Recruitment

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ClerVolc is inviting applications for four PhD projects to start 1 October 2021

Details of each project are given below.

The annual (three-year) salary for a ClerVolc PhD in 1920 euros per month, before any tax. The ability to speak French is not necessary, but any successful non-francophone candidates will be required to attend courses in the language. Overseas applicants will be required to apply for a student visa, aided by the university.

Please note that only candidates who have either completed a Masters degree (or equivalent, to be specified), or who are in the processes of studying for one and will graduate this year, are eligible to apply for PhD grants in the French system.

Application procedure

Interested candidates are invited to:

  1. Contact the main supervisor (see below for email addresses) in order to discuss the project.
  2. Send the following package to the main supervisor (with copy to tim.druitt@uca.fr) 
    • A curriculum vitae including name, nationality, contact details, education history with dates and annual exam results, employment history, and any other information.
    • Photocopies of official certificates for undergraduate and Masters years, with courses taken and grades.
    • A letter of motivation no longer than two pages.
  3. Arrange for letters of support by two academic referees to be sent confidentially by email to the main project supervisor. These letters should be sent directly by each referee and should not transit via the candidate.

The application package and references may be in French or English. They should reach us no later than 20 May 2021. Incomplete or late applications will not be taken into account.

Candidates for each project will be shortlisted, and the shortlisted applicants will be invited to an online interview at the beginning of June for a decision immediately afterwards.

In the event of enquiries or problems, please contact ClerVolc scientific director T. Druitt (tim.druitt@uca.fr).

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The four projects

Cerium isotopic composition of the upper mantle

Supervisors : Régis Doucelance, Maud Boyet, Pierre Bonnand (Laboratoire Magmas et Volcans)

Contact: Regis.Doucelance@uca.fr

Mid-ocean ridge basalts (MORB) have homogeneous isotopic compositions (Sr, Nd, Pb) compared to those of ocean island basalts (OIB). This is generally interpreted as reflecting (1) the homogenization by mechanical mixing of the upper mantle source of MORB, and (2) the great variety of materials (oceanic crust, sediments, sub-continental lithosphere, …) recycled within the deep source of OIB.

The isotope 138La decays into 138Ce by beta emission with a half-life of 292.5 Gyr. The recent development of 1013Ω amplifiers allows precise determinations of Ce isotope compositions in mantle-derived samples, complementary to those of Nd isotope compositions. Coupled together, they can provide information on the shape of the Light REE pattern of the source material. The 138Ce/142Ce isotope ratios measured in MORB show unexpectedly “large” variations, similar to those recorded in OIB (1 epsilon-Ce-unit in MORB against 1.5 units in OIB, to be compared for example with the 3 and 12 epsilon-Nd-units variations observed for MORB and OIB, respectively). To date, the origin of these variations has not been identified.

The PhD project aims to increase the number of MORB samples analyzed for La-Ce systematics, and to establish the Ce isotope composition of the upper mantle. The selected candidate will combine radiogenic (Ce-Nd-Hf-Sr-Pb) and stable (Fe, Cr, Ca) isotopes. The results will be used to better understand the isotopic heterogeneity of the MORB mantle source. 

The methods involved include clean room chemistry, mass spectrometry (ICPMS, TIMS, MC-ICPMS), isotope dilution techniques.

From magma ascent to deep-seated-ocean lava flow emplacement: the case study of the ongoing (since 2018) submarine eruption offshore Mayotte 

Supervisors : Oryaëlle Chevrel, Lucia Gurioli, Etienne Medard (Laboratoire Magmas et Volcans)

Contact : Oryaelle.CHEVREL@uca.fr

Following the seismic crisis that has been impacting Mayotte Island (Western Indian Ocean) since May 2018, a lithosphere-scale magmatic event gave birth to a 820-m high submarine volcano 50 km from the island, and produced about 6.4 km3 of magma on the ocean floor at 3,300 m depth. Studying volcanism in a deep submarine environment is a real challenge. However, since May 2019, several scientific cruises, led by the French scientific community, have collected a large number of samples (obtained by sea floor dredges) and time-series of bathymetric data through the 2018-2021 period. Samples from the flank of the active volcano, and from distal ponded lava, lava domes and currently active distal fractures, give us a unique dataset to track the spatial and temporal variations in lava effusive rate, facies and syn-eruptive degassing. 

The goal of the PhD project is to provide a complete picture of the volcanic system, integrating pre-eruptive conditions of the deep-seated mantle reservoir, magma ascent and degassing/cooling- induced vesiculation and crystallization along the conduit, and magma emplacement on the ocean floor. 

The student will first perform a detailed quantitative textural analysis of the samples to quantify (i) degassing variations in time and space, (ii) magma ascent time and velocity, and (iii) physical properties/parameters of the erupted material that control the ascent dynamics and submarine emplacement. 

These textural analyses will be integrated with experimental petrology and volcanology measurements to (i) constrain the degassing evolution of the magma towards the surface, (ii) determine the link between decompression rate and vesicle and crystal number densities in mafic magmas, (iii) establish the temperature dependence of viscosity for these compositions (basanite to phonolite), and (iv) quantify the effect of crystal and bubble growth on viscosity during ascent and submarine lava emplacement. 

Finally, bathymetry of the growing volcano and lava flow morphology will be examined to (i) correlate sample texture to lava flow morphology, (ii) relate lava flow morphology diversity to effusion rate variation, and iii) constrain submarine lava flow emplacement dynamics (duration, advance rates, effusion rates). 

We seek a highly motivated student with a good background in geology and in volcanology including skills for rock analyses (textural analyses, petrography, petrology). More importantly, the student must have a team spirit as he/she will work with several scientists in the lab as well as on board scientific cruises. Indeed, the student is expected to take part in one or two scientific cruises around Mayotte.

Formation of the Early Earth atmosphere: numerical modelling of the accretionary contribution

Supervisors: Julien Monteux and Ali Bouhifd (Laboratoire Magmas et Volcans), A. Bianco and L. Deguillaume (Laboratoire de Météorologie Physique).

Contact: julien.monteux@uca.fr

Meteorite impacts have contributed to the evolution of the primitive atmosphere of growing telluric bodies by providing material to the impacted body, by degassing deep material and by eroding the pre-existing atmosphere. The competition of these three phenomena can thus lead to an increase in the thickness of the atmosphere or its erosion. The effectiveness of degassing or impact erosion is a function of the impact velocity, the size of the impactor, the size of the impacted body, the chemical composition and rheological properties (porosity, viscosity) of the materials involved. The degassing of volatile elements is still under-constrained and attributed to the combined action of high temperatures, to an amplification of the diffusion or to the development of micro-fractures. As the combination of these mechanisms is poorly known, scaling laws for obtaining the fraction of material degassed by giant impact from laboratory experiments on small volumes are difficult to obtain. In addition, there are still great uncertainties about the accretive history of the planets and the Earth in particular what was the role of the impacts that followed the Moon forming impact on the formation of the early atmosphere of the Earth. The partitioning of volatile elements in the Earth’s mantle is now increasingly constrained at high pressure and high temperatures. In addition, thanks to numerical modelling, we can now more precisely describe the internal consequences of large meteorite impacts. The scaling laws obtained from hydrocode models show that the pressure below the point of impact can reach several tens of GPa on volumes comparable to that of the impactor. Then the increase in pressure experienced on impact decreases rapidly as you move away from the point of impact. The accretion parameters could thus play a major role in the evolution of the thickness of this atmosphere. However, at present, no quantitative model can integrate the effect of the outgassing of the mantle, initial gases and volatile compounds produced by the impact during the accretion of the planets.

Comparative planetology and numerical simulations are powerful tools for understanding and reconstructing the phenomena that may have occurred more than 4.5 billion years ago. During this project the successful candidate will develop numerical models in order to constrain the primitive evolution of the atmosphere and the primitive Earth mantle and in particular to propose a model of volatile element composition of the primitive atmosphere after accretion of the Earth as a function of the accretion properties (rate, duration, characteristic sizes). By considering realistic chemical compositions, he/she will determine for each impact the degassed volume as well as its composition. Once a robust parametrisation will have been derived, he/she will develop a multi-impact approach to model the global evolution of the early atmosphere accounting for the outgassing of a realistic late accretionary episode. This will allow to monitor the evolution of the early atmosphere in terms of composition and thickness. He/she will test whether selective degassing is a viable model to explain the spectrum of volatile elements (C, H, N and O) or if other processes need to be considered (nucleus formation, nature elementary bricks or the erosion of the earth’s surface via impacts). At the end of this project, he/she will propose a numerical model in which it will be possible to choose the characteristics of the accretion of the Earth in terms of impactor size, impact velocity and accretion rate and to know the characteristics of the primitive atmosphere generated by such accretion. The information obtained by these models will be the thickness but also the composition of this atmosphere in elements such as C, H, N, O and S. 

Equilibrium and Kinetic fractionation of sulphur isotopes in magma

Supervisors : Kenneth Koga, Estelle Rose-Koga (Laboratoire Magmas et Volcans)

Contact: Ken.Koga@uca.fr

Sulfur has four stable isotopes, 32S, 33S, 34S, and 36S, of which 32S is the most common with 95% abundance. In general, chemical reactions at high temperature do not discriminate one isotope from another; all S-bearing, equilibrium, phases have the same isotopic composition, in such case. Reactions at low temperature can usually discriminate the isotopes. However, fractionations of these four isotopes are observed among material derived from volcanic eruptions, usually attributed to fractionation during volcanic degassing. While exact mechanism of this fractionation is not well understood, there can be several possible explanations. 

The proposed thesis project aims to quantify equilibrium and kinetic fractionation of sulfur isotopes in various geological settings especially related to volcanic eruptions, by laboratory high-temperature, high-pressure, experiments. Because of lack of laboratory determined kinetic parameters, the majority of current geochemical interpretations of sulfur isotope data preclude kinetic processes. The project is conceived to fill such obvious gap. 

In addition to master-level education in Earth Sciences, following backgrounds would be particularly useful in carrying this research: familiarity with thermodynamic and kinetic theories, ease with programing computations using widely available scripting programs such as MATLAB, Python, R, and other similar software, experience and/or enthusiasm working in experimental petrology lab (meaning heating and pressing rock powders), as well as tenacity to seek the highest possible data quality.

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A fifth PhD project financed by the Clermont-Auvergne I-site is also proposed. The application procedure is the same as for the above.

Thermochemical heterogeneities in the deep Earth’s mantle 

Supervisor: Denis Andrault (Laboratoire Magmas et Volcans)

Contact:  denis.andrault@uca.fr 

 Seismic tomography and analytical geochemistry provide clear evidences that the deep mantle is constituted of different reservoirs. Major ones are -the so-called “mean mantle”, -the subducted slabs, -the large low-shear-velocity provinces (LLSVP) and -the small ultra-low velocity zones (ULVZ). Both LLSVP and ULVZ are at the contact with the core, however, the LLSVP extend vertically up to ~1000 km. High-resolution seismic imaging is also detecting hot plumes that are rising up through the mantle until they eventually induce intraplate volcanism. 

This PhD work will be dedicated to the interpretation of the LLSVP and ULVZ and more generally to the thermochemical structure of the lowermost mantle. This subject remains largely controversial for several reasons: (i) The comparison between the 1D seismic profiles (such as PREM) and the elastic properties of minerals suggests a larger fraction of the major mineral, the (Mg,Fe)(Si,Al)O3 bridgmanite, with increasing the mantle depths. However, the coexistence of a bridgmanitic lowermost mantle and a peridotitic upper-mantle is incompatible with large-scale mantle convection, which should mix the mean-mantle efficiently, as shown by all geodynamical models. (ii) It was proposed that LLSVP were formed by major mantle overturns early in the Earth’s history, which would have brought large volumes of magmas to the lowermost mantle. Still, the seismic properties of LLSVP are not compatible a basaltic composition. (iii) The nature of the ULVZ received interpretations as divergent from each other as -a chemical reaction with the core, -the graveyard for subducted basalt, -the relic of a basal magma ocean… 

New experiments will be performed to refine our knowledge on the mineralogy and composition of the LLSVP and ULVZ. We will also try to understand the mechanism yielding to the generation of hot-plumes. With the laser-heated diamond anvil cell (LH-DAC), we will reproduce the conditions of pressure and temperature prevailing in the entire lower mantle and synthesize samples representative to the conditions of the primitive Earth’s, when the different reservoirs segregated from each other, as well the conditions of the present-day Earth. 

The recovered samples will be analyzed using the last generation of high-resolution scanning electron microscope coupled with a focused ion beam for their preparation (FIBSEM). Synchrotron-based techniques of X-ray diffraction and X-ray fluorescence are also available, as well as various types of mass spectrometries.