Ambiguity of crustal geotherms: A thermal-conductivity perspective

Abstract

The paper addresses the effects of pressure (P) and temperature (T) on thermal conductivity (λ) applied to crustal geotherms. A subset of the ca. 50 experimentally derived numerical P―T relations available in literature for typical crustal rock types was selected for this purpose. The relations comprise those from physical-contact laboratory methods (PCM), for P and T, as well as from a non-contact technique, the laser-flash analysis (LFA), for T. The effects on geotherms are quantified for two crustal models: (I) a simplified two-layer “Proterozoic” crust, which consists of granitic rocks in the upper part and granulitic rocks in the lower part and (II) a multi-layer “Phanerozoic” crust. We show that neglecting P–T correction of ambient λ leads to underestimation of T that is larger with increasing surface heat flow (qs). In the Phanerozoic crust, the T difference at the crust–mantle boundary is 130/50 K [PCM/LFA], for qs 60 mW m–2, and 240/100 K, for qs 80 mW m–2. In the Proterozoic crust, with qs typically <60 mW m–2, the T difference between P–T-corrected versus uncorrected geotherms is minor (55/55 K). The T uncertainty shown by the envelope of min–max solutions of λ correction differs with respect to type of crust and typical heat flow. Lower values (max. < 20/<10 K) are inferred for Proterozoic crust and low to moderate qs (<60 mW m–2) compared to the compositionally complex Phanerozoic-type crust (about 210/110 K) with typical qs of 80 mW m–2. A combined LFA(T)/PCM(P) correction of λ provides lower T than a P–T correction with PCM alone.