Volatile element depletion in Earth and carbonaceous chondrites

Volatile elements such as sulfur, zinc, indium and potassium are depleted relative to the Sun in all known inner solar system materials, including Earth, Mars, Moon and differentiated and undifferentiated meteorites. The Sun comprises 99.8 wt.% of our solar system and is thus representative of its chemical composition. CI chondrites are the only meteorites with an extremely similar chemical composition as the Sun for non-atmophile elements and therefore serve as a reference in cosmochemical studies. The degree of volatile element depletion is different between and even within groups of meteorites and planetary bodies, indicating complex fractionation processes. In this work, volatile element abundances in carbonaceous chondrites are studied to elucidate volatile element fractionation processes in the solar nebula and to assess the potential of volatile element depletion patterns as tracers for planetary building blocks. Furthermore, an analytical protocol for the precise determination of volatile element abundances in geological materials is presented. Carbonaceous chondrites are primitive material sampling the solar nebula. They are made of two main components (i) chondrules, µm- to mm-sized silicate melt droplets and (ii) matrix, an unequilibrated assemblage of extremely fine-grained minerals. Carbonaceous chondrites are the meteorites that are least depleted in volatile elements. In Chapter 2, data for the bulk chemical composition of 27 carbonaceous chondrites of different groups analyzed via sector-field inductively-coupled-plasma mass-spectrometry (SF-ICP-MS) are presented. All studied chondrites show the same characteristic “hockey stick” volatile element depletion pattern, where volatile elements with 50% condensation temperatures between 800 and 500 K are equally depleted relative to CI chondrites. The relative abundances of these ‘plateau volatile elements’ are characteristic for each group and covariate with the matrix abundances of the respective host chondrites. Hence, all carbonaceous chondrites likely contain a CI-like matrix component which accounts for the majority of volatile elements. The same “hockey stick” volatile element depletion pattern is observed for the Earth. The silicate Earth exhibits a more fractionated volatile element depletion pattern because core formation led to the redistribution of elements according to their geochemical characters. However, lithophile plateau volatile elements (zinc, indium, chlorine, bromine and iodine) are unfractionated from each other relative to CI chondrites. This abundance plateau accounts for the disputed high abundance of In in the silicate Earth without the need of exotic building blocks or secondary volatile loss and suggests the accretion of 10-15 wt.% CI-like material before core formation ceased. Finally, the newly recognized hockey stick volatile element depletion pattern allows more accurate estimates of volatile element abundance in the core and bulk Earth, which are provided in Chapter 3. Terrestrial basalts and mantle rocks, lunar rocks and achondrites are much more depleted in volatile elements than carbonaceous chondrites. Abundances of volatile elements in these rocks can only be determined precisely after the addition of enriched isotope tracers and chemical separation. Chapter 4 comprises a new analytical protocol for the determination of copper, zinc, gallium, silver, cadmium, indium, tin and thallium mass fractions via isotope dilution ICP-MS. Results for 21 reference materials of different lithologies and three carbonaceous chondrites are presented and discussed. The results for Orgueil CI1, Murchison CM2 and Allende Smithsonian CV3 are in good agreement with bulk chemical analyses obtained in Chapter 1.