Thermal structure and variability of the lower continental crust: dataset and models based on project DIVE boreholes 5071_1_A and 5071_1_B

Knowledge of the thermal conductivity (TC) and internal heat production (A) of rocks forming the lower continental crust (LCC) is essential for any thermal study of the Earth's lithosphere, as they define the geotherm. Many heat flow models use sparsely sampled data of thermal properties along with simplified layer structures, leading to poorly constrained geotherms and large uncertainties in temperature calculations. To improve heat flow calculations of the LCC, we present new high-resolution TC data (total data points: 13080) of felsic, (meta-)mafic and ultramafic lithologies of the LCC of the Ivrea-Verbano Zone (IVZ, Northern Italy). The measurements are done on a representative set of drill cores from the scientific drilling project DIVE (Drilling the Ivrea Verbano ZonE; ICDP; www.dive2ivrea.org; Pistone et al. 2017) in which two boreholes 5071_1_A (final depth: 909.5 m), located in Megolo, and 5071_1_B (final depth: 578.5 m), located in Ornavasso, have been realised. A total number of 74 drill core samples with a total length of ~ 28 m from the two boreholes have been measured at high spatial resolution using an optical TC scanner (OTCS, Popov et al., 1999). Measured TC for lithologies of the upper part of the LCC (1_B) ranges between 1.79 – 4.97 W/m·K for amphibolites, 2.02 – 6.63 W/m•K for kinzigites, 1.53 – 5.34 W/m•K for calcsilicates, and 2.12 – 5.70 W/m•K for leucocratic veins. The lithologies of the lower part of the LCC (1_A) show TC that ranges between 1.71 – 2.75  W/m•K for stronalites, 1.83 – 2.54 W/m•K for gabbros, 1.55 – 2.11 W/m•K for gabbronorites, 1.70 – 2.36 W/m•K for garnet granulites, 1.66 – 2.40 W/m•K for intermediate gabbronorites, 1.56 – 2.13 W/m•K for anorthosites, and 2.16 – 3.54 W/m•K for pyroxenites. The results show a significant variability of TC within the same lithology and between different lithologies, explained by the spatially variable mineral contents and grain sizes. Measured concentrations of heat-producing elements (U, Th and K) of 33 selected drill cores were obtained using powder-based gamma spectrometry. The results show that the concentrations are lithology-dependent and decrease towards mafic and ultramafic rocks. These data are compared to spectral-gamma borehole logs to evaluate the radiogenic heat production along both boreholes. TC and A are used as input parameters for 7 types of probabilistic, steady-state 1D heat flow models with synthetic lithology columns with variable layer thicknesses (d) that are randomly assigned and emulate the lithology characteristics seen in the boreholes and the IVZ. By carrying out many realisations, the effect of high-resolution TC sampling on heat flow uncertainties is quantified. The adaptive nature of our models allows us to test the parameter sensitivity of TC, A, and d. The first results show that higher spatial variability on thermal properties structure cause larger model uncertainties in the temperature calculations compared to cases with more homogeneous structure.