A new compositionally-based thermal conductivity model for plutonic rocks

Abstract

Thermal conductivity is a physical parameter crucial to accurately estimating and understanding temperature related processes within the lithosphere. Direct measurements however are not always practical and so recent research has turned to indirect estimates, often developing regionally-specific models from non-causal physical properties. In this study I report on 340 new thermal conductivity measurements of (mostly) plutonic rocks using an optical scanning device, coupled with major element geochemistry and modal mineralogy to produce broadly applicable empirical relationships between composition and thermal conductivity. Predictive models for thermal conductivity are developed using (in order of decreasing accuracy) major oxide composition, CIPW norms and estimated modal mineralogy. I find that SiO2 content is the dominant elementary control on thermal conductivity due not only to its relationship with quartz but also its relatively large abundance over the entire compositional range. The feldspars are the major control on thermal conductivity for both mineralogy-based models, with particular emphasis on the transition from Na-rich albite to Ca-rich anorthite. Four common mixing models (arithmetic, geometric, square-root and harmonic) are tested and, while the results are similar, the geometric model produces the best fit. The preferred model uses five commonly reported oxides (SiO2, Al2O3, FeO, Na2O and K2O) plus loss on ignition to predict thermal conductivity across the entire compositional spectrum of plutonic rocks to within 0.27 W m-1 K-1. Since geochemistry cannot always be sampled directly, a comparison of thermal conductivity and oxide-based estimates of P-wave velocity and density reveal systematic trends across the compositional range. Relating thermal conductivity, via a direct compositional control, to these widely available surface measurements allows for a more universal predictive model of thermal conductivity than has been previously established.