Investigating records of Earth’s precursors in the early Solar System using Cr, Ni and Zn isotope compositions of meteorites

Overview

The Earth and other terrestrial planets are products of the early Solar System. The early Solar System was a rapidly changing environment where pre-solar dust and gas mixed, the earliest solids and planetesimals formed and eventually planets accreted [see 1]. The formation processes of Earth which occurred at these times set important physical and geochemical parameters which ultimately controlled its subsequent evolution [e.g. 2]. Thus, understanding the sources and processes of terrestrial accretion is crucial to having a robust starting point to investigate characteristics and features which occurred later including mantle dynamics and habitability. Unfortunately, the material from which the Earth is made is locked away within our planet and the accessible portion of the silicate Earth has been homogenised by 4.5 billion years of mantle stirring. Therefore, the only way to examine the accretion phase of our planet is to examine samples of the early Solar System as proxies for terrestrial building blocks to properly place Earth within this context.

Meteorites are samples Solar System bodies, and many are samples of bodies which have survived from the early Solar System. These are our only record of the earliest sources and processes of the Solar System and are a crucial repository of information for understanding our planet in a Solar System context. Different regions of the early Solar System received a different mixture of pre-Solar material [3,4,1]. This pre-Solar material contains grains with different nucleosynthetic histories which had different isotopic compositions. The precise mix of pre-Solar material, and so the isotopic composition, is characteristic of the region in which the material formed. Therefore, by measuring the isotope compositions of samples from the early Solar System the origins and relationships between the material from which they formed can be examined [3-5].

Over the past 20 years the isotopic compositions of a range of early Solar System samples and a range of elements have been mapped out [see 1]. Material has been identified as coming from the outer Solar System (carbonaceous chondrites) and from the inner Solar System (ordinary and enstatite chondrites). Several models have recently been proposed to explain these observations including the early formation of Jupiter to keep these reservoirs separate [5,6] and the planetesimal formation triggered by a migrating snow line [7].

This project aims to trace the movement of material between different regions of the early Solar System at the scale of both bulk meteorites and chondritic components. This will be achieved by measuring the isotopic compositions of multiple element systems (Ni, Cr, Zn) in a range of chondrites and iron meteorites to fill in gaps for meteorite groups which have not been extensively studied and by measuring a range of chondritic components. These data will be used to test hypotheses of dynamical models of the early Solar System. For example, the separation of the inner and outer solar system regions caused by the Jupiter barrier [e.g. 5,6] or the formation of planetesimals at a migrating snow line. Ultimately, using the additional understanding gained about early Solar System models to test the theories of delivery of material, especially volatile rich material crucial for life, to the terrestrial planet forming region and validity of models for the bulk composition of the Earth [e.g. 7].

Methodology

The aim of this project is to better place Earth and the terrestrial planet reservoir in the context of the early Solar System. This will be achieved by analysing the isotope composition of a range of meteorites, meteorite components and terrestrial samples to trace mixing of material between different regions of the early Solar System. These results will allow a more complete picture of the building blocks of Earth and the accretionary phases of terrestrial planets. Meteorite samples will be primarily obtained from the Natural History Museum, London though some samples are held at other museums around the world and will be requested. The samples will be characterised using SEM and ICPMS at St Andrews. Chemical purification methods using ion exchange resins will be applied to achieve clean aliquots for a range of elements (Cr, Ni, Zn). Analytical development will be undertaken to improve efficiency, for example automated chemical separation for Zn using ESI PrepFAST. Finally, isotopic compositions will be determined using MC-ICPMS. These results will then be compared with predictions of new and existing models for the evolution of the early Solar System.

Project Timeline

Year 1

Literature review and compilation of existing data; training on current techniques for Ni, Cr and Cu analytical techniques; training on handling and separating meteorite and their components; analytical development to improve precision for smaller sample sizes; write and defend Year 1 Research Proposal.

Year 2

Major analytical work measuring the isotope compositions of a range of iron meteorites, chondritic meteorites and chondritic components.

Year 3

Complete analytical work; modelling and interpretation of results; prepare research manuscripts for publication; writing up thesis; attend international conferences to present results.

Year 3.5

Prepare research manuscripts for publication; writing up thesis; attend international conferences to present results.

Training
& Skills

– Training in meteoritics
– Training in use of clean laboratories
– Training in the purification and measurement of a range of isotope systems (Cr, Ni, Zn)
– Training in analytical development of both chemical purification and mass spectrometry
– Interpretation and modelling of isotope and elemental data to place new constraints on the evolution of the early Solar System and formation of terrestrial planets.
– Presentation of research in publications and at international conferences

References & further reading

[1] R. C. J. Steele. 2021. Nucleosynthetic heterogeneities in meteorites. Encyclopedia of Geology (Second Edition), pages 242–257.[2] N. Dauphas. 2017. The isotopic nature of the Earth’s accreting material through time. Nature , 541 (7638), 521–524.[3] A. Trinquier, et al. 2007. Widespread 54Cr heterogeneity in the inner Solar System. ApJ , 655, 1179–1185.[4] R. C. J. Steele, et al. 2012. Neutron-poor nickel isotope anomalies in meteorites. ApJ , 758 (1), 59.[5] J. A. Nanne, et al. 2019. Origin of the non-carbonaceous–carbonaceous meteorite dichotomy. EPSL , 511, 44 – 54.[6] T. Kleine, et al. 2020. The non- carbonaceous–carbonaceous meteorite dichotomy. Space Sci. Rev, 216, 55.[7] T. Lichtenberg, et al. 2021. Bifurcation of planetary building blocks during solar system formation. Science, 371 (6527), 365.

Further Information

For further information please contact Robert Steele – rcjs@st-andrews.ac.uk; 01334 463491

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