Thermal characterization of the lower continental crust: first results from the DT-1B borehole of project DIVE (Ivrea-Verbano zone, Italy)

The thermal state of the Earth’s interior is a key factor in controlling various geological processes. However, our knowledge of the geotherm and its temporal and spatial variability is usually poorly constrained, as it is typically based on point-wise data. Specifically, for the lower continental crust (LCC), data on rock’s thermal properties are scarce, and therefore temperature estimates are uncertain.

 

We collect new data and provide new insights in this domain, realized in the frame of project DIVE (Drilling the Ivrea-Verbano zonE; www.dive2ivrea.org), which aims at a better understanding of the physical and chemical evolution and formation of the LCC. The first borehole in Ornavasso (DT-1B) has been successfully completed and reached a depth of 578.5 m with 100% core recovery; it provides continuous drill cores of mainly felsic metasedimentary and metamafic lithologies. The second borehole in Megolo di Mezzo (DT-1A) is ongoing and planned to be completed in Spring 2024.

 

The first results on the thermal characterization of lower crustal rocks are based on 17 fresh cores from DT-1B, sampling all the lithologies present in the borehole. We performed continuous, high-resolution (2 mm) thermal conductivity (TC) measurements using an Optical TC Scanner (Popov et al., 1999), profiling over 10 metres of rock cores in total. Our results show that TC can exhibit large variations even within a given lithology, as a result of mineralogical variability, indicating that this approach provides more representative results compared to conventional methods (e.g. needle-probe technique). We also measured the concentrations of heat producing elements (U, Th, K) using powder-based gamma spectrometry, and use (spectral) gamma borehole logs to evaluate the variability of heat production (A) in the borehole. The correlation of both TC and A with other petrophysical properties is analyzed.

 

Based on the new measurements, we investigate the consequences on LCC geotherms. The small-scale TC variations affect heat flow calculations and have implications for their uncertainty. These are quantified through model calculations as part of an upscaling procedure employing harmonic averaging. We aim to quantify the effect of continuous TC profiling and how our approach influences the level of uncertainties by applying many realizations of heat flow calculations. The probability distribution of heat flow can be determined by using Bullard’s approach (Bullard 1939; Beardsmore & Cull, 2001) and by randomly selecting rock’s thermal property data while calculating the geotherm. Further samples from DT-1B and a new set of samples from DT-1A will provide a representative dataset for the LCC.

 

 

References

 

Beardsmore, G. R. (Graeme R., & Cull, J. P. (James P. (2001). Crustal heat flow: a guide to measurement and modelling. Cambridge University Press.

 

Bullard, R. (1939). Heat flow in South Africa. Mon. Not. R. Astr. Soc., Geophys, 173, 229–248.

 

Popov, Y. A., Pribnow, D. C., Sass, J. H., Williams, C. F., & Burkhardt, H. (1999). Characterization of rock thermal conductivity by high-resolution optical scanning. Geothermics, 28(2), 253–276.