Investigating High-temperature Mass Dependent Chromium Isotope Fractionation

Chromium (Cr) is an element of interest due to its physical properties; it has 4 isotopes which are created in different ways, differentiates into the core depending on fugacity and temperature, has multiple oxidation states in the BSE and is slightly volatile. All of these could cause fractionation of isotopes, which may be preserved throughout geological time. The study of these may then allow us to learn about processes of planetary formation.

My research will be into understanding the causes behind these differences in the Cr isotope ratio. Mass dependent occurs due to preference for heavier isotopes in a certain phase, or due to the velocity difference that occurs of isotopes of different size at the same energy. For Cr mass independent fraction refers due to difference in the starting material, or the decay of other isotopes.

At the moment the main causes of fractionation in different samples is poorly understood. The starting place will be to obtain an accurate value for the bulk silicate Earth. Recent studies have disagreed and been hindered by large uncertainties on measurements and the choice of sample. Obtaining high quality measurements and careful sample selection, which will not have fractionation from the mantle, will allow us to compare the Cr isotope value of Earth to other solar system bodies. During the investigation of mantle rocks it may also be possible to identify methods of fractionation that are common for our planet.

Following this I will also investigate the Cr isotope value of other solar system bodies. Work has already been carried out an a range of meteorites which shows that small differences occur between classes, and lunar samples that show variations in the Cr value occurring during formation of basalts. The investigation into mineral separates for both of these cases, may be a useful way to look to investigate the fractionation of Cr in these samples. For lunar samples measurements of mass independent fractionation may also be carried out, as this is an area that has not been studied in detail.

Method development and evaluation will form an additional part of my research. During the deconvolution of data for both mass dependent and mass independent fractionation assumptions are made. It would be of interest to see how robust these assumptions are, especially now that larger variations in mass dependent fractionation are being recorded.

Development of a chemistry that allows the separation of two different spiked would be a useful development. Samples studied are very precious, so being able to gather two measurements will mean that with the small amount of material we have, we will be able to greater increase our understanding of the solar system. Creation of a new method will have to be carefully considered so that accuracies of data are not compromised.

Laboratory work will be carried out in the Department of Earths Science Metal Free Labs. The main technique will be the double spike, which allows high precision measurements of samples, taking out any effects that may be caused during the separation of samples and other processing. Measurements are to be carried out on the departments Thermal Ionisation Mass Spectrometer.