The Volatile Legacy of the Early Earth

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

Abstract

In response to the NERC Theme Action (TA) we propose a consortium among scientists at seven UK institutions and with three international partners centred on the 'The Volatile Legacy of the Early Earth'. Earth's habitability is strongly linked to its inventory and cycling of volatiles, which today are coupled to plate tectonics, but we still have little notion as to how our planet found itself in this near-ideal 'Goldilocks' state where the volatile mix is 'just right'. Was it simply a matter of being at the right solar distance with the right supply of volatiles? Or were the details of the chemistry and dynamics of early accretion and differentiation crucial to the eventual outcome? Such questions are of critical importance for understanding our own planets development, and given the burgeoning field of exo-planet discovery, they gain extra piquancy for gauging the probability of life elsewhere. In this proposal we investigate how the early evolution of volatiles on Earth set the stage for habitability.

Planets grow by collisions and these violent events may lead to loss of the volatiles carried within the impacting bodies. We will explore with numerical modeling the conditions under which the volatiles are retained or lost in planetesimal collisions. We will also assess the likelihood that volatiles were delivered to Earth 'late', namely after the maelstrom of major collisions was finished and the planet was largely constructed, by studying the element S and notably its geochemical twin, Se. We will constrain the process of loss to the core and the isotopic signature imparted by this process. We will further use isotopic measurements as finger-prints of the origin of modern Se, and will find out whether it corresponds to any known meteorite type, or if it was possibly delivered by comets. The Moon provides further clues to the origin of the Earth, and Interrogating the significance of the recently refined volatile inventory of the Moon requires new experiments under appropriate conditions.

The energy generated by planetary collisions inevitably results in large-scale melting. The solubility and chemical nature of volatiles within a magma ocean controls whether or not gases are carried into the interior of the planet or left in the atmosphere. Volatiles retained in the magma ocean may become part of a deep mantle volatile cycle or become permanently sequestered in deep reservoirs. We will redress this issue with a series of experiments that simulate conditions of the early magma ocean. We will further investigate the stability of phases in the lower mantle that can potentially hold volatile elements if delivered to great depths by solubility in a convecting magma ocean. Using seismic and modeling techniques, we will assess if any remnants of such stored volatiles are currently 'visible' in the deepest mantle. The influence of the core on volatile budgets is potentially great because of its size, but volatile solubility is poorly known. We will examine the solubility of hydrogen, carbon and nitrogen in liquid metal at high pressures and temperatures.

In this consortium we will also create a cohort of PhD students and supervisors who work as part of a large team to piece together the evidence for Earth's volatile evolution using inclusions trapped in diamonds. These may be the key 'space-time' capsules that can link experimental and theoretical work on early Earth evolution to present-day volatile budgets and fluxes in the deep Earth.

The questions raised in this proposal are complex and require a wide range of information in order to provide meaningful answers. It is our goal to establish a much-improved understanding of how Earth initially became a habitable planet, and to build a solid foundation on which further UK research can continue to lead the way in this exciting field. This will be the ultimate legacy of this consortium, and through links to other consortia, of the entire Theme Action.

Planned Impact

The initial conditions for the formation of Earth have made it the habitable planet that it is. Understanding how volatile elements like water and carbon were delivered to the inner planets of our solar system is a key piece of the puzzle that links astrophysics to geology. Volatile elements are obviously why we have life on Earth, but also why we have plate tectonics and volcanism. A less obvious consequence of water and other volatile elements is the formation of precious minerals and economically valuable ore bodies. For these reasons our 'impact plan' is focused on the role of the early Earth in economic geology. We target two sets of end users - the general public and the mining industry in its widest sense.

Beneficiaries

Outreach and education: educators; school children; general public
Industry: mining companies; local, regional and national government that may benefit from economic geology and enhanced exploration practices; the mineral exploration community, including surveyors, geophysicists, geologists and engineers.

Delivery of Benefit

Teaching resources: for children at two key stages (7-9 yrs and 14-15 yrs) delivered through Bristol's 'Your planet Earth' education series. A suite of teaching tools that explain how Earth processes have led to the formation of key economic mineral resources such as copper, diamonds, and gold will be developed. This will highlight the finite nature of natural resources and encourage more responsible use.
Exhibits at science fairs: posters, animations and hands-on exhibits will be developed. This will not only show how mineral resources are formed and how their existence is a consequence of Earth evolution, but also highlight where common minerals in household products come from.
Exploiting existing collaborations with the mining industry: for example, BHP-Billiton has recently awarded Bristol a 5-year project to work on porphyry copper. Synergy with such projects can be used to better explore more fundamental questions of ore formation and precious metal genesis.

Workshop on Earth formation and economic geology: Speakers from industry and academia will be invited, with structured discussion sessions to encourage collaborations. This will be facilitated through key project partners who have extensive industry expertise.
Development of broader engagement with the natural resources industry: The impact plan will be used to help develop a broader Bristol-based project that will be funding through a NERC accelerator grant. Industry interaction through research and workshops will help develop stronger ties through secondment programmes for young researchers (i.e., PDRAs and PhD students) to spend some time at a company, and vice versa where industry scientists spend time at the University.

Publications

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Description There is a tremendous amount of work in progress throughout the consortium, some highlights of which are:

Paper ready for submission by project PDRA on the effects of impacts on crustal erosion and volatile loss during planetesimal formation

Paper nearly ready for submission by project PDRA on the phase relations of carbonate at lower mantle conditions

Paper published in Nature on melting of carbonated eclogite in the deep mantle

Paper being prepared on the behaviour of volatile elements during segregation of earth's metallic core
Exploitation Route Through published outputs
Sectors Other