Radiogenic heat production inthe continental crust accounts for a significant portionof Earth’s total heat budget. Past crustal heat production estimateshave relied on surface heatfluxand an assumedmantle heat flux. However, few direct constraints on mantle heat flow—and hence Moho temperature—exist. Here we constraincrustal heat production in the Tanzanian and Siberian cratons using combined U-Pb andtrace-element petrochronology of accessory minerals from lower crustal xenoliths. Wederivepermissible Moho temperaturesfromtitanite, rutile,and apatitein xenoliths erupted from Quaternary volcanoes in the Tanzanian craton and adjacent Mozambique belt and the Devonian Udachnaya kimberlite pipe in the Siberian craton. Near-Moho depth xenoliths from Tanzania include a set ofgarnet two-pyroxene granulites that experienced peak pressures >1.6 GPa during Proterozoic continent collision. Titanite yields a range of U-Pb dates that, along with Sr, Nd, and Zr measurements, suggest prolonged recrystallization in the lower crust. Rutile and apatite do not contain measureable radiogenic Pb despite appreciable amounts of U. These data indicate that the Moho beneath the craton and belt is cooler thanthe Pb closure temperatureof titanite, but hotter thanthat of rutile and apatite. Using regional surface heat fluxes, we obtain crustal heat production values <0.6 μW/m3for the craton and 0.4–1.1 μW/m3for the belt. Preliminary analyses of rutile and apatite in amphibolite-and granulite-faciesxenoliths from Udachnaya revealcomplex age patterns that suggest prolonged residence below their respective Pb closure temperatures. Based on these data and available heat flow measurements, we infera very low heat production for the Siberian craton (<<0.5μW/m3). Collectively, thedata for both localities pointto exceedingly low crustal heat production in these cratons, which lie at or below the lower limit previously estimated for Archean crust.