Image looking down at Vesta’s north pole
This image of Vesta was taken from the last sequence of images of NASA’s Dawn spacecraft, looking down at Vesta’s north pole. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/ID.
Asteroid 4 Vesta is the second largest body in the Asteroid Belt, second to the dwarf planet Ceres. Unlike most asteroids, Vesta is differentiated into a silicate crust, mantle and core, much like Earth. The HED meteorites (Howardite-Eucrite-Diogenite) that sample Vesta comprise the largest fraction of achondrites sampled in meteorite collections, making Vesta reasonably accessible for the study of planetary-scale magmatic differentiation processes. The HED meteorites are a suite of crustal igneous rocks that likely represent crystallisation from a magma ocean (Greenwood et al. 2005, 2014), and subsequent episodic magmatic intrusion and eruptions from ~4547-4459 Ma (Zhou 2013). HEDs are traditionally thought to have crystallised from volatile poor parent magmas (Mittlefehldt 2013) , but recent studies on apatites in eucrites report H2O contents of up to ~2600 ppm (Barrett et al. 2016) and D/H ratios comparable to carbonaceous chondrites, terrestrial basalts and some lunar lithologies (Sarafian et al. 2014) . The primary goal of this project is therefore to utilize the HED meteorites to interrogate the behavior of volatiles during magma ocean crystallization, and we envisage there will be implications for planetary differentiation more widely.
The main body of work for the project will be the compilation of a halogen abundance and noble gas isotope inventory of the chemically evolved materials that are the products of differentiation from Vesta. Both the atmophile noble gases (Ar, Kr, Xe) and hydrophile halogens (Cl, Br, I) are expected to be present in relatively low abundances in the HED materials. Halogens and noble gases are powerful geochemical tracers for investigating volatile evolution in planetary bodies because their volatility means that they are significantly affected by the physical processes of differentiation and impacts that occurred on early planetary bodies.
The first step in the project will be sample acquisition from Museum and Antarctic holdings (e.g., ANSMET, NIPR). The basaltic eucrites and diogenites are particularly advantageous because of the large number of samples in collections (i.e., ~46% of the achondrite Antarctic Meteorite Collection are HED meteorites, HED n=328). This will allow us to carefully select ideal target materials. For example, we will avoid those samples that have undergone high degrees of terrestrial weathering, those with extensive thermal metamorphism and those that are heavily-brecciated. The next step will be careful petrological characterization of the sample suite, including basaltic (cumulate and non-cumulate) eucrites and olivine- rich diogenites. In addition to standard petrography, micro-analysis using the automated mineral mapping software QEMSCAN will yield detailed and high-resolution mineral, chemical and textural information. This will be coupled with mineral chemical data from electron microprobe, including Cl mapping, to assess terrestrial contaminants and aid identification of host halogen phases. As a complement to the QEMSCAN mapping, there is the prospect of carrying out X-ray microtomography to quantify the distribution of petrological features consistent with volatile exsolution, such as vesicles (there are several moderately vesicular eucrites and diogenites, for example). The halogen abundance measurements will be made using the uniquely sensitive neutron−irradiation noble gas mass spectrometry (NI−NGMS) technique (Ruziè-Hamilton et al. 2017), pioneered at the University of Manchester and currently the only lab in the UK capable of measuring ppb−level halogen concentrations in bulk rock samples and minerals. The technique works by converting the halogens in the sample into noble-gas proxy isotopes, 38ArCl , 80,82KrBr, and 128XeI , which are easier to measure by conventional mass spectrometry. Noble gas analysis on bulk samples will beaccomplished using the HELIX-MC at Manchester, a multi-collector mass spectrometer capable of measuring He, Ne, Ar, Kr and Xe. In addition to bulk rock work, mineral separates from diogenite cumulates and coarser grained basaltic eucrites will also be prepared for halogen analysis, to examine the effects that crystallization of discrete phases have on volatile partitioning.
A key part of the project will be to synthesise the petrological observations and datasets from the halogen and noble gas analyses to construct robust petrogenetic models for the behavior of volatile elements during magma fractionation, degassing and solidification on Vesta. These models will enable us to much better constrain the way in which volatiles respond to the planetary evolution processes that are common to the terrestrial planets. The work outlined above will most likely suit a student with interests and experience in mineralogy and geochemistry from their earlier degree(s).
Greenwood, R. C., Franchi, I. A., Jambon, A. & Buchanan, P. C. Nature 435, 916–918 (2005).
Greenwood, R. C. et al. Earth Planet. Sci. Lett. 390, 165–174 (2014).
Zhou, Q. et al. Geochim. Cosmochim. Acta 110, 152–175 (2013).
Mittlefehldt, D. W. Chemie der Erde - Geochemistry 75, 155–183 (2015).
Barrett, T. J. et al. Meteorit. Planet. Sci. 51, 1110–1124 (2016).
Sarafian, A.R., Nielsen, S. G., Marschall, H. R., McCubbin, F. M. & Monteleone, B. D. Science 346, 623–626 (2014).
Ruziè-Hamilton et al. (2016) Chem. Geol.,437, 77–87.
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