Thermal convection in a volumetrically heated, infinite Prandtl number fluid with strongly temperature-dependent viscosity: Implications for planetary thermal evolution

Research areas:
Year:
1998
Authors:
Journal:
JOURNAL OF GEOPHYSICAL RESEARCH-SOLID EARTH
Volume:
103
Number:
B8
Pages:
18171-18181
Month:
AUG 10
BibTex:
Abstract:
Parameterized models of the thermal evolution of planets are usually
based on the assumption that the lithosphere-convecting mantle boundary
can be defined by an. isotherm at a temperature below which viscosity is
infinite on geologic timescales. Recent experimental results argue
against this assumption. We have investigated both the definition of the
lithosphere-convecting mantle boundary and the power law relation
describing convecting heat transfer, based on numerical experiments of
thermal convection in a volumetrically heated fluid with
temperature-dependent viscosity. Other recent studies have treated only
the heating from below, but volumetric heating is likely to be the
dominant mode of heating in planetary mantles, either as a consequence
of radioactive heating or as a proxy for secular cooling. Convection can
occur either in the whole box or be located under a stagnant lid. In the
lid regime, convection is driven by a temperature contrast depending on
the rheology of the fluid and the interior temperature. This result, in
agreement with experimental studies, indicates' that boundary between
the stagnant lid and the convecting layer (similar to the
lithosphere-convecting mantle boundary) cannot be defined as a fixed
isotherm. During thermal evolution of planets, the viscosity contrast in
the convecting mantle remains constant, not the temperature at the
bottom of the lithosphere. We present an example showing that the
evolution of planets is strongly dependent on the criterion chosen to
define the lithosphere-convecting mantle boundary. For reasonable values
of the activation energy for thermally activated creep, the temperature
defining the lithosphere-convecting mantle boundary, the mantle
temperature, and the thickness of the lithosphere could be larger than
expected from previous models which treat the base of the lithosphere as
a fixed isotherm.