Periodic Reporting for period 2 - NONUNE (Evolution of the Non-Uniformitarian Earth)
Reporting period: 2022-07-01 to 2023-12-31
The project seeks to uncover evidence to constrain the two largest scale events in the history of the Earth. One focus of attention is the crystallisation of a primordial magma ocean, which is believed to be an inevitable initial stage of the planet following its energetic accretion. Do any vestiges of this event remain or have subsequent processes removed all record of this tumultuous beginning? The second main objective is to determine when the signature process of the Earth, plate tectonics, commenced.
These are ‘blue skies’ science questions but seek understanding of the fundamental processes that shape our unique planet. As such, we believe the answers are of intrinsic interest to all.
We will use new tools and technologies to tackle these difficult problems. So we have initial objectives in a) refining our ability to analyse the isotopic compositions of the main element constituents of the outer Earth (silicon and magnesium), which can preserve a record of magma ocean crystallisation b) developing a means to date single grains of potassium feldspar, one of the major minerals of the Earth’s crust, that have been eroded and accumulated in sedimentary rocks. Such grains provide a representative sample of the Earth’s crust through time, but need to be dated in order to be used. We will employ novel methods to allow these ages to be determined.
Once our analytical tools are developed we will look for diagnostic differences in the Mg and Si isotope compositions of a range of samples of mantle to look for residual signatures of the magma ocean. We expect this evidence, if still present, to be stored in the deep mantle. This part of the Earth can be indirectly accessed by studying melts generated from upwelling mantle plumes, such as beneath volcanically active islands such as Iceland. For the potassium feldspar grains we will combine our innovative dating methods with further analysis of the Pb isotopic compositions on the same grains, which carry a diagnostic signature if the formed through processes intrinsically associated with plate tectonics. These combined measurements will therefore track the history of plate tectonics. Ultimately we will use dynamic models of the Earth’s surface and interior to model our isotopic compositions and so obtain holistic view of how the Earth functions.
These are ‘blue skies’ science questions but seek understanding of the fundamental processes that shape our unique planet. As such, we believe the answers are of intrinsic interest to all.
We will use new tools and technologies to tackle these difficult problems. So we have initial objectives in a) refining our ability to analyse the isotopic compositions of the main element constituents of the outer Earth (silicon and magnesium), which can preserve a record of magma ocean crystallisation b) developing a means to date single grains of potassium feldspar, one of the major minerals of the Earth’s crust, that have been eroded and accumulated in sedimentary rocks. Such grains provide a representative sample of the Earth’s crust through time, but need to be dated in order to be used. We will employ novel methods to allow these ages to be determined.
Once our analytical tools are developed we will look for diagnostic differences in the Mg and Si isotope compositions of a range of samples of mantle to look for residual signatures of the magma ocean. We expect this evidence, if still present, to be stored in the deep mantle. This part of the Earth can be indirectly accessed by studying melts generated from upwelling mantle plumes, such as beneath volcanically active islands such as Iceland. For the potassium feldspar grains we will combine our innovative dating methods with further analysis of the Pb isotopic compositions on the same grains, which carry a diagnostic signature if the formed through processes intrinsically associated with plate tectonics. These combined measurements will therefore track the history of plate tectonics. Ultimately we will use dynamic models of the Earth’s surface and interior to model our isotopic compositions and so obtain holistic view of how the Earth functions.
In the first half of the project we have made very significant progress in developing the innovative set of new techniques proposed. This includes notable success in realising the novel isotopic measurements, modification of models of planetary convection and crustal production and progress with first principles mineral physics calculations.
We have been able to characterise, to high precision, a homogeneous baseline Mg isotopic composition of the Earth’s upper mantle against which any signature of magma ocean relicts can be detected. We have also been able to quantify the subtle changes in Mg isotopic composition caused by melting and so the bias caused by using melts to access the composition of samples of the deepest mantle in upwelling plumes. Unfortunately, we have discovered that an additional process, diffusion, compromises our ability to use melts to sample the mantle from which they are derived. This problem should not affect our Si isotope measurements which we are now commencing.
We can now date potassium feldspar minerals and analyse their Pb isotopic composition in the same detrital grains. We have applied this ground-breaking methodology to several sample sets. We have made a proof of concept study in NW England, analysed grains in major modern rivers from India and China and studied in a 2 billion year old sedimentary sequence in Scotland. The latter study looks very promising. As expected for this time period, the analysed samples to reflect a plate tectonic regime involved in their formation. The challenge is now to move further back in time. We have tested our potassium feldspar ages in whole rock samples up to 3.5 billion years where it can be compared against other dating methods. This work has highlighted potential problems in resetting some of the ages of the detrital grains. We will explore ways to address this problem in the next stage of the project.
We have extended calculations of Mg isotope fractionation between a magma ocean and the minerals that crystallise from it to appropriate great depths. We have also implemented new styles of crustal deformation and growth in our planetary convection model. These features have been used to reproduce modern observables on the surface of Venus, which helps calibrate behaviour for studying the early Earth.
We have been able to characterise, to high precision, a homogeneous baseline Mg isotopic composition of the Earth’s upper mantle against which any signature of magma ocean relicts can be detected. We have also been able to quantify the subtle changes in Mg isotopic composition caused by melting and so the bias caused by using melts to access the composition of samples of the deepest mantle in upwelling plumes. Unfortunately, we have discovered that an additional process, diffusion, compromises our ability to use melts to sample the mantle from which they are derived. This problem should not affect our Si isotope measurements which we are now commencing.
We can now date potassium feldspar minerals and analyse their Pb isotopic composition in the same detrital grains. We have applied this ground-breaking methodology to several sample sets. We have made a proof of concept study in NW England, analysed grains in major modern rivers from India and China and studied in a 2 billion year old sedimentary sequence in Scotland. The latter study looks very promising. As expected for this time period, the analysed samples to reflect a plate tectonic regime involved in their formation. The challenge is now to move further back in time. We have tested our potassium feldspar ages in whole rock samples up to 3.5 billion years where it can be compared against other dating methods. This work has highlighted potential problems in resetting some of the ages of the detrital grains. We will explore ways to address this problem in the next stage of the project.
We have extended calculations of Mg isotope fractionation between a magma ocean and the minerals that crystallise from it to appropriate great depths. We have also implemented new styles of crustal deformation and growth in our planetary convection model. These features have been used to reproduce modern observables on the surface of Venus, which helps calibrate behaviour for studying the early Earth.
Our development of detrital potassium feldspars as a completely new tool to study the evolution of the continental crust is beyond the state of the art. The key aspect of this work, obtaining ages on a single crystals, relies on the recent harnessing of collision cells to isotope ratio plasma mass-spectrometers and is a very promising application of this new technology. We argue that there is potential in this new approach to provide as significant, yet complementary, information about crustal evolution as provided by the revolutionary development of zircon micro-analysis over the last 30 years.
There are lessons to be learned on how best to use the information contained in detrital feldspar analyses. Not least, it may well be that their propensity to have their ages reset by tectonic events is more useful than recording initial formation ages, which is already robustly recorded in zircons. Exploring the power of detrital potassium feldspar analyses will be the focus of much of our attention in the second half of the project. The results on the style and timing of continental growth that they provide will act as observables to constrain the geodynamic model of the evolving Earth.
The other major set of results to be generated with be high precision Si isotopic compositions of mantle samples. We are embarking on this work. This will go hand in hand with new theoretical estimates of Si isotope fractionation at the extreme conditions of the Earth’s deep interior, calibrated at low pressures by some petrological experiments made in the laboratory. This work will provide us with definitive constraints on whether or not relicts of a primordial magma ocean are preserved to present day. In turn we will explore if this result is consistent with our dynamical model of the Earth’s mantle.
There are lessons to be learned on how best to use the information contained in detrital feldspar analyses. Not least, it may well be that their propensity to have their ages reset by tectonic events is more useful than recording initial formation ages, which is already robustly recorded in zircons. Exploring the power of detrital potassium feldspar analyses will be the focus of much of our attention in the second half of the project. The results on the style and timing of continental growth that they provide will act as observables to constrain the geodynamic model of the evolving Earth.
The other major set of results to be generated with be high precision Si isotopic compositions of mantle samples. We are embarking on this work. This will go hand in hand with new theoretical estimates of Si isotope fractionation at the extreme conditions of the Earth’s deep interior, calibrated at low pressures by some petrological experiments made in the laboratory. This work will provide us with definitive constraints on whether or not relicts of a primordial magma ocean are preserved to present day. In turn we will explore if this result is consistent with our dynamical model of the Earth’s mantle.