Thermal properties of the lower continental crust: Variabilities and link to other rock properties based on DIVE drill cores and boreholes (Ivrea-Verbano Zone)

Scarcity of thermal property data on lower crustal rocks and their variability is a major hindrance in constraining the continental geotherm. As part of the ICDP project DIVE, two boreholes have recently sampled 1.5 km of rocks across a lower crustal metasedimentary and a mafic-ultramafic section of the Ivrea-Verbano Zone. Complete drill core recovery and borehole logging allowed us to generate a new, representative dataset of thermal properties ranging from high-resolution measurements to larger scale profiles. Thermal conductivity under ambient con-
ditions varies considerably within every lithology type: felsic lithologies are more variable and show higher averages (~3.0 W/(m⋅K)) than mafic lithologies (~2.4 W/(m⋅K)). Radiogenic heat production is generally low and varies considerably between mafic-ultramafic lithologies (~0.05 μW/m3), felsic lithologies and granulitefacies metasediments (~0.5 μW/m3), and amphibolite-facies metasediments (~1.5 μW/m3). Overall, metasediments are the largest contributor to the heat budget of the lower crust, where heat production seems to be related to the protolith, not to the metamorphic grade. Measurements of specific heat capacity, density, and thermal
diffusivity also reveal primary differences between felsic and mafic lithologies. There is an inverse semilogarithmic correlation between density and heat production. The variability of all measured thermal properties is significant at all spatial scales, and increases with increasing scales. Thermal conductivity and diffusivity vary by a factor of ca. 4 within each borehole, and heat production by a factor of 10 between the boreholes. These findings prompt for detailed sampling and comprehensive assessment of thermal property variabilities for applications involving the thermal field.