Tracing signatures of discreet nucleosynthetic contributions to the early solar system using nickel isotopes

• Main Author: Steele, Robert C. J.
• Published: University of Bristol 2012
• Subjects: 523.24
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.556724

The Solar System is formed from material processed by a wide variety of nucleosynthetic sources (Burbidge et al., 1957). Many studies have investigated the nucleosynthetic origins of the Solar System by examining the isotopic anomalies within meteorites and their constituent components. However, debate continues as to the astrophysical source these anomalies represent and the early Solar System processes that results in their inhomogeneous distribution. Nickel isotopes offer an opportunity to examine further the sources and mixing of early Solar System materials. Nickel is an abundant element in most meteorite groups (normally wt. %) with five stable isotopes that are dominantly produced in different astrophysical environments. A new high resolution mass spectrometric technique has been developed and has been examined for potential analytical artefacts; none are found to have a significant effect. Currently, this technique produces the highest precision «10 ppm) data for all Ni isotope including the least abundant, 64Ni, which has previously only been reported in two studies (Cook et al., 2006; Dauphas et al., 2008). New Ni isotope data are presented for a range of bulk chondritic and iron meteorites which, when normalised to 58Ni/61 Ni and presented as part per ten thousand difference (%00) from NIST SRM 986, show ranges of 0.18, 0.35 and 0.9 %00 for c6oNiH, c62NiH and c64NiHrespectively. A strong positive correlation is observed between c62NiH and c64NiH with gradient 2.98 ±0.21, which is within error of the slope produced between the terrestrial composition and the average of early measurements of calcium, aluminium-rich inclusionss (CAls) (Birck and Lugmair, 1988). Interestingly, this gradient is also within error of the slope expected for isotopic anomalies on 58Ni. 'Absolute' Ni isotope ratios determined on two meteorites (Orgueil Cl and Butsura OC) to <0.3 %00 (2 s.e.) confirm that anomalies reside on 58Ni. This finding of neutron-poor anomalies has significance for the interpretation of anomalies in other elements. The nucleosynthetic origins of these meteorite anomalies have been quantitatively investigated using a holistic approach which considers potential anomalies from all isotopes, including those used for normalisation. I argue that such an approach should be taken for all elements. It has been found that no bulk astrophysical environment can provide a robust case as the source of Ni isotope anomalies in meteorites. However, a more promising result is found in the model composition of the Si/S shell of type II supernovae (SN II) (Rauscher et al., 2002). The Si/S zone of all masses of SN II investigated (12-40 Md can meet the Ni isotope constraints set by the measurements of early Solar System materials. This finding removes the need for input from the rare type la supernova, which is considered unlikely to be associated with star forming regions. Although this places narrow constraints on the nucleosynthetic source for material containing anomalous Ni isotopes, this Si/S zone cannot account for all the Ni isotope anomalies (e.g. f:6oNiSf is not perfectly correlated with f:58NiSf) nor the anomalies of all other elements. This suggests a major control on isotopic anomalies observed in meteorites may be which zones of supernovae are sampled by robust pre-solar carriers and how these carrier phases are differentially mixed in by nebula processing. More insight in to these processes and sources could gained by identifying the carrier phases of isotopic anomalies.