Michaelmas: Week 6

Essay: Describe, using appropriate examples, how basaltic and granitic magmas are generated, the different tectonic environments in which these processes occur, and the type of rocks formed as a result.

Thin sections: Describe the thin sections ‘super 3’ and ‘super 7’.  Refer to the short guide I have written to help you.  These sections are in an envelope in my department pigeon hole (outside reception, on the left) – please be very careful with them and put them back in the envelope and back in my pigeon hole after you have finished working with them.

Remember that after identifying phases you need to decide what type of rock you are looking at, and give a summary of its history.

PhD positions for 2018 start

I am involved in several exciting projects at the University of Cambridge, each of which is part of the Cambridge Earth Systems Science DTP (projects I am involved in) and will base the student in the Department of Earth Sciences.  Several of the projects below are cross-disciplinary and link areas of the geosciences and astronomy.  My research life is based both in the Institute of Astronomy and Department of Earth Sciences.

If you have any questions about these projects, please feel free to contact me or one of the other listed supervisors.


Tracing mantle carbon

The project will ask the question ‘How reliable are magmatic archives as recorders of mantle carbon content?’. There are significant limitations to the most ubiquitous approaches of estimating the carbon content of Earth’s mantle: Using seawater noble gas contents provides a bulk upper mantle estimate, not easily resolving the constituent mantle sources; whilst C/trace-element ratios in melt inclusions are frequently lowered by C degassing. This project will provide new constraints on the distribution and cycling of C within the mantle by employing C isotopes as an independent tracer of degassing, and by targeting for analysis eruptions with geochemical signatures from: the lower mantle (Iceland); crustal recycling (The Canaries); and the archetypal depleted upper mantle.

Lead supervisor: Oliver Shorttle [ESC-homepage, IoA-homepage]
Co-supervisors: John Maclennan


Cooking the crust: 4.4 billion years of weathering-driven continental maturation?

The crustal composition has likely evolved over time because of weathering. This project will quantify this over Earth history. This has previously been neglected, but could be a major control on crustal composition with implications for the evolution of the surficial environment and for the igneous processes which generate crust. The student will measure stable isotope ratios of Mg and Li on river sediments to characterise average crustal compositions and variability, whilst associated “model” ages will be determined from radiogenic isotope systems such as neodymium. The Li and Mg stable isotope systems are highly-sensitive to water rock interaction, such as weathering, and their fractionations are most prominent at low temperature. Therefore, when combined, these three isotope systems will enable the weathering-driven maturation of the crust to be constrained.

Lead supervisor:  Ed Tipper
Co-supervisor: Oliver Shorttle  [ESC-homepage, IoA-homepage]


Carbon and sulfur cycling in the Earth’s mantle over the last 4 Ga: new clues from novel stable isotopes

This project will explore carbon and sulfur cycling and mantle source region heterogeneity using a combination of novel stable isotope and geochemical tracers. Through this it will be possible to explore the interplay between partial melting processes, mantle chemical and mineralogical heterogeneity and volatile element cycling. Key ocean island localities will be targeted, which possess abundant evidence for recycled components in their source regions. The study will also extend its reach back over Earth history by considering komatiites, with this project’s analyses contributing important new results to the debate about the origin (thermal vs. compositional) of these enigmatic high-degree melts.

Lead supervisor: Helen Williams
Co-supervisors: Oliver Shorttle [ESC-homepage, IoA-homepage]


The alternative chemistry of deep planetary interiors (Lead Supervisor: Simon Redfern

Ab initio structure prediction methods have now reached a maturity that allow them to be used to model the enthalpic stabilities of phases across pseudo-binary composition sections, The project uses this development to search for new structures in key silicates, oxides and carbonates at high pressures, where “unusual” chemical configurations may be stabilised. Such computational predictions demand experimental verification using high-pressure structural techniques such as vibrational spectroscopy and X-ray diffraction of samples pressurised in the diamond anvil cell. This project seeks to identify such unexpected phases by first adopting particle swarm structure prediction methods based on quantum mechanical computational results, combined with experimental studies of these structures for key candidate silicate, oxide and carbonate chemistries.

Lead supervisor: Simon Redfern
Co-supervisors: Oliver Shorttle [ESC-homepage, IoA-homepage]


Beyond Imaging: Optical Spectroscopy for Mineral Characterization

During this project, we wish to answer the following question: (1) how does the optical absorption signature of minerals such as olivine vary at the micron scale, and what are the key components leading to this variation? (2) Are defects such as dislocations in olivine (a sign of compression and deformation) perceptible with optical tools, and to what extent? (3) How does optical absorption and cathodoluminescence of minerals be used as complementary techniques?

Lead supervisor: Emilie Ringe
Co-supervisors: Oliver Shorttle [ESC-homepage, IoA-homepage]


Stable isotope tracers for the evolution of the crust-mantle system

This project will explore the changing composition of the Earth’s mantle and crust from the perspective of novel isotopic tracers. Recently, differences in the 238U/235U in OIB and MORB have been related to recycling of surface material and the different ages of the OIB and MORB sources (Andersen et al. 2015). Similar effects might be expected for stable isotope systems such as the transition metals (e.g. δ98Mo) and δ138Ba (e.g.Freymuth et al. 2015, Nielsen et al. 2018). Mantle-derived samples representing a time-series of Earth’s history will be used to characterise the isotopic evolution of the mantle in these systems and relate them to the history of plate tectonics and the formation of continental crust via geochemical modeling.

Lead supervisor: Helen Williams
Co-supervisors: Heye FreymuthOliver Shorttle [ESC-homepage, IoA-homepage]

Michaelmas: week 4

Long-format answer: Explain why the concept of structure, rather than chemical formula, is more useful in the study of silicate minerals. Use olivine, pyroxene, amphibole and mica as examples in your discussion.

Question sheet 1 [no need to hand in]: I thoroughly recommend working through the question sheets provided with this part of the course.  The first one has you downloading the crystal maker software and using it to examine the structural elements of crystals.  This is an extremely valuable learning tool so do get familiar with it now.

 

Michaelmas: week 2

Calculation question: Attempt the isostasy question in the 2011 Tripos Part 1A practical exam (it is section 2, parts a-c).

Complete the answer on clean sheets of A4 rather than trying to fit it into the text boxes that come with the exam paper.  You can download past tripos papers from the NST Part 1A Earth Sciences page on Moodle, just follow the ‘past exam papers’ link from the navigation pane on the left (or follow this link).  Note that the structure of the 1A Earth Sciences exams was different before 2012, read your course guide for more details (or look at the most recent exam papers), but some of the older questions set are often still relevant and useful for revision.

Map exercise: Complete map exercise 2 from the examples booklet.

 

Michaelmas: week 0

1 page answer only: Make an information table on the distinctions between sedimentary, igneous and metamorphic rocks. Use columns for the different rock groups and rows for the different distinguishing factors. Put descriptive factors first then process factors later. Incorporate diagrams if appropriate.

Constraining mantle carbon: CO2-trace element systematics in basalts and the roles of magma mixing and degassing

Mixing and degassing systematics

The systematics of mixing and degassing magmas, shown schematically (top) and as seen in the melt inclusion record (bottom). Primary melts are generated with correlated C-trace element systematics (left). Degassing only affects carbon, reducing its concentration in saturated (high C) melts (middle). Subsequent mixing of this variably degassed suite of inclusions can generate the appearance of no degassing having occurred.

The mantle is an important, yet poorly understood, part of Earth’s carbon cycle, which interacts with Earth’s surface through volcanism and subduction. The CO2 flux in to and out of the mantle regulates the mass of CO2 in Earth’s crust and hydrosphere, exerting control over the evolution of Earth’s climate and carbon availability for life. However, carbon’s volatility, and therefore tendency to degas from magmas and emanate at Earth’s surface diffusely, has made identifying the present-day mantle carbon distribution difficult.

Droplets of magma trapped within crystals as they grow deep in the crust offer a chance of observing CO2 concentrations in magmas prior to degassing. The behaviour of CO2 during magma evolution is encoded in the covariation of CO2 and trace element concentrations. In a small number of datasets, a correlation between CO2 and either Ba or Nb has been reported; consequently identical behaviour, in particular a lack of degassing, has been inferred. These, apparently undegassed, datasets underpin our understanding of carbon distribution in the mantle.

In this paper, we argue that many of the melts supplied from the mantle should be oversaturated in CO2 vapour at the pressure of magma storage, whilst others will be sufficiently depleted in CO2 that they should be strongly undersaturated. Such a population of melts will tend to partially degas at the earliest stages of melt evolution, before subsequent mixing and fractionation. We show that positive correlations between CO2 and both Ba and Nb are a natural consequence of this process. Furthermore, our new model makes specific predictions about the covariance of CO2 with a gamut of trace elements, if partial degassing and mixing has taken place.

Since we demonstrate that positive correlations between CO2 and trace element concentrations are arise from partial degassing and mixing, we cannot use this metric alone as a criterion for identifying whether a dataset has been affected by degassing. Mantle carbon contents, derived by assuming such melts preserve primary CO2 concentrations, are likely to be underestimates. We find the maximum CO2/Ba ratio in a dataset is the best proxy for mantle carbon content.


Online [publisher, open access]: https://doi.org/10.1016/j.epsl.2017.09.047

Reference: Simon Matthews, Oliver Shorttle, John F Rudge, John Maclennan. Constraining mantle carbon: CO2-trace element systematics in basalts and the roles of magma mixing and degassing. Earth and Planetary Science Letters (2017).

Olivine-hosted melt inclusions as an archive of redox heterogeneity in magmatic systems

Olivine hosted melt inclusions from the AD 1783 Laki eruption. These inclusions were trapped early in the magma’s life, preserving many chemical signals from the subsequent reprocessing, which occurred as the magma cooled and crystallised in the crust. However, our new study shows the susceptibility of tracers of magma oxidation state in the melt inclusions to resetting, even following eruption.

The amount of oxygen in magmas affects their physical and chemical properties, and ultimately their impact on chemical cycles linking planetary oceans and atmospheres to their deepest interiors.  A key archive of information on oxygen in magmas is the abundance of Fe2+, reduced iron, compared with Fe3+, oxidised iron. The abundance of these two forms of Fe evolves as magmas are stored in the Earth’s crust, meaning that the primary Fe2+/Fe3+ that magmas have when they enter the crust from the mantle will not be preserved at the point a magma comes to erupt.

An important information source for getting back at the chemical state of primitive magmas, before their crustal evolution, is in melt inclusions — small pockets of melt trapped as crystals grow.  However, in this study we show that even these archives of early magma history are susceptible to chemical resetting.  Firstly, in magma chambers at high temperature diffusion can occur, resetting all the melt inclusions to record the same activity of oxygen in the magma (oxygen fugacity).   Secondly, even after eruption, as the magma is flowing along the surface of the Earth, changes in the oxygen fugacity of the surrounding magma can propagate through to the melt inclusions changing theirFe2+/Fe3+ ratio.

These results mean that to reconstruct the oxygen fugacity of primitive magma we have to 1) select our samples very carefully, and 2) characterise the crustal and eruptive processes that could have reset the melt inclusions.


Online [publisher]: https://doi.org/10.1016/j.epsl.2017.09.029

Reference: Margaret Hartley, Oliver Shorttle, John Maclennan, Yves Moussallam, Marie Edmonds. Olivine-hosted melt inclusions as an archive of redox heterogeneity in magmatic systems. Earth and Planetary Science Letters (2017).

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