Mr. Luke Daly1,2
1The Institute for Geoscience Research (TIGeR), Department of Applied Geology, Curtin University, GPO Box U1987, Perth, WA, Australia
2School of Geographical and Earth Sciences, University of Glasgow, Glasgow, G12 8QQ, UK
The invention of the light microscope by Hans and Zacharias Jansen 1580-1638 and telescope by Hans Lipperhey in 1608 using two glass lenses revolutionised our understanding of the natural world. Using the telescope, Gallileo made observations of the Solar System and the Milky Way proving the Earth was not the centre of the Solar System. While Antoni van Leeuwenhoek’s (1632-1723) use of the microscope revealed the existence of bacteria, cells and eventually lead to the destruction of the idea of spontaneous generation and revolutionised biology and medicine.
Fast forward 400 years and these simple observational tools were improved to push back the limits of what is observable from the extremely small to the incredibly far away. Large telescopes such as the Atacama Large Millimeter Array were built that are able to observe the structure of protoplanetary disks around newly forming stars (i.e. Brown et al., 2004), while electron microscopes are now able to observe the atomic lattice of materials (i.e. Hansen et al., 2002). Planetary Science uses these techniques to try to answer the question of how our Solar System formed through astronomical observations and microscopic exploration of minerals within meteorites. Meteorites are particularly important as they preserve a record of the first materials that formed in the early Solar System (Hutchison, 2004).
Through analyses of the chemistry, crystallography and isotopic composition of meteorites, we have improved our understanding of the processes that formed our Solar System. Analysis of mineral grains in meteorites can determine the shock pressures meteorites experienced (i.e. Stöffler & Keil, 1991), the cooling rate of the nebula disk (i.e. Berg et al., 2009), mechanisms for dust migration in the nebula (i.e. Ciesla, 2010), identification of pre-solar grains (Nittler, 2003; Zinner, 2003) and much more. However, meteorites contain several phases that are between 1-1000 nm. These sub-micrometre grains are approaching the limit of the resolution of established techniques. To extract bulk compositions, trace element abundances and isotopic data from grains this small, so we can use them to interpret Solar System processes, requires near atomic resolution. Enter the Atom Probe (Figure 1).
The atom probe has been in use since 1967 (Müller et al., 1968) and has been applied extensively in materials science to examine the properties of conducting materials, particularly metal alloys at the atomic scale. However, it was not until the local electrode atom probe (LEAP) was combined with a pulsing laser, that insulating materials could be routinely analysed (Kelly & Larson, 2012). This combination allows nonconductive materials, which includes the majority of geological and meteoritic samples, to be analysed.
Samples are prepared for atom probe analysis using a focussed ion beam that fires a stream of Ga ions into the sample to cut away a small wedge. This wedge is further sliced and attached to a silicon post where it is milled to a needle-like shape with a tip diameter of 100 nm (Thompson et al., 2007) (Figure 2). These samples are placed into the atom probe under high vacuum at low temperatures ~50-80 K. The samples are then subject to a high voltage, just under the voltage required to field evaporate ions from the sample. A focussed laser is then rapidly pulsed over the sample at 100-250 kHz. The laser provides just enough energy to ionise a single atom in the specimen. This ion is then field evaporated from the specimen and fired across a potential difference towards a position sensitive detector. The time of flight between laser pulse and detection, allow us to determine the mass of the ion as well as its original position in space (Kelly, 2011). Single samples can generate over 100 million detections where each detection represents a single atom. The resulting data set is a 3D atomic reconstruction of the sample (Figure 3). The mass-to-charge-state-ratios enable the identification of the element and isotopic species. The atom probe is a highly sensitive and is able to detect 10 appm (atomic parts per million) concentrations of elements (Kelly & Larson, 2012). The combination of atomic resolution, bulk and trace geochemistry and isotopic abundances make it a powerful tool for the analysis of planetary materials.
Previous studies on terrestrial samples such as Valley et al. (2014) have used the atom probe on Hadean zircons to conduct Pb-Pb geochronology. The resulting ages were consistent with SHRIMP analyses, while also revealing that Pb becomes concentrated in nanometre-sized nuggets (Valley et al., 2014). Further studies have also shown that similar results can be obtained from baddeleyite grains that have been shocked during impacts (White et al., 2017). The implications of these studies mean we can reliably determine ages from grains that would be too small for analyses using other methods. In addition, Heck et al. (2014) and Stadermann et al. (2011) analysed nanodiamonds from meteorites with atom probe microscopy. It is possible that some of these nanodiamonds formed before the Solar System itself. However, to demonstrate this requires the observation of unusual isotopic signatures typical of formation during the death of supernovae or red giants (Nittler, 2003; Zinner, 2003). However, as most nanodiamonds are ~10 nm across isotopic analysis of single grains has not been possible. Preliminary atom probe results from Heck et al. (2014) suggest that the carbon isotopes are anomalous. However, there are some additional analytical challenges that must still be overcome to prove that these anomalies are present in the grain and not an artefact of the analysis.
At Curtin University, we have been using the newly installed Geoscience Atom Probe to look at another small mineral phase known as refractory metal nuggets. Refractory metal nuggets are metallic alloys comprised of the highly siderophile elements such as osmium, iridium and platinum and are usually < 1 micrometre in diameter (Palme & Wlotzka, 1976; Wark & Lovering, 1976) (Figure 4). They are found throughout the most primitive meteorites (Daly et al., 2017a) and are thought to be the first phase to condense from the nebula (Berg et al., 2009) close to the newly formed sun (MacPherson et al., 2005). However, refractory metal nuggets are so chemically and crystallographically complex, it has been suggested that some may have a pre-solar origin (El Goresy et al., 1978; Daly et al., 2017a; Daly et al., 2017b).
Our analyses of refractory metal nuggets using atom probe tomography reveal that these grains are not only comprised of metals from the highly siderophile element group, such as osmium iridium and platinum but also contain trace abundances of sulphur. This association is unexpected as sulphur is volatile compared to the highly siderophile elements and they have condensation temperatures that differ by > 1000 K (Lodders, 2003). Furthermore, sulphur is thought to condense further out in the protoplanetary disk between 1-3 astronomical units (AU) (Ciesla, 2015) compared to refractory metal nuggets which are thought to form close to the sun (MacPherson et al., 2005). This observation coupled with the petrography of the sample indicate that these refractory metal nuggets were ‘free floating’ prior to their incorporation into their current host inclusion, and suggests that these refractory metal nuggets may have migrated early in the Solar Systems history, in and out of the protoplanetary disk (Daly et al., 2017c). Therefore, atom probe microscopy has revealed, through analysis of some of the smallest grains in meteorites, evidence for large-scale migration of particles during the formation of the Solar System.
Excitingly, this technique is still in its infancy for meteoritical samples and has a huge potential to unlock some of the biggest secrets of our Solar System by analysing the very smallest objects. Watch this space.
Acknowledgements
The Brian Mason Travel Award is sponsored by the International Meteorite Collectors Association for the 2016 79th Annual Meeting of the Meteoritical Society. This research was funded by the Australian Research Council via their Australian Laureate Fellowship program. This work was conducted within the Geoscience Atom Probe Facility at Curtin University, which is part of the Advanced Resource Characterization Facility (ARCF). The Advanced Resource Characterisation Facility is being developed under the auspices of the National Resource Sciences Precinct – a collaboration between CSIRO, Curtin University and The University of Western Australia – and is supported by the Science and Industry Endowment Fund (SIEF RI13-01). The authors acknowledge the use of Curtin University’s Microscopy & Microanalysis Facility, whose instrumentation has been partially funded by the University, State and Commonwealth Governments, Reddy acknowledges support from the ARC Core to Crust Fluid System COE (CE11E0070).
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