Admissions > PhD by research > Research Projects > Controls on the composition of the Earth and terrestrial planets

Controls on the composition of the Earth and

terrestrial planets

Supervisors: Tim Elliott, Chris Coath and Sara Russell (Natural History Musuem)

There are distinct differences in the isotopic compositions of the Earth, Mars and asteroids (as sampled by meteorites) that cannot be explained by solar-system processes (e.g. 1-4). These so-called mass-independent isotopic variations reflect variable contributions of materials from different stars that comprise to the solar system. Although these isotopic differences are subtle, they co-vary with key elemental abundances, most notably the abundances of volatile elements (5). Thus understanding the processes that imperfectly mix, or indeed, unmix, the different stellar components in the solar system is key to understanding the bulk composition of the Earth and other planets. In order to address this major problem, a first step is to understand the original stellar source (e.g. AGB, SNII, SNIa) that gave rise to the anomalous material.

Figure: Remnant of Keppler's SuperNova
(NASA image) spewing freshly synthesised
material into the inter-stellar medium.
Such material is eventually incorporated
into nebular disks and ultimately planets.
Elements from Type la Super Novae such
as this have very different isotopic signatures
to those from other stellar explosions.

An inherent problem in this quest lies in an assumption that needs to be made in conventional isotopic analyses. In order to remove the effects of mass fractionation during measurement, one isotope ratio is used to ‘internally normalise’ all others ratios. Any resulting anomalies rely on which ratio is chosen for normalization and thus is assumed to have a known, terrestrial value. This results in an uncertainty as to which isotopes are truly anomalous and hence what is their most likely stellar source. This ambiguity can be circumvented by an approach termed double spiking (see recent review of this technique in 6). This is not a new approach (see 7) but the novelty lies in being able to make such measurements to very high precision, as is possible using modern multi-collector inductively coupled plasma mass-spectrometry (MC-ICPMS). Thus we wish to build on work we have already undertaken in the Ti isotopic system (5) to examine ‘absolute’ isotope ratios and therefore deduce the stellar origin of the Ti isotope anomalies. Titanium is a rich isotopic system with five stable isotopes. Our previous work was presented as showing anomalies of 50Ti and 46Ti, which is striking as 46Ti and 50Ti are dominantly produced in very different stellar sources. However, one other possible interpretation of the data would be as 50Ti anomalies and coupled 49Ti-excess and 47Ti-deficits. These two scenarios have very different implications for the nucleosynthetic origin of the anomalous material. This question can be readily resolved by high-precision double spiking, which is the core of this project. Once the stellar source of the Ti is resolved by this approach it should be possible to predict the likely mineralogical carrier of this isotopic anomaly (e.g. 8) and we will search for this using the state-of-the-art field emission gun electron microprobe in the School of Earth Sciences and the Nano-SIMS at the Open University Milton Keynes.

This project will provide training in cutting-edge analytical cosmochemistry and will also involve developing skills in meteorite petrology. There is a possibility of collaboration with colleagues at the American Museum of Natural History in New York with expertise in condensation calculations.

References

  1. S. Niemeyer, Systematics of Ti Isotopes in Carbonaceous Chondrite Whole-Rock Samples. Geophys. Res. Lett. 12, 733 (1985).
  2. N. Dauphas, B. Marty, L. Reisberg, Molybdenum evidence for inherited planetary scale isotope heterogeneity of the protosolar nebula. Astrophysical Journal 565, 640 (2002).
  3. A. Trinquier, J. L. Birck, C. J. Allègre, Widespread 54Cr heterogeneity in the inner solar system. Astrophysical Journal 655, 1179 (2007).
  4. M. Regelous, T. Elliott, C. D. Coath, Nickel isotope heterogeneity in the early Solar System. Earth Planet. Sci. Lett. 272, 330 (2008).
  5. A. Trinquier et al., Origin of nucleosynthetic isotope heterogeneity in the solar protoplanetary disk. Science 324, 374 (2009).
  6. J. F. Rudge, B. C. Reynolds, B. Bourdon, The double spike toolbox. Chem. Geol. 265, 420 (2009).
  7. F. R. Niederer, D. A. Papanastassiou, G. J. Wasserburg, Absolute Isotopic Abundances of Ti in Meteorites. Geochim. Cosmochim. Acta 49, 835 (1985).
  8. A. V. Fedkin, B. S. Meyer, L. Grossman, Condensation and mixing in supernova ejecta. Geochim. Cosmochim. Acta 74, 3642 (2010).

 

Last updated: 24/11/11