75-8 Thermal Properties of Proterozoic Basement Gneisses from the Sveconorwegian Orogen, Southeastern Norway: Effects of Texture and Temperature

Session: Mineralogy, Geochemistry, Petrology, and Volcanology Student Session (Posters)

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Understanding geothermal gradients is essential for accurate subsurface modeling as they – along with mineralogy – control the rheological properties of the mid-lower crust. For this reason, developing an accurate model of the thermal properties of the basement is critical as these determine the local geothermal gradient. However, thermal properties are impossible to observe directly at depth and therefore must be characterized through laboratory testing. Here we examine how the deformation and texture of gneiss and amphibolite sampled from exposures of the Proterozoic basement of the Sveconorwegian orogen in southeastern Norway affect the thermal diffusivity (D), thermal conductivity (k), and heat capacity (C) from 25°C to Moho temperature and beyond. 

We measure the heat capacity and thermal diffusivity using laser-flash analysis, while the thermal conductivity is calculated with the formula k = DρC, where ρ is density. Density is measured by a helium pycnometer. Current results show a range for D of 1.1-2.1 mm²/s at 25°C and 0.8-1.0 mm²/s at 300°C, while k ranges 2.5-4.8 W/(m*K) at 25°C and 1.7-3.2 W/(m*K) at 300°C. For an individual sample, D decreases by approximately 10-20% from 25°C to 300°C, showing high variation even in similar samples. Thermal diffusivity at room temperature is highest in the most quartz-rich lithologies, and these also show the greatest decrease in D on heating. In our single high temperature experiment, we observe a D of 1.261 mm²/s at 25°C and 0.388 mm²/s at 1150°C in a fine-grained amphibolite. 

Most samples do not retain their original D after heating, with larger drops usually occurring in more deformed samples, potentially due to microcracking. We observe the most dramatic change in D in a well-foliated augen gneiss, where D dropped from 1.38 mm²/s to 1.09 mm²/s. We theorize the drop could be caused by the smaller grain size observed in the more deformed samples, or by the presence of hydrous minerals, which can explode when heated at the surface, but not at depth in the crust. However, further investigation must be carried out to find the true cause of this drop and the exact temperature it occurs at. Future work will also emphasize testing at higher temperatures.    

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

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