Utilising Rb-Sr Isotopes to Understand Early Solar System Volatile Depletion

Project Background
Planetary bodies in the inner solar system are depleted in volatile elements compared with CI chondrites and the Sun. Some of the most extreme volatile depletions are found in the parent bodies of achondrites (angrites and eucrites), while volatile depletion is common in most chondrites relative to CI chondrites, indicating that volatile loss is a ubiquitous feature of planetary formation. Two competing models exist for the timing of this volatile loss, which occur over strikingly different timescales (e.g. Halliday and Porcelli, 2001; Hans et al., 2013). Model 1 is incomplete condensation from hot nebula gas during planetary accretion, which occurs on a timescale of less than 1 Ma. Model 2 is evaporative loss during planetary accretion and differentiation, which can be prolonged over several millions of years. The 87Rb-87Sr isotope system provides a means to constraining the timing of volatile loss because Rb is a moderately volatile lithophile element (Tc~800 K), whereas Sr is a refractory lithophile element (Tc~1464 K). Therefore Rb/Sr and 87Sr/86Sr ratios are influenced by the degree and timing of volatile depletion events. However, there is considerable evidence that early solar system material have significant nucleosynthetic Sr isotope anomalies and mass-dependent stable Sr isotope variations (Charlier et al., 2012, 2017; Brennecka et al., 2013; Hans et al., 2013). Therefore comparing the difference in initial 87Sr/86Sr ratios between early solar system materials is not straightforward and requires 84Sr/86Sr, 87Sr/86Sr and 88Sr/86Sr ratios to be measured properly via double-spike Sr isotope measurements to resolving timing of volatile loss.

Project Aims and Methods
The project will be based around obtaining high-precision double spike Sr isotope data on CAIs, chondrules and other components in meteorites. This project will utilise a double-spike Sr technique already set up in Bristol, utilising high-precision multi-dynamic thermal ionization measurements (TIMS) that provide accurate Sr isotope determinations (e.g., Henshall et al., 2018). This will allow us to define the initial 87Sr/86Sr of the solar system, free from inferences about nucleosynthetic inputs. Additionally, single chondrule data will help understand the timing of their formation and potentially record volatile depletion during dust heating and cooling and the initial accretion of planetary bodies. There is also potential to undertake high-temperature evaporation experiments using a novel gas-flow levitation system to understand mass-dependent Sr and Rb isotope fractionation and quantify volatile loss in chondrules. Combining these data will allow us to build a model for volatile depletion in the early solar system.