Crustal differentiation on Earth and Mars from Potassium and Thorium multi-scale distributions

Abstract:

Potassium (K) and Thorium (Th) are useful tracers of crustal differentiation and secondary processes as these elements concentrate in melts during magmatic processes but are not equally mobile during alteration. They may be mapped by airborne or orbiting gamma ray spectrometers measuring the natural activity of ²³²Th and ⁴⁰K. Such data are commonly used in geological mapping and mineral exploration on the Earth and have been produced for other planets. However, the direct comparison of spatially averaged concentrations measured remotely with bulk rock analyses of surface samples may be challenging. In the case of Mars, there is a notable shift between K, Th concentrations in Martian meteorites with those from orbital data [1]. We observe a similar discrepancy when airborne radiometric surveys on portions of the continental crusts are statistically compared with bulk analyses of rock samples. We argue that the generally right-skewed characteristics of the distributions of K, Th concentrations for the terrestrial crust may be extrapolated to the Martian crust. We illustrate how such distributions transform with resolution in remote sensing data, and evolve as normal laws as expected from the central limit theorem. This well-known statistical effect offers a simple explanation for the shift between K, Th concentrations in meteorites and orbital data. The shift would reflect the existence of disseminated occurrences of differentiated rocks on Mars, adding to other pieces of evidence for magmatic diversity [2-4]. The characteristics of the distribution of K and Th within the GRS footprint appears to be critical for a correct interpretation or these data, but such distribution cannot be directly documented on Mars due to the lack of appropriate data. However, interpretation of airborne remote sensing data sets on Earth will clearly benefit from the characterization of the spatial organization of K and Th concentrations at the outcrop scale (1 – 100 m), which is now possible by using portable gamma ray spectrometers.

[1] Taylor, G. J et al. JGR-Planets 111, E03S06 (2006). [2] Carter, J. & Poulet, F. Nature Geoscience 6, 1008-1012 (2013). [3] Baratoux, D. et al. JGR-(Planets) 119, 1707-1727 (2014). [4] Sautter et al. Nature Geoscience. DOI: 10.1038/NGEO2474 (2015).