What do pallasite meteorites tell us about processes deep in the interior of differentiated planet(essimal)email@example.com
|Figure 1. Pallasite meteorites may represent the core-mantle boundary of planetesimals, or the mixing of the mantle of a planetesimal with metal derived from an impactor. They comprise roughly equal proportions of ultramafic silicates and iron-rich metal (± sulphides and phosphates)|
Theoretical and numerical studies indicate that chemical heterogeneity in the outer core region profoundly influences the dynamics and evolution of Earth's core and mantle, and the behaviour of the geomagnetic field. However, we presently don't know much about this heterogeneity. For example, tantalizing hints of geochemical heterogeneity in the outermost core are given by anomalous seismic signals at the core-mantle boundary, but this region of our planet is by no means accessible for direct geochemical investigation (despite what movies such as “The Core” would have you believe). Atomistic modelling provides some constraints on what chemical composition could be expected in the deepest portions of the mantle and geophysical modelling techniques can be used to predict geochemical heterogeneity within the outermost core. Pallasite meteorites (Figure 1) may be derived from differentiated planetesimals, subsequently smashed into fragments by collisions with large bolides early in the history of the solar system. These meteorites may provide indirect evidence with which processes deep within the Earth can be tested. This project will use a combination of geochemical investigations of pallasite meteorites, geophysical modelling of the composition of the Earth’s outermost core and atomistic modelling of equilibrium metal / silicate compositions to investigate the nature and origin of pallasite meteorites and what information they contain regarding differentiation processes in planetary bodies.
Figure 2. It is by looking at the remains of planet-planet collisions from early in the history of the solar system that we hope to test hypotheses relating to the Earth’s core-mantle boundary using natural samples
Although Hollywood would have you believe otherwise, it is simply not possible to directly sample the Earth’s deepest geochemical reservoirs. The deepest we have drilled to date is 12.26 km (the Kola Superdeep Borehole, Russia), leaving us approximately 2900 km short of the core-mantle boundary and 5100 km away from the solid inner core.
Pallasite meteorites comprise roughly equal proportions of olivine (± low Ca-pyroxene) and iron, similar to that found in fractionated iron meteorites. Historically, they have been thought to represent remnants of the core-mantle boundary of planetesimals that were assembling at the same time as the formation of Earth. These planetesimals grew to a sufficient size for the segregation of an iron-rich core to take place, but were subsequently broken into meteorite-sized chunks by collisions with other planetesimals. However, this hypothesis was recently challenged (Tarduno et al., 2012) and the possibility that the metal component comes, not from the differentiated body itself but, from the impactor has been suggested. This means that a cloud hangs over the utility of pallasite meteorites in the investigation of core-mantle processes on planetary bodies, specifically, the Earth. Pallasite meteorites are rare (only 61 found to date, and only 4 of those being observed falls; we will have samples from 12 main group pallasites available for this project) but they may offer insights into planetary differentiation and impact processes, and offer glimpses back through 4.6 Ga of time to when the parent bodies of these meteorites were intact.
The unique nature of pallasite meteorites means that many different hypotheses relating to the deep Earth can be investigated. The first part of the project would entail detailed characterization of the pallasite meteorites and, potentially, the identification of mineral phases which may prove useful for geochronology, i.e. for dating some of the oldest materials in the solar system. Once characterized, there are several potentially high impact hypotheses that could be explored.
|Figure 3. The 525 km diameter asteroid Vesta is the second largest body in the asteroid belt today (9% of the mass of the entire asteroid belt). Vesta is thought to have undergone core-differentiation within the first 10 million years of the solar system’s history. Pallasite meteorites may well represent fossil analogues of planetesimals ranging in size from Vesta to the diameter of the Earth.|
For example, textural evidence preserved within pallasites suggests that their derivation from a single parent body is unlikely, yet it is not established how many parent bodies are required to satisfy the textural observations made to date, nor the nature of the multiple bolides that would be inferred to have been necessary to fragment more than one planetary body.
Recent seismic investigations (e.g. Helffrich, 2014) suggest that there is a geochemically stratified region in the outermost core that is responsible for the observed deflections of seismic waves at around 2900 km depth. These deflections are not consistent with a simple iron-liquid outer core and the chemical heterogeneity responsible has been attributed to pressure-related diffusion preferentially concentrating light elements (O, S, Si, amongst others) in the outermost regions of the Earth’s outer core (Gubbins and Davies, 2013). Geochemical investigations that compare the composition of iron meteorites (innermost planetary cores) with pallasites could potentially test these hypotheses with natural samples.
Certified pallasite samples are readily available from both commercial suppliers and museum collections (Natural History Museum, UK; The Smithsonian Institution, Washington DC), so this project does not rely on the fortuitous location of new pallasite samples.
The willingness and potential to undertake extensive delicate chemical analyses under clean laboratory conditions is essential for this project. In addition, a high degree of numeracy will be advantageous, as will a desire to build and test geophysical and / or atomistic models relating to the chemical and physical properties of the Earth’s deepest geochemical reservoirs.
It is by no means expected that the successful candidate would already have significant experience in any of these fields - thorough training in cutting edge geochemical methods, atomistic and geophysical modelling will be an integral part of this project and will be tailored to the successful student as required – we realize that these are not necessarily off-the-shelf skills, but the right candidate will acquire them!
At the University of Leeds we have active and vibrant research groups who focus on the deep Earth and high temperature geochemistry. The Deep Earth Research Group is one of the largest groups of scientists studying the structure and dynamics of Earth’s core and mantle in the world while the High Temperature Geochemistry Group offers outstanding experimental and analytical facilities with which the elemental and isotopic characteristic of Earth and planetary materials can be determined in world class clean laboratories with state of the art instrumentation. Research topics within these groups include the dynamics and structure of the Earth’s magnetic field and convection in the outer core, material properties under high pressure and temperature, global seismology, water-rock interactions in mid-ocean ridges and subduction zones, the Earth’s deep water cycle, high pressure metamorphism, and the geochemistry of ore fluids. Please contact any of the supervisors to discuss further PhD opportunities.
Gubbins and Davies (2013) Physics of the Earth and Planetary Interiors 215, 21-28 http://dx.doi.org/10.1016/j.pepi.2012.11.001;
Helffrich (2014) Earth and Planetary Science Letters 391, 256-262 http://dx.doi.org/10.1016/j.epsl.2014.01.039;
Tarduno et al. (2012) Science 338, 939-942
Related undergraduate subjects:
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