The Response of the Earth to the Terminal Cataclysm

Lead Research Organisation: University of Bristol
Department Name: Earth Sciences

Abstract

The Earth grew via a series of impacts of smaller planets. The last major event saw a Mars-sized object hit the ~90% complete Earth. This impact involved sufficient energy to launch enough material into orbit to form the moon and melt the remaining Earth. If not before, this global melting event allowed the excess melt in the Earth to sink to the centre and form the core. This finished the main stage of construction but some 600 million years afterwards, a series of smaller, but not insubstantial impacts hit the Earth. This has been dubbed the Terminal Cataclysm and was originally recognised from the clustering of cratering ages on the moon at 3.9 billion years before present. Although less physically tangible, the influence on the Earth has been profound, at least from a resources perspective. Precious metals such as gold and platinum have an extremely high preference for residing in a metallic melt relative to silicate melt or crystal phases (silicate minerals comprise the outer layers of the Earth). Metallic melts than sank to produce the core, thus carried with them the vast majority of the Earth's initial inventory of Au and Pt. The Au and Pt that is currently accessible to humanity arrived with the Terminal Cataclysm, after core formation had locked away the original hoard. Since the Terminal Cataclym did not trigger further core formation, this so-called Late Veneer remained within the outer, silicate portion of the Earth. The precious metals from the late veneer are now well distributed throughout the Earth and this project will follow this redistribution from the Terminal Cataclysm. In doing so we will learn about the way in which the mantle (convecting, silicate portion of the Earth) behaves in the first half of Earth History. We also think that the Terminal Cataclysm may have triggered the onset of the modern form of convection we currently see as plate tectonics. We will investigate this idea by seeing if other ancient chemical signatures were homogenised at the same time as the Late Veneer. A positive result would suggest a link between the two processes and implicate the Terminal Cataclysm in starting this process.
The method by which this work will be done is by high precision measurements of the isotope ratios of some of the so-called siderophile elements, namely W (as founded in old-style light bulbs and cutting tools) and Mo (used as an industrial lubricant). The isotope ratios of the meteorite material delivered by in the late veneer has different isotope ratios to the Earth before the Terminal Cataclysm. The differences are small but our new techniques have suitable precision to detect these. The strength of using isotope ratios to trace this process is that they are essentially changed only by mixing and little by other geological processes. By examining the isotope ratios of Mo and W in a sequence of increasingly young rocks, we hope to trace the mixing of the late veneer with the rest of the mantle.

Planned Impact

There are several potential beneficiaries of this research outside the academic community (although as a Blue Skies proposal, the latter is should be the primary focus and will be addressed through the usual channels).
Analytical development. This high precision analyses of the proposed work will showcase the capabilities of the new generation of plasma multi-collector mass-spectrometers. We have close links with the instrument manufacturer and will make Application Notes on this work. This will raise awareness of such capabilities to other users of these instruments, such as nuclear and forensic laboratories.
Our previous NERC-funded work on Mo isotopes is led to a partnership funded by Shell Research to investigate Mo isotopes as a means of tracing oil. The honing of analytical skills in Mo isotope analyses in this project will help in potential future commercialisation of this research.
The planetary scale tales addressed in this research make ripping yarns that are of interest to TV producers and amateur scientists alike. We have a good track record of engaging with both these communities and will continue to do so. To support the dissemination that arises from such work, we will further develop the laboratory website as part of this project in order to more clearly house and display accessible summaries of this and other projects for interested browsers.
 
Description The basis of this proposal was our findings of variable W isotopes on Earth. We interpreted this observation as a consequence of the delivery of a 'Late Veneer' of meteorites to the Earth after core formation. This same process has been invoked to account for the precious metal distribution on the planet. Another group in the US similarly found variable W isotopic compositions on Earth but interpreted these quite differently- in terms of ancient magmatic processes from very early in Earth History. We have made a simple test of this hypothesis. Our original study was on rocks which also showed anomalous (positive) 142Nd isotope signatures. In a magmatic fractionation scenario rocks with negative 142Nd isotopic signatures would be expected to have negative W isotopic compositions. Conversely, in our Late Veneer scenario, they would be expected to show positive W isotopic signatures (as in the rocks with positive 142). We have made this test on ancient samples from the Acasta gneisses with show variably negative 142Nd. All show positive W isotopic compositions, providing strong support for our initial hypothesis.
We have also refined techniques for measuring mass-dependent and mass independent Mo isotope ratio in low abundance Archean samples.
The work of mass-dependent Mo variations has proven highly successful in modern arc settings.
Exploitation Route As part of this proposal we worked with S. Mojzsis (University of Colorado at Boulder). In a desire to continue such work Mojzsis was successful in applying to the Templeton Foundation (USA) to further collect rocks of great antiquity in N. Canada and Greenland. The PI is a named collaborator in the work and will actively participate in this follow on project.
Sectors Education,Environment

 
Description The antiquity of the samples and the link to the origin of precious metals makes the narrative of this proposal of great interest to the public. The work being undertaken as part of the grant has formed the basis of many public presentations. This is also a hot topic in academic research and has given a high profile to UK science in several international meetings.
Sector Education
Impact Types Cultural

 
Description Dialogue with on-line popular science publications 
Form Of Engagement Activity A press release, press conference or response to a media enquiry/interview
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Public/other audiences
Results and Impact I talked to three publications that disseminated information on scientific findings concerning the developing field of W isotopes in the Earth's mantle as investigated in this grant. The most important piece was for Quanta magazine, run by the Simmons Foundation, which involved an interview with me and for part of a major article:
https://www.quantamagazine.org/20161222-earth-history-geophysics/
I also provided comment and material for "Inside Science"
https://www.insidescience.org/content/evidence-earths-early-years-may-still-lurk-mantle/3956"
and Physics Today
Year(s) Of Engagement Activity 2016
URL https://www.quantamagazine.org/20161222-earth-history-geophysics/