The co-evolution of Earth’s continental crust and hydrosphere: a Si isotope perspective

Overview

The oldest rocks on Earth today are found as parts of the continental crust. These rocks record both the earliest processes that formed new crust, and also the processes of weathering and alteration on Earth’s surface: over 4.0 Ga of Earth history. Of particular interest are the times before, leading up to and just after the so-called Great Oxidation Event (GOE; ~ 4.0-2.0 Ga). The rock record from the Hadean, Archaean and Palaeoproterozoic is sparse compared to more recent geological periods, yet this time span – around 50% of Earth’s history – is thought to include the establishment of subduction-driven plate tectonics and the rise of atmospheric oxygen and life on Earth [1].

One isotope system that could inform us about the evolution of both of these important phenomena in terms of their effects on the (upper) continental crust is silicon – THE defining element of the silicate Earth [2]. Recent Si isotope studies have identified that the earliest examples of continental crust – so-called “TTGâ€ rocks – have distinctive Si isotope compositions compared to continental crust formed in the last 500 Ma: crucially, these ancient TTG Si isotope signatures seem to suggest that marine-derived silica (i.e. cherts) plays an important and apparently ubiquitous role in the source rock of ancient continental crust [3, 4, 5].

The possible identification of significant quantities of marine-derived silica in the rock record as far back as 3.8 Ga suggests that there was wide-spread ocean basins before this time, and that silicification was an important process on the early Earth. However, these recent studies pose a number of pertinent research questions:

1. Within our current petrogenetic understanding, is it possible to form TTG crust by melting a mixture of chert and basaltic material?
2. The Si isotope data imply that TTGs have a consistently heavy Si isotope composition compared to modern day crust – but direct analysis of Archaean cherts show these to be isotopically very variable. Is incorporation of marine chert the only way to make TTG rocks isotopically heavy?
3. Are ALL TTGs isotopically distinct from modern continental crust, or is there a secular change – possibly reflecting a change in tectonic processes?

Silicon isotopes also behave relatively predictably during continental chemical weathering – for instance during the formation of clay-rich sediments [6, 7]; whereby increasing chemical denudation of the source rock leads to increasing negative isotopic signatures. In simple terms, the more intense the weathering degree, the more fractionated the Si isotopes will become. Again, however, this is during surface weathering in our modern-day atmosphere, rich in oxygen, and poor in methane and (relatively) poor in CO2. Can we see a secular effect of the evolution of the composition of Earth’s atmosphere on the Si isotope composition of chemical sediments?

This study aims to investigate aspects of the broad question: why has the average Si isotope composition of the Earth’s continental crust varied over time? In particular, what effect has the evolution of plate tectonics and the hydrosphere/atmosphere had on the silicon budget of the continental crust? To do this, the project will focus on two broad lithologies:

1. Ancient cratonic basement rocks – e.g. TTGs – consisting of meta-igneous and metasedimentary rock units – from Greenland and Scotland.
2. Glacial diamictites and associated siliciclastic strata deposited at key time intervals sourced globally.

Throughout the project there will be opportunities to sample both ancient continental crustal rocks, and glacial diamictites in the field, which straddle or accompany significant global terrestrial environmental changes such as Snowball Earth events spanning the GOE. The project has the potential to characterise a new geological proxy for weathering intensity and/or continental crust formation.

Click on an image to expand

Image Captions

1280px-Lewisian_Gneiss,_Achmelvich_Bay.jpg – Metamorphosed Archaean continental crust – the Lewisian Gneiss, Achmelvich Bay, NW Scotland.
TTG formation.jpg – Schematic cartoon of a prevailing view of how formation of TTG crust differs from modern day subduction (from Polat, A., 2012. Growth of Archean continental crust in oceanic island arcs, Geology, 40(4), 383-384)

Methodology

The main aim of this project is to constrain the bulk Si isotope composition of the Earth’s continental crust over the period from ~4.0 to recent, with an ultimate goal of using variations (or lack thereof!) in Si isotope composition to make broad inferences about early continental crust petrogenesis as well as weathering strength and style. This will be accomplished by measuring a broad range of continental crust-derived materials – this can involve field work, mapping/logging, sample preparation, in collaboration with the Continental Geochemistry Group at the University of California, Santa Barbara, who established the use of glacially-milled rock powders (diamictites) as proxies for continental crust isotopic composition [8, 9].

The student will begin by working on the samples collected, prepared and characterised by Gaschnig et al. [9]. In addition, new ‘composite’ samples will be generated by sampling tillite horizons at different localities, separating matrix from clasts and combining each sampled horizon in equal quantities. These will then be characterised for major and trace element compositions at St Andrews and Durham University, and all samples will require Si isotope analysis – by high resolution multi-collector inductively coupled plasma mass spectrometry (MC-ICPMS) – which will be performed by the student at St Andrews and Durham. The student will then be able to take the project in their own direction – either by focussing on ancient metamorphic rocks (TTGs) or on other glacially-generated sediments. To better interpret the isotopic data colelcted, the student also will have an opportunity to unravel the petrogenesis of ancient metamorphic rocks via the application of thermodynamic modelling software (ThermoCalc) and training in this will occur at the University of St Andrews. The potential for the Si isotope system to be used as a proxy for weathering degree/intensity could be explored in other extreme climatic events on Earth – and following the initial whole-Earth investigation, there is the potential for the student to investigate Si isotope variations in detail in the glacial deposits formed after more recent global glaciation events.

Project Timeline

Year 1

Literature review and compilation of existing data for ancient and modern continental crust and sampling proxies; identification of localities and sampling strategy; acquisition of extant samples and field work planning; training in rock sampling and characterisation techniques, elemental and stable isotope analysis; fieldwork to take place in summer between Years 1 and 2; write and defend Year 1 Research Proposal.

Year 2

Characterisation of collected samples from field work; continued Si stable isotope analysis of all samples; begin stable isotope modelling of data. Prepare data for presentation/publication; attend international geochemistry conference (e.g. Goldschmidt).

Year 3

Completion of isotope work and interpretation and modelling of data, writing up. Presentation of results at a national/international conferences; complete thesis.

Year 3.5

Finalising any outstanding measurements; main focus on writing journal articles and thesis chapters.

Training
& Skills

• Field sampling of the appropriate ancient sedimentary deposits; sampling characterisation techniques
• Training in the measurement of Si stable isotopes using high precision MC-ICP-MS at St Andrews, as well as routine elemental sample characterisation.
• Interpretation and modelling of isotope and elemental data to place new constraints on the terrestrial Si isotope cycle through geological time.
• Application of thermodynamic modelling software (Thermocalc)
• Participation and presentation of research at both national and international geochemistry conferences.
• Assimilation and presentation of data in the form of journal articles and other written scientific media

References & further reading

[1] Canfield, D. (2005) The Early History of atmospheric oxygen. Annu. Rev. Earth. Planet. Sci. 33, 1-36.[2] Savage, P.S. et al. (2014) High temperature silicon isotope geochemistry. Lithos 190-191, 500-519.[3] Trail, D. et al (2018) Origin and significance of Si and O isotope heterogeneities in Phanerozoic, Archean, and Hadean zircon. PNAS, 115 (14) 10287-10292[4] Andre, L. et al (2019) Early continental crust generated by reworking of basalts variably silicified by seawater. Nature Geoscience 12, 769-773.[5] Deng, Z. et al (2019) An oceanic subduction origin for Archaean granitoids revealed by silicon isotopes. Nature Geoscience 12, 774-778.[6] Savage, P.S. et al. (2013) The silicon isotope composition of the upper continental crust. Geochim. Cosmochim. Acta, 109, 384-399.[7] Opfergelt, S., & Delmelle, P. (2012) Silicon isotopes and continental weathering processes: assessing controls on Si transfer to the ocean. Comptes Rendus Geoscience 344(11-12), 723-738.[8] Gaschnig, R., et al. (2014) Onset of oxidative weathering of continents recorded in the geochemistry of ancient glacial diamictites. EPSL 408, 87-99.[9] Gaschnig, R., et al. (2016) Compositional evolution of the upper continental crust through time, as constrained by ancient glacial diamictites. Geochim. Cosmochim. Acta, 186, 316-343.

Further Information

For further information please contact Paul Savage (pss3@st-andrews.ac.uk; 01334 464013) or Kevin Burton (k.w.burton@dur.ac.uk; 0191 3344298).

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