Earth, Environmental, and Planetary Sciences as told by potassium isotopes

In this talk, I will introduce potassium isotopes, a novel isotope tracer, and discuss its applications in the fields of Earth, Environmental and Planetary Sciences.

High-precision stable K isotopes is an emerging non-traditional isotope system which has been significantly developed in recent years. Geochemically, K is a highly incompatible lithophile element, and a highly soluble, biophile element. The isotopic fractionation of K is relatively small during magmatic processes such as partial melting and fractional crystallization, whereas during low-temperature and biological processes its fractionation is considerably larger. This significant fractionation has made K isotopes a promising tracer for a variety of Earth and environmental processes, including chemical weathering, low-temperature alteration of igneous rocks, reverse weathering, and the recycling of altered oceanic crusts and sediments into the mantle during subduction. Overall, sorption and interactions of aqueous K with different clay minerals during cation exchange and clay formation are likely to be of fundamental significance in generating much of the K isotope variability seen in samples from the Earth surface and samples carrying recycled surface materials from the deep Earth. Yet, the magnitude of this fractionation is both process- and mineral-dependent and comprehensive quantification of pertinent K isotope fractionation factors is currently lacking and urgently needed. Applied to biological processes, significant K isotope fractionation has been observed during biological activities such as plant uptake, demonstrating the potential utility of K isotopes in the study of the nutrient cycle and its relation to the climate and various ecosystems, enabling new and largely unexplored avenues for future research.

Of significant importance to the cosmochemistry community, K is a moderately volatile element with large variations in K/U ratio observed among chondrites and planetary materials. As this indicates different degrees of volatile depletion, it has become a fundamental chemical signature of both chondritic and planetary bodies. This volatile depletion has been attributed to various processes such as solar nebula condensation, mixing of volatile-rich and -poor reservoirs, planetary accretional volatilization via impacts, and/or magma ocean degassing. While K isotopes have the potential to distinguish these different processes, current results are still highly debated. A good correlation between the K isotope compositions of four differentiated bodies (Earth, Mars, Moon, and Vesta) and their masses suggests a ubiquitous volatile depletion mechanism during the formation of the terrestrial planets. Nevertheless, some of the K isotopic variation among chondrites and differentiated bodies may also be attributed to inherited signatures of mass-independent isotopic anomalies, the products of stellar nucleosynthesis.