that an important mitotic function for γ-tubulin may lie within the spindle, where augmin and γ-tubulin function cooperatively to amplify the number of microtubules.