Volatilitätskontrollierte Fraktionierung refraktär-lithophiler Elemente in Meteoriten und der Erde

Abstract

During the condensation of the elements in the early solar system, volatility-controlled fractionations occurred. Such fractionations of refractory lithophile elements in individual chondrite components and between bulk chondrites, achondrites and planets are subject of this thesis. The contents of rare earth elements (REE) as well as Nb, Ta, Zr and Hf were analysed by means of laser ablation inductively coupled plasma mass spectrometry. Individual chondrite components were measured in-situ using the CV chondrite Leoville. Whole rock samples were prepared from bulk chondrites, achondrites and terrestrial samples by laser melting during aerodynamic levitation. The investigation of diverse components of the Leoville chondrite showed that refractory inclusions have volatility-controlled fractionated REE group-II-patterns and subchondritic Nb/Ta ratios. They hence formed from a residual gas from which an ultrarefractory component had been isolated before. Most chondrules have unfractionated REE patterns and unfractionated Zr/Hf and Nb/Ta ratios. However, some type-1-chondrules, the Al-rich chondrules and the chondrite matrix show fractionated REE patterns. This indicates an additive of refractory constituents with REE group-II-pattern. The analyses of bulk chondrites indicates that carbonaceous chondrites have characteristic volatility-controlled REE patterns (ultrarefractory or group-II) compared to the CI chondrite Orgueil. This could be explained by the occurrence of refractory components with fractionated rare earth elements. The majority of ordinary, enstatite and rumuruti chondrites has relatively unfractionated REE patterns. The results provide evidence that ordinary, enstatite and rumuruti chondrites as well as achondrites, Mars, Moon and Earth have small negative Tm anomalies compared to the CI chondrite Orgueil. On the basis of their relative contents of heavy rare earth elements (HREE), the inner solar system objects were divided into two groups: a carbonaceous and a non-carbonaceous chondrite reservoir which also includes the achondrites, Mars, Earth and Moon. It was assumed that the objects of the non-carbonaceous chondrite reservoir reflect solar system HREE ratios. In contrast, carbonaceous chondrites have variable Tm anomalies that are caused by the input of fractionated refractory components into the region where they formed. CI chondrites, that are generally regarded as the chemically most primitive chondrite group, would have a positive Tm anomaly of 4.8 ± 0.9 % and therefore do not agree with the solar system. A fraction of 0.2 wt% of a refractory component with group-II-pattern in CI chondrites could explain the observed Tm anomaly.