Tracing volatile element fractionations in the early Solar System: Evidence from Ge isotopes in meteorites and planetary bodies
by Elias Wölfer
Date of Examination:2025-02-17
Date of issue:2025-08-05
Advisor:Prof. Dr. Thorsten Kleine
Referee:Prof. Dr. Andreas Pack
Referee:Prof. Dr. Thorsten Kleine
Persistent Address:
http://dx.doi.org/10.53846/goediss-11425
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Abstract
English
Volatile elements are variably depleted in all meteoritic and planetary materials, including Earth and other terrestrial planets, relative to the bulk solar composition. The origins and underlying mechanisms of these depletions remain highly topical problems in present-day planetary sciences. Much of the debate centers around the question to what extent the depletions among different planetary materials reflect prior nebular fractionations or, alternatively, planetary devolatilization processes during the accretion and the subsequent thermal evolution of planetary bodies. This thesis investigates these issues by determining the concentration and mass-dependent isotopic composition of the moderately volatile element (MVE) Germanium in a range of variably volatile-depleted planetary materials. For this purpose, an analytical Ge double spike routine was developed, which allows for simultaneous high-precision measurements of Ge concentrations and isotopic compositions using multi-collector inductively coupled plasma mass spectrometry (MC-ICPMS). Nebular fractionation processes were examined by analyzing chondrite meteorites. The volatile element budget and isotopic composition of ‘non-depleted’, primordial material was obtained by analyzing Ivuna-type (CI) chondrites and a sample from the Cb-type asteroid Ryugu. CI chondrites and Ryugu have similar elemental and mass-dependent isotope systematics of the MVEs Ge, Te, and Zn, which are distinct from those of all other carbonaceous chondrites. These results reinforce the notion that CI chondrites and Ryugu formed from the same precursor materials, and indicate that for moderately volatile (and more refractory) elements, their compositions were not significantly modified by processes in the solar protoplanetary disk or on the parent bodies. For these elements, CI chondrites/Ryugu thus likely record the average chemical and mass-dependent isotopic composition of most parts of the disk and the bulk Solar System. Compared to Ryugu and CI chondrites, other carbonaceous chondrites display group-specific Ge elemental and isotopic variations. In line with observations for other MVEs these correlations are best explained by variable mixing between a volatile-rich, isotopically heavy CI-like matrix and a volatile-poor, isotopically light chondrule (precursor) component common to all carbonaceous chondrite groups. Consistent with their chondrule-rich nature, enstatite and ordinary chondrites have lighter Ge isotope compositions than the more volatile-rich carbonaceous chondrites. Planetary volatile element fractionation processes—and their delamination from nebular processes—were investigated by analyzing the Ge elemental and isotopic composition of differentiated planetary materials and metal- and sulfide-melt degassing experiments. The experiments reveal that evaporation of Ge increases with increasing temperatures, decreasing pressure, in the presence of S, and is accompanied by a heavy Ge isotope enrichment in the residue. Such features are also seen for non-magmatic iron meteorites, supporting near-surface impact processes as a likely scenario for their formation. In contrast, the systematic study of variably volatile-depleted magmatic iron meteorites (i.e., those sampling planetary cores) demonstrates that the combined effects of primary nebular, and subsequent planetary volatile element depletions are required to account for their Ge elemental and mass-dependent isotope compositions. In this context, the parent bodies of the magmatic iron meteorites—like the chondrite parent bodies—are inferred to have initially formed as mixtures of volatile-rich and volatile-poor precursor assemblages. Starting from this nebular baseline, evaporation during planetary differentiation and collisional exposure of molten cores then set the final volatile budgets of the iron meteorites. Finally, the combined Ge elemental and isotopic data of differentiated and undifferentiated meteorites in concert with new Ge data for terrestrial rocks are used to gain insight into the (volatile) accretion history of the Earth. The Ge isotope compositions of the bulk silicate Earth (BSE) is uniform, falls within the chondritic range, and is best accounted for as a 65:35 mixture of CI and enstatite chondrite-derived Ge. This mixing ratio is distinct from the 30:70 ratio inferred for Zn, reflecting the different geochemical behavior of Ge (siderophile) and Zn (lithophile), and indicating the late-stage accretion of volatile-rich CC materials to Earth. On dynamical grounds it has been argued that Earth accreted CC material through a few, Moon-sized embryos, and so the Ge isotope results imply that these objects were volatile-rich, presumably because they were either undifferentiated or accreted volatile-rich objects themselves before being accreted by Earth.
Keywords: Germanium; Moderately volatile elements; Volatile depletion; Iron meteorites; Chondrites; Degassing experiments; Mass-dependent isotope fractionation
