Core-Mantle Boundary Heat Flow

Core-Mantle Boundary Heat Flow The transition from perovskite (Pv) to postperovskite (pPv) varies with temperature and depth (pressure) as indicated by the dashed line on the left. If the temperature at the core-mantle boundary exceeds the temperature for post-perovskite stability, the steep increase in temperature with depth in the lower mantle thermal boundary layer will result in two intersections with the phase boundary. These intersections would be manifested in paired velocity increases and decreases as shown, and laterally varying “lenses” of post-perovskite as depicted on the right. Such paired discontinuities have been observed and used to estimate thermal gradients and heat flow based on the temperature calibration from mineral physics. (Reprinted by permission from Macmillan Publishers Ltd.: J.W. Hernlund, C. Thomas, and P.J. Tackley, 2005. A doubling of the post-perovskite phase boundary and structure of the Earth’s lowermost mantle, Nature, 434:882–886, doi:10.1038/nature03472, ©2005).
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About 25 years ago, seismologists discovered a seismic velocity discontinuity several hundred kilometers above the core-mantle boundary. This boundary remained enigmatic until 2004, when mineral physicists discovered that the dominant lower mantle mineral, silicate perovskite, transforms at corresponding pressures and temperatures to a new phase called post-perovskite. This discovery has stimulated great activity in seismology, mineral physics, and geodynamics. A calibrated phase change enables bounds to be placed on the absolute temperature at great depth in the Earth, with ~ 2500°C being estimated for the seismic discontinuity. Experiments and theory predict a steep positive pressure temperature gradient at the perovskite-post-perovskite transition, but it is possible that an even steeper thermal gradient in the hot thermal boundary layer above the core can intersect the transition twice, producing a lens of post-perovskite sandwiched between perovskite. Seismic observations support this model, with paired velocity increases and decreases observed at different depths. These velocity changes provide two estimates of temperature at closely spaced depths, enabling an estimate of the temperature gradient if a steady-state conductive boundary layer is assumed. Assuming a value of thermal conductivity then yields a direct estimate of the local heat flux from the core to the mantle. Several such estimates have now been published, finding values close to the average heat flux at the surface. Extrapolated globally, these studies imply that as much as a quarter of the surface heat flow comes from the core, though the uncertainties are large— particularly in the estimation of the thermal conductivity. These seismically derived constraints on heat flow have broad implications for mantle convection, core cooling, inner core growth, and other fundamental Earth processes driven by the global heat flux.


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