Finding the missing evidence for Earth's magma ocean: a novel stable isotope approach

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

Abstract

Earth's present belies its violent past. Catastrophic impacts during the Earth's first 500 million years generated enough energy to melt the planet's interior, creating planetary-scale volumes of melt, or "magma oceans". Their subsequent cooling and crystallisation would have set the chemistry of the Earth and its future long-term habitability. However, we do not know exactly where and how the Earth's magma oceans crystallised, what their composition was and whether remnants of early magma ocean material remain present in the Earth's deep interior, potentially acting as important reservoirs for volatiles and precious metals.

A key piece of information may reside in the deep Earth: as the magma ocean cooled it would have started to crystallise, with the dense newly formed crystals sinking to the base of Earth's mantle. This would have generated strong chemical layering in the mantle, which could persist to today. This project focuses on finding the chemical evidence for these piles of dense magma ocean crystals, and thus identifying a key missing piece of evidence for Earth's earliest history.

As the deepest mantle is inaccessible to direct sampling, we must rely on nature to do this for us. This occurs when regions of the mantle heat up, buoyantly rise and melt, ultimately producing volcanism; a phenomenon exhibited at Iceland, Hawaii and other "mantle plumes". We can use the chemistry of these lavas to probe the composition of the material that melted to form them, thereby gaining a window into the deep Earth. The chemical signals in both modern and ancient lavas have resulted in the paradigm of isolated and "primordial" regions of the Earth's interior, often presumed to be located at the very base of the Earth's mantle, at the boundary with the planet's central metallic core. It has been suggested that the mineralogy and composition of these deep mantle domains has allowed them to resist being entrained into the convecting mantle for billions of years, where they may store volatile- and heat-producing elements. Do these regions of the Earth's mantle have their origin in magma ocean crystallisation? Has magma ocean material always remained isolated from the convecting mantle? Can residual frozen melts or crystalline material left over from magma ocean crystallisation be transported into the upper mantle, and if so, can it melt and contribute to the chemistry of modern and ancient primitive lavas?

To answer these questions, we need chemical tracers that, 1) respond directly to the type of minerals that would have formed during the crystallisation of a deep magma ocean, 2) are resistant to alteration when volcanic rocks are weathered at Earth's surface so that they can be applied to ancient lavas, and 3) reflect the bulk properties of the mantle that these lavas were derived from.
We propose to use iron (Fe) and calcium (Ca) stable isotopes as tracers. Reconnaissance measurements of 3.7 billion year old rocks shows that these tracers are robust to the rocks' weathering history. The data also contain the tantalising suggestion that these volcanics were derived from melting material residual from a former magma ocean. We will use these tracers to explore the Earth's magma ocean history and its role in defining the chemical and physical state of the planet today. Important steps are:

1) Constraining the partitioning of Fe and Ca isotopes during magma ocean crystallisation. We will do this by high-pressure laboratory experiments, where we will simulate the conditions of magma ocean crystallisation and analyse the crystal residues that we produce.

2) Undertaking new Fe and Ca isotope analysis of volcanics ranging from 3.7 billion years old to the present.

3) Develop a series of thermodynamic models to track the Fe and Ca isotope effects of magma ocean crystallisation and to predict the composition of volcanics derived from the entrainment and melting of these magma ocean crystal piles in the upper mantle.

Planned Impact

Industry and the general public will both benefit from our proposed research. The nature of the economic and societal benefits of our research to these groups is detailed below. The strategies that we propose to use to engage these groups and ensure that they achieve these benefits are described in the Pathways to Impact.

Industry: Instrument manufacturers and developers

This research involves making precise stable isotope (Fe, Ca) measurements of small samples at high precision using two different types of instrument: a multiple collector inductively coupled plasma mass spectrometer (MC-ICPMS; Thermo Fisher Scientific Neptune Plus) and thermal ionisation mass spectrometer (TIMS; Thermo Fisher Scientific Triton Plus). Such measurements are of great interest to many other research fields (nuclear, medical, archaeology, environmental and pollution science) and to companies (e.g. Cameca, Agilent, Nu Instruments, Thermo Fisher Scientific, Perkin Elmer, Elemental Scientific) developing analytical instruments and accessories for sale. We have already collaborated with Thermo Fisher Scientific and are in discussion with them about future collaborations involving new method development and analytical demonstrations. This benefits this company and others in several ways: in the short-term, in the form of instrument sales resulting from successful demonstrations, and in the medium/long-term, in the form of developing the next generations of instruments with input from users.

General Public

There is considerable public interest in the Earth's planetary origins. However, the research methods employed by scientists to study the inaccessible interior of the Earth and so gain clues about its origin are not well understood. The diverse nature of the research tools employed in this project (high-pressure experiments, analysis of natural samples, modelling) provides us with an excellent opportunity to inform the public about the cross-disciplinary techniques and methods of Earth Science research, as well as the results. The benefit to the public as a whole will therefore be information about how our planet has formed and evolved as well as insights into how scientists work and integrate information from many different sources to test their ideas. This latter part is particularly important, as we are focused on engaging, inspiring and informing schoolchildren on the potential of STEM subject choices and careers in the UK. Our objective is to build their understanding and curiosity in Earth Sciences and the general process of scientific research. We have planned an extensive program of engagement with these groups (detailed in the Pathways to Impact) and we anticipate that the societal benefits will be achieved both in the short term, during the life-time of this grant, as well as in the longer term.

Publications

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