Temperature-Dependent Thermal Properties of Troctolite

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

Presenting Author:

Authors:

Abstract:

Accurate knowledge of thermal properties of rocks (e.g. thermal diffusivity D and thermal conductivity k = DrC_P, where r is density and C_P is heat capacity) is critical for understanding heat flow in the crust. Thermal properties are themselves temperature- dependent and vary widely between different minerals (e.g. at room temperature, D₂₉₈ is ~5 mm²s^-1 for quartz, ~2.5 mm²s^-1 for olivine, and <1 mm²s^-1 for most plagioclase feldspars; Hofmeister et al., 2015, Treatise on Geochemistry, 2nd Edition, Chapter 2.23). Some of the most extreme variations in thermal properties are therefore found in layered mafic intrusions, where mineralogical compositions of different layers can vary widely, and temperatures vary from the basalt liquidus during intrusion, to ambient for present-day surface exposures. Troctolite, a layered mafic cumulate rich in plagioclase and olivine, provides an ideal case to study such thermal behavior due to its contrasting mineral properties and common occurrence in layered intrusions. 

We studied a troctolite from the Sonju Lake Intrusion of the Duluth Complex (Minnesota, USA), with a density of ~3100 kgm^-3. Fifteen discs were prepared from a single hand sample and measured using a NETZSCH LFA 467 HyperFlash, together with a Pyroceram standard, to maximum temperatures (T_max) between 300 °C and 1250 °C. Thermal diffusivity always decreases with increasing T, from ~0.7-0.8 mm²s^-1 at 25˚C to ~0.4 mm²s^-1 at 950˚C. Thermal conductivity decreases less steeply with increasing T, from ~2 Wm^-1K^-1 at 25˚C to ~1.4 Wm^-1K^-1 at 950˚C. Such large variations in thermal properties with temperature are important to consider in thermal models of cooling intrusions, and of crustal geotherms.   

Samples heated above 400˚C were affected by a permanent decrease in D₂₉₈ of ~15-20%. This drop is tentatively attributed to microfracture formation, potentially caused by a combination of thermal expansion mismatch between plagioclase and olivine, and breakdown of hydrous secondary phases, suggested by up to a few weight percent mass loss during heating.