We have compared several methods of studying thermochemical convection
in a Boussinesq fluid at infinite Prandtl number. For the representat
ion of chemical heterogeneity tracer, marker chain, and field methods
are employed. In the case of an isothermal Rayleigh-Taylor instability
, good agreement is found for the initial rise of the unstable lower l
ayer; however, the timing and location of the later smaller-scale inst
abilities may differ between methods. For a simulation of entrainment
by thermal convection of a dense layer at the bottom of the mantle we
found good agreement for a few overturn times. After this, differences
between the results can be large. We propose intrinsic differences be
tween the methods and possibly chaotic mixing effects may be the cause
of the lack of detailed agreement. The comparison shows that high res
olution is necessary for a reasonable thermochemical study. This will
pose severe restrictions on the applicability of these methods to thre
e-dimensional situations.