Why are plume excess temperatures much less than the temperature drop
across the lowermost-mantle thermal boundary layer?
Abstract
While temperature drop across the mantle’s basal thermal boundary layer
(TBL) is likely $>$1000 K, the temperature anomaly of
plumes believed to rise from that TBL is only up to a few hundred
Kelvins. Reasons for that discrepancy are still poorly understood and a
number of causes have been proposed. Here we use the ASPECT software to
model plumes from the lowermost mantle and study their excess
temperatures. We use a mantle viscosity that depends on temperature and
depth with a strong viscosity increase from below the lithosphere
towards the lower mantle, reaching about $10^{23}$ Pas above the
basal TBL, consistent with geoid modelling and slow motion of mantle
plumes. With a mineral physics-derived pyrolite material model, the
difference between a plume adiabat and an ambient mantle adiabat just
below the lithosphere is about two thirds of that at the base of the
mantle, e.g. 1280 K vs.\ 835 K. 3-D models of isolated
plumes become nearly steady-state $>$ 10-20 Myr after the
plume head has reached the surface, with excess temperature drop
compared to an adiabat for material directly from the CMB usually less
than 100 K. In the Earth, plumes are likely triggered by slabs and
probably rise preferrably above the margins of chemically distinct
piles. This could lead to reduced excess temperatures, if plumes are
more sheet-like, similar to 2-D models, or temperature at their source
depth is less than at the CMB. Excess temperatures are further reduced
when averaged over the plume conduit or melting region.