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Smectite, variety hectorite
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Mineralogical Society Distinguished Lecturer Programme 2017–18

The Distinguished Lecturers for academic year 2017-18 will be Dr Dan Smith (University of Leicester) and Dr Helen Williams (Cambridge University)

Dr Dan Smith

Lecture A: Tellurium Tomorrow: Solar Power, Supply, Demand and Waste of a Rare Material

Tellurium is one of the least abundant elements in the Earth’s crust, yet society has a burgeoning need for a reliable supply of this semi-metal, particularly as a key ingredient in solar panels. Although established uses in alloys and computer components have declined, rapid growth in solar power has dramatically increased the consumption of tellurium. Tellurium is almost exclusively produced as a by-product of refining other metals (chiefly copper), and at present, industry is poorly positioned to increase supply in line with growing demand. Alternative sources exist however; a number of mineral deposit types are notably enriched in tellurium. Why some deposits are enriched in Te in particular is puzzling; this lecture will unpick some reasons why this rare element becomes enriched in particular environments.

A continuing challenge for tellurium supply has been in the “how” of recovering it. There are a number of metallurgical problems with the extraction of the element even from enriched ores. This talk will outline some novel solvents being developed at the University of Leicester that have the potential to radically change the way we process ore minerals, and transform the supply of rare metals such as tellurium, all while reducing the economic and environmental impacts of mineral processing.

 

Lecture B: Wet Magmas and Copper Fertility

Porphyry deposits are the principle source of the world's copper and molybdenum, and contribute significantly to global gold, silver and critical metal production. Porphyry deposits are the products of magmatic-hydrothermal systems associated with wet magmas, often (but not exclusively) in arc settings. The mineralising hydrothermal system is shallow (1-6 km) but must be tapping into volatiles and metals from a huge volume of magma at greater depth. Water-rich magma is a key building block of porphyries, and a number of recent studies have focussed on how magmas evolve in the crust, how their volatiles are stored and released, and whether there are distinctive signatures of "fertility" or ore-forming potential. Fertility indicators have the potential to improve our ability to locate and discover new deposits, including those with minimal expression at the surface. Can we identify "smoking guns" for these vital resources? Or are we analysing "red herrings"?


Dr Helen Williams

Lecture C: Tracing fluid transfer across subduction zones using iron and zinc stable isotopes

Subduction zones are the main site of volatile element transfer between the downgoing plate, the overriding mantle wedge and the Earth's deep interior. The breakdown of serpentine minerals within the downgoing slab and the fluids released play a fundamental role in volatile cycling as well as the redox evolution of the sub-arc mantle. Constraining subduction-related serpentinite devolatilisation is essential in order to better understand of the nature and composition of slab-derived fluids and fluid/rock interactions.

Iron and Zn stable isotopes are recently-established geochemical tracers can trace fluid composition and speciation as isotope partitioning is driven by changes in oxidation state, coordination, and bonding environment. In the case of serpentinite devolatilisation, Fe isotope fractionation should reflect changes in Fe redox state and the formation of chloride and sulfide complexes; Zn isotope fractionation should be sensitive to complexation with carbonate, sulfide and sulfate anions.

This study involved targeting samples from Western Alps ophiolite complexes, interpreted as remnants of serpentinized oceanic lithosphere metamorphosed and devolatilized during subduction.  A striking negative correlation is present between bulk serpentinite Fe isotope composition and proportion of ferric iron, with the highest grade samples displaying the heaviest Fe isotope compositions and proportion of oxidised iron. The same samples also display a corresponding variation in Zn isotopes, with the highest grade samples displaying isotopically light compositions. The negative correlation between Fe and Zn isotopes and decrease in ferric iron content can explained by serpentinite sulfide breakdown and the release of fluids enriched in isotopically light Fe and heavy Zn sulphate complexes. The migration of these highly oxidizing sulfate-bearing fluids from the slab to the slab-mantle interface or mantle wedge has important implications for the redox evolution of the sub-arc mantle and the transport of metals from the subducting slab.

 

Lecture D: The iron isotope composition of the Earth's lower mantle: implications for mantle mineralogy and differentiation processes

Core formation is a key process in the evolution of terrestrial planets that imparts characteristic geochemical fingerprints on the remaining silicate mantle. Recent studies have exploited the potential of high-pressure, high-temperature equilibrium stable isotope fractionation driven by differences in element redox state and bonding environment to constrain planetary differentiation processes and the composition of planetary cores. 

The caveat to these studies is that there must exist some means of directly comparing the predicted isotopic variations with the observed compositions of planetary mantles. This is difficult for the Earth because most primitive magmas are derived from the upper mantle and may not provide a true window into metal-silicate equilibrium at the base of a magma ocean. For example, the heavy Fe isotope compositions of terrestrial basalts relative to basaltic meteorites from Mars and Vesta can be interpreted in multiple ways: as a function of the low Ni content of the Earth's core, planetary accretion processes, partial melting or the disproportionation of ferrous iron into ferric iron and metallic iron during bridgmanite crystallisation in the lower mantle.

Use this form to apply for a lecture at your institute in 2017 - closing date 1st September 2017. Applications by Societies are welcomed.