Download Core formation in the early Earth: the lasting geochemical legacy of

Core formation in the early Earth: the lasting geochemical legacy of
textural instabilities
Dr Geoffrey Bromiley (School of GeoSciences, University of Edinburgh), Professor
Paul Attfield (School of Chemistry, University of Edinburgh), Dr Ian Butler (School of
GeoSciences, University of Edinburgh)
Primary supervisor contact: [email protected]
Project background – Earth and the other rocky planets of the inner solar system consist of
iron-rich cores surrounded by thick shells of ‘rock’ (silicate mantle and crust). Recognising
how and when metallic cores formed in these bodies is key to understanding formation and
evolution of our solar system. However, core formation also has far greater implications for
many areas of Earth Science research, because it left a lasting geochemical signature on the
rocky planets. Our ability to model processes which have changed the Earth through time
depends on our understanding of the chemistry of the deep Earth. If we do not know the
geochemical legacy of core formation, we cannot truly understand the most important
processes which have shaped planets like the Earth since their formation 4.4 billion years
ago.
It is generally believed that core formation in the inner solar system initiated once the outer
portions of planets had melted to create deep, silicate magma oceans. Iron-rich liquid would
have rapidly sunk to the bottom of magma oceans and ponded, eventually sinking through
the underlying solid portion of planets as large diapers (see box). This model predicts rapid
chemical equilibration
Left: artist’s impression of the formation of the Earth
between liquid iron and
(astroblog.wordpress.com). Top right: model of core
liquid silicate at the base of
formation showing settling of iron (black) at the base of a
magmas oceans, which
deep magma ocean (orange). Iron diapers then sink
would have ‘reset’ the
through the remaining solid portion of the mantle (green)
chemistry of the mantle.
to form a core. Bottom right: SEM image from high PT
However, despite
experiment showing textural instability in liquid silicate
considerable effort in
(white) iron (grey) boundary.
modelling conditions of
chemical equilibrium from
observed mantle
geochemistry, many
problems remain.
Increasingly, researchers
are developing models
which invoke varying
degrees of disequilibrium, or
complex, multi-stage core
formation processes, to
explain geochemical
signatures of Earth’s mantle.
Much of our understanding of the legacy of core formation comes from detailed geochemical
investigations of the Earth and other bodies, and high-pressure/temperature (HPT)
experiments designed to constrain element and isotope behaviour during core formation. By
contrast, few studies have explored physical mechanisms for core formation. In this project
we will investigate a fundamental aspect of models which has remained essentially
unexplored: the morphological stability of textures during core segregation.
Key research questions - Using a novel experimental approach we will seek answers to the
following: How stable is the interface between core-forming liquids and silicates during coreformation? What influence does textural stability have on core-mantle chemical reequilibration during various stages of core formation? What influence does textural instability
have on the efficiency of core formation? Are geochemical signatures in Earth’s mantle
consistent with chemical disequilibrium arising from textural instability?
Methodology – You will perform experiments in model systems under the extreme pressuretemperature conditions of core formation using piston-cylinder and multi-anvil apparatus.
Textural development in samples will be studied by high-resolution electron microscopy and
3-D X-ray microtomography, with in-situ composition analysis by electron microprobe.
Additional in-situ time-resolved studies of morphological instability will be performed at ESRF
(Grenoble, France) and DIAMOND (Oxfordshire) following application for beamtime at these
international synchrotron facilities.
Training - A comprehensive programme will be provided comprising both specialist scientific
training and generic transferable and professional skills. Specific training will include use of
high-PT experimental equipment and microanalysis using scanning electron microscopy,
electron probe microanalysis, and X-ray microtomography with 3-D textural analysis. You will
also have the opportunity to network and present your research at local and international
conferences, be involved in both E3 DTP training events and the NERC funded “Volatile
Legacy of the Early Earth” consortium, and be a member of the cross-disciplinary Edinburgh
Centre for Science at Extreme Conditions (CSEC).
Requirements
The student should have a good first degree in an area of Earth Science or physical sciences
and should have good numeracy skills. Experience in analytical techniques and/or sample
synthesis and general lab skills would be advantageous.
Further reading
1. Otsuka and Karato (2012) Nature 492:243-247 (experimental investigation of core-mantle
textural instability in present-day Earth)
2. Rudge et al. (2010) Nature Geoscience, 3:439-443 (example of a disequilibrium modelling
for core formation)
3. Wood (2008) Philosophical Transactions A, 366:4339-4355 (example of an equilibrium
modelling for core formation)
Project summary
You will investigate the fundamental processes of core-formation in the early solar system,
exploring effects that instabilities in liquid metal-silicate interfaces had on the geochemistry of
the rocky planets.