Page 28 - Geothermal Energy Systems Exploration, Development, and Utilization

4 1 Reservoir Definition
1.1.3
Mantle Convection and Heat Loss beneath the Lithosphere

Heat from the mantle is released through the overlying lithosphere. Spatially
averaged heat flow data over oceans and continents show a strong discrepancy
between oceanic and continental mantle heat losses. Among the 46 TW of total
heat loss, only 14 TW is released over continents. In terms of heat losses, two major
differences between continental and oceanic lithospheres must be explained.
First, oceanic lithosphere can be considered as a thermal boundary layer of the
convective mantle since it does participate in convective motions. Actually, oceanic
heat flow data show a similar decrease from mid-oceanic ridges to old subducting
lithosphere as that deduced from theoretical heat flow variation from upwelling-
to downwelling parts of a convecting system (Parsons and Sclater, 1977). Second,
heat production within the oceanic lithosphere is negligible when compared to that
of the continental lithosphere, enriched in radioactive elements. It follows that the
oceanic lithosphere can be considered as a ‘‘thermally inactive’’ upper boundary
layer of the convective mantle. In other words, the appropriate thermal boundary
condition at the top of the oceanic mantle corresponds to a fixed temperature
condition, which is indeed imposed by oceanic water.
Contrary to oceanic lithosphere, continental lithosphere is not directly subducted
bymantle downwellings and behaves as afloating bodyof finite thermal conductivity
overlying a convective system (Elder, 1967; Whitehead, 1976; Gurnis, 1988; Lenardic
and Kaula, 1995; Guillou and Jaupart, 1995; Jaupart et al., 1998; Grign´ eand
Labrosse, 2001; Trubitsyn et al., 2006). Even if atmospheric temperature can be
considered as a fixed temperature condition at the top of continents, it does not
apply to their bottom parts (i.e., at the subcontinental lithosphere–asthenosphere
boundary) since heat production within continents create temperature differences
at depths. Depending on crustal composition, heat production rates can vary from
one continental province to the other, and lateral temperature variations at the
conducting lithosphere–convecting asthenosphere boundary are thus expected. It
follows that thermal boundary condition at the base of the continental lithosphere
may be difficult to infer since thermal regime of continents differs from one case
to the other. However, as it is suggested below, some large-scale trends in thermal
behavior of continental masses can be drawn and thus a subcontinental thermal
boundary condition may be inferred.

1.1.3.1 Mantle Heat Flow Variations
Since radiogenic heat production is negligible in oceanic lithosphere, heat flow
through the ocean floor corresponds to mantle heat flow at the bottom of the
oceanic lithosphere. This suboceanic heat flow varies from several hundreds of
milliwatts per square meter at mid-oceanic ridges to about 50 mW m −2 over
oceanic lithosphere older than 80 Myr (Lister et al., 1990). When thermal effects
of hydrothermal circulation are removed, this variation is well explained by the
cooling plate model.