Elsevier

Chemical Geology

Volume 145, Issues 3–4, 15 April 1998, Pages 395-411
Chemical Geology

Thermal structure, thickness and composition of continental lithosphere

https://doi.org/10.1016/S0009-2541(97)00151-4Get rights and content

Abstract

Global compilations of surface heat flow data from stable, Precambrian terrains show a statistically significant secular change from 41±11 mW/m2 in Archean to 55±17 mW/m2 in Proterozoic regions far removed from Archean cratons. Using the tectonothermal age of the continents coupled with average heat flow for different age provinces yields a mean continental surface heat flow between 47 and 49 mW/m2 (depending on the average, non-orogenic heat flow assumed for Phanerozoic regions). Compositional models for bulk continental crust that produce this much or more heat flow (i.e., K2O>2.3–2.4 wt%) are not consistent with these observations. More rigorous constraints on crust composition cannot be had from heat flow data until the relative contributions to surface heat flow from crust and mantle are better determined and the non-orogenic component of heat flow in the areally extensive Phanerozoic regions (35% of the continents) is determined. We calculate conductive geotherms for 41 mW/m2 surface heat flow to place limits on the heat production of Archean mantle roots and to evaluate the significance of the pressure–temperature (PT) array for cratonic mantle xenoliths. Widely variable geotherms exist for this surface heat flow, depending on the values of crustal and lithospheric mantle heat production that are adopted. Using the average K content of cratonic peridotite xenoliths (0.15 wt% K2O, assuming Th/U=3.9 and K/U=10,000 to give a heat production of 0.093 μW/m3) and a range of reasonable crustal heat production values (i.e., ≥0.5 μW/m3), we calculate geotherms that are so strongly curved they never intersect the mantle adiabat. Thus the average cratonic peridotite is not representative of the heat production of Archean mantle roots. Using our preferred estimate of heat production in the cratonic mantle (0.03 wt% K2O, or 0.019 μW/m3) we find that the only geotherms that pass through the xenolith P–T data array are those corresponding to crust having very low heat production (<0.9 wt% K2O). If the lithospheric mantle heat production is higher than our preferred values, the continental crust must have correspondingly lower heat production (i.e., bulk crustal K, Th and U contents lower than that of average Archean granulite facies terrains), which we consider unlikely. If the xenolith P–T data reflect equilibration to a conductive geotherm, then Archean lithosphere is relatively thin (150–200 km, based on intersection of the P–T array with the mantle adiabat) and the primary reason for the lower surface heat flow in Archean regions is decreased crustal heat production, rather than the insulating effects of thick lithospheric roots. On the other hand, if the xenolith P–T points result from frozen-in mineral equilibria or reflect perturbed geotherms associated with magmatism, then the Archean crust can have higher heat producing element concentrations, lithospheric thickness can range to greater depths and the low surface heat flow in Archean cratons may be due to the insulating effects of thick lithospheric roots. An uppermost limit for Archean crustal heat production of 0.77 μW/m3 is determined from the heat flow systematics.

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):qs=qr+DAwhere 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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