Thermal structure, thickness and composition of continental lithosphere
Introduction
Heat flowing from the surface of the earth can be divided into three components (Vitorello and Pollack, 1980): (1) heat from radiogenic decay of heat producing elements (HPE, mainly K, Th and U) in the lithosphere,1 (2) heat conducted through the lithosphere from the underlying convective mantle and (3) `orogenic' heat, convectively transported from magmas and fluids that enter the lithosphere from below during orogenic events. If these contributions to heat flow can be distinguished, it may be possible to place constraints on lithospheric composition (both crust and mantle) from heat flow data.
Nearly three decades ago it was discovered that surface heat flow correlates positively with heat production in particular heat flow provinces (Birch et al., 1968; Lachenbruch, 1968; Roy et al., 1968):where qs is the surface heat flow, qr is the reduced heat flow, D is the slope of the line (and broadly reflects the depth distribution of heat producing elements) and A is the heat production at the site where the heat flow is measured. The reduced heat flow was originally identified as the heat that originates from below the radiogenically-enriched upper crustal layer (Roy et al., 1968) and includes a mantle and deep crustal contribution to heat flow. Some subsequent workers have identified reduced heat flow with mantle heat flow in order to separate crust and mantle contributions to heat flow and place constraints on the heat producing element content of the continental crust.
However, recent work has shown that such an interpretation is likely to be in error, due to the combined effects of lateral heterogeneities in thermal conductivity and heat production within the crust (Jaupart, 1983; Furlong and Chapman, 1987; Pinet and Jaupart, 1987) and the possible effects of thick lithospheric mantle roots on heat flow from the convective mantle (Ballard and Pollack, 1987; Nyblade and Pollack, 1993). Therefore, constraints on crust composition from surface heat flow data are not as robust as was originally assumed by Taylor and McLennan (1985), who relied on the earlier heat flow models to derive their continental crust composition.
This paper consists of two parts. In the first part we review the constraints that heat flow data place on the composition of the continental crust. In the second part we investigate the bounds on heat producing elements abundances in the lithosphere of Archean cratons by comparison with surface heat flow and the temperature distribution in the lithosphere.
Section snippets
Composition of the continental crust
Table 1 lists models of the K, Th and U content for the bulk continental crust. These compositional estimates have been derived from observations of seismic velocities of the crust (Christensen and Mooney, 1995; Rudnick and Fountain, 1995; Wedepohl, 1995; Gao et al., 1998), chemical composition of granulite facies terrains (Weaver and Tarney, 1984; Shaw et al., 1986) and from heat flow observations combined with models of how the crust grows (Taylor and McLennan, 1985; McLennan and Taylor, 1996
Composition and thermal structure of cratonic lithospheric mantle
The lithospheric mantle represents another potential source of heat within the continents and, although the concentrations of HPE are clearly much lower in mantle than in crustal rocks, the great thickness of lithospheric mantle that may exist beneath some crustal regions (e.g., cratons) makes mantle lithosphere a potentially important contributor to surface heat flow (Jordan, 1988).
Unfortunately, the lithospheric mantle is much less accessible than the continental crust so its heat production
A final word on Archean crustal heat production
In Fig. 6a, the curve corresponding to 0.7 μW/m3 crustal heat production intersects the adiabat at a depth of ∼450 km. This is deeper than even the deepest seismic models of lithospheric thickness, and would extend the lithosphere into the Earth's transition zone, which begins at ∼410 km. The minimum lithospheric thickness for a crust with this heat production is ∼300 km (Fig. 7), assuming no heat production in the lithospheric mantle. From the relationship between crustal heat production and
Conclusions
The average surface heat flow for stable regions of the continents, where heat flow is likely to be mainly conductive, ranges between 47 and 49 mW/m2 (depending on the non-orogenic heat flow in Phanerozoic crust). Crustal compositional models that produce this much or more surface heat flow are too radiogenic (e.g., the models of Shaw et al., 1986and Wedepohl, 1995). The remaining crustal models produce less heat flow than this upper limit and are therefore compatible with the heat flow data.
Acknowledgements
We thank Joe Boyd for sharing his mineral chemistry data for Kaapvaal and Siberian mantle xenoliths and Carl Agee for discussions on the temperature of transition zone phase changes. We are grateful to Henry Pollack, Dallas Abbott, Scott McLennan and Anton Hales for thoughtful reviews and Andy Nyblade and Geoff Davies for discussions, all of which have led to improvements in the manuscript. This work has been supported by NSF grants EAR 95-06510 to RLR and EAR 95-06517 to WFM.
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