High-precision measurements of heavy p-process isotopes in early solar system materials

This work investigates the abundances of rare, neutron poor isotopes of some heavy elements in meteorites. The elements in the solar system that are heavier than H and He were predominantly produced in stars. The Sun cannot have produced these elements during its lifetime as a small main sequence star, and their abundances must therefore have been inherited from interstellar matter that was produced by pre-existing massive stars. The elements lighter than iron were produced by nuclear fusion, whereas elements heavier than iron were produced predominantly by neutron capture reactions on pre-existing nuclei. This mechanism is referred to as the s-process when the produced nuclei decay before an additional neutron is captured, which is predominantly the case in asymptotic giant branch stars. When, in contrast, neutrons are captured before decay is possible, heavier nuclei may be formed (i.e., the r-process) and these conditions are achieved in core-collapse type supernovae. A group of rare, neutron-poor isotopes cannot have formed by s- or r-process nucleosynthesis and these are referred to as the p-process isotopes. The predominant mechanism by which p-process isotopes are produced includes photodisintegrations of pre-existing nuclei in core collapse type supernovae. However, this mechanism typically underproduces the solar system abundances of the light p-process isotopes, possibly implying decoupling between the nucleosynthetic sources of heavy and light p-process isotopes in the solar system. The non-radiogenic, mass independent isotopic compositions of some heavy elements are variable between different types of early solar system materials, reflecting isotopic heterogeneities between different nucleosynthetic carrier phases in the nascent solar system. Whereas the distributions of the most abundant isotopes, typically consisting of s- and r-process nuclides, have been relatively well constrained in most types of early solar system materials, studies for heavy p-process isotopes have long been impossible because of analytical difficulties due to their low relative abundances (typically less than 1 %). These isotopes became of particular interest since the recent discovery of 180W heterogeneities in iron meteorites, which were interpreted to predominantly reflect heterogeneity of heavy p-process components in the early solar system. In this dissertation, the systematics of heavy p-process isotopes in different solar system materials are examined. As examples, the abundances of heavy p-process 174Hf, 180W and to lesser extent 190Pt in different types of early solar system materials are explored. Based on new data for 180W in meteorites, an alternative explanation for the 180W heterogeneities is proposed, showing for the first time that measurable amounts of 180W are produced by radioactive decay of 184Os. In Chapter II, analytical protocols for high-precision measurements of heavy p-process isotopes 174Hf, 180W and 190Pt based using multicollector inductively-coupled plasma mass spectrometry (MC-ICP-MS) are presented. These protocols largely rely on the use of newly available Faraday amplifiers equipped with 1012Ω resistors that were used for collecting the isotopes of interest, as well as isotopes with which main isobaric interferences were monitored. It was found that measurement precisions strongly depend on signal intensity and become dramatically worse at < 50 mV on the low-abundance isotope of interest. At higher intensities, measurement precisions are typically ~1 parts per 10,000 or better, a ca. tenfold improvement compared to most earlier studies employing conventional 1011Ω resistors. As part of this work, absolute isotope abundances of 174Hf (0.16106 ± 0.00006 %); 180W (0.11910 ± 0.00009 %); and 190Pt (0.01286 ± 0.00005 %) in standard reference materials were determined throughout approximately 10 analytical sessions for each target element, ranging over ~3 years. These are the most precise estimates of the terrestrial abundances of these isotopes available so far. For precisely determining 174Hf, 180W and 190Pt in natural materials, ultra-clean separation schemes for Hf and W from silicate-rich matrices, and for Pt from iron meteorite matrices, have been developed. It is shown that iron meteorite samples from the IAB, IIAB and IIIAB groups, one sample each, have indistinguishable 190Pt from terrestrial Pt. Data on 174Hf and 180W in natural materials are discussed in depth in Chapters III and IV, respectively. Chapter III reports high-precision data for 174Hf in different types of chondrites, eucrites, one silicate inclusion of a IAB type iron, and one calcium-aluminium rich inclusion (CAI) from the Allende CV3 chondrite. Some chondrites as well as the IAB silicate inclusion exhibit elevated mass-bias corrected 174Hf/177Hf that are correlated with 178Hf/177Hf and anti-correlated with 180Hf/177Hf. This feature is interpreted to reflect the presence of small neutron capture effects (< -43 ± 14 ppm on 180Hf/177Hf) caused by exposure to cosmic rays. Chondrite and eucrite samples that appear to be unaffected by neutron capture reactions exhibit indistinguishable 174Hf from the terrestrial composition. This indicates that the p-process contribution to 176Hf (~3% p-process; 97% s-process) in these materials is also indistinguishable from terrestrial Hf, and therefore the 176Lu-176Hf chronometer in these materials was unaffected by a heterogeneous distribution of p-process Hf. In contrast, the CAI sample yields mass-bias corrected 174Hf/177Hf and 180Hf/177Hf that are lower (200 ± 78 ppm) and higher (32 ± 9 ppm) than the terrestrial Hf compositions, respectively. Whereas the low value for 174Hf/177Hf may reflect a p-process deficit in the reservoir from which CAI formed, the elevated 180Hf/177Hf remains unexplained. The observations that p-process 174Hf and possibly 190Pt do not show resolvable heterogeneities in bulk meteorites are difficult to reconcile with the previously proposed models that 180W heterogeneities in iron meteorites are caused by a heterogeneous distribution of p-process W. In Chapter IV, combined 180W and Os-W concentration data are presented that indicate that 180W heterogeneities in iron meteorites are better explained by radiogenic ingrowth from the decay of the rare nuclide 184Os. Alpha decay of 184Os has been theoretically predicted, but was previously never observed in experiments. A combined 184Os-180W isochron for iron meteorites and chondrites yields a decay constant value of λ184Os(α) of 6.49 ± 1.34 × 10-14 a-1 (half life 1.12 ± 0.23 × 1013 years), which is in good agreement with theoretical estimates. It is furthermore demonstrated that terrestrial silicate samples display a deficit in 180W relative to chondrites by 1.16 ± 0.69 parts in 10,000, reflecting the expected long-term evolution of the silicate Earth with subchondritic Os/W since core formation occurred ~4.5 Ga ago.