Dr Sami Mikhail
Lecture A: Diamonds illuminate the nature of Earth’s deep and dynamic carbon cycle
Carbon is omnipresent yet very mobile within the Earth system. The predominant mechanisms driving exchange between Earth’s layers are diffusion, volcanism, and chemomechanical mixing (i.e., plate tectonics). A minor technical issue is that >90% of Earth carbon is stored within the inaccessible interior (mantle + core). This means that tracing Earth’s planet-wide carbon cycle is challenging, to say the least. However, there are windows into Earth’s mysterious interior, and they’re made of diamonds.
Diamond is a chemically simple mineral comprised largely of carbon with trace amounts of nitrogen (~0.025%). The formation of these crystals sometimes traps fluid and solid inclusions (metasomatic fluids and mantle minerals, respectively). Decades of diamond geoscience has shown us that the carbon cycle is dynamic with evidence for the interaction of subducted volatiles with indigenous mantle carbon during complex tectonothermal events, such as the subduction of crustal material and/or plume-lithosphere interaction.
My work focuses on tracing the origin of diamond-forming carbon. I work on the cheapest and ugliest of the diamond family – known as the diamondites. Despite only being worth around $1 per carat, diamondites host abundant mantle minerals and fluids. Therefore, diamondites are very valuable, geologically-speaking. These unsightly samples comprise fine- to medium-grained diamond intergrown with garnet, minor clinopyroxene, and accessory phases including but not limited to, rutile, sulphide, magnetite, cohenite, Mg-chromite, and fluid inclusions.
Using the major and trace element geochemistry alongside He-C-N-O stable isotope analysis from a suite of diamondites originating from the Orapa mine in Botswana we can tease out information relating to how the formation of diamondites ties into the broader carbon cycle. These data show that diamondites provide evidence for remobilisation of existing mantle carbon by subducted volatiles, ultimately resulting in a hybridised fluid. This diamondite-forming media is indistinguishable from the fluids which precipitate gem-quality diamonds in terms of major and trace element geochemistry. However, while the nature of the parental fluid(s) share a common lithophile element geochemical affinity, the origin(s) of diamond-forming carbon-rich mantle fluids do not always share a common origin. Therefore, it is wholly conceivable that the economically useless diamondites are evidence of a distinct and temporally unconstrained tectono-thermal diamond-forming event beneath Southern Africa.
Lecture B: Is Earth’s atmosphere growing, or shrinking?
The chemistry of a planet’s atmosphere is an archive of surface and subsurface processes, inclusive of volcanism, weathering, meteoritic influx, and biological processes. In addition, Earth is a dynamic planet where subduction zones cycle material back into Earth’s interior. Over the course of Earth’s history these processes have modified the mass and chemistry of Earth’s atmosphere. What is still open for debate, however, is in which direction did the mass of Earth’s atmosphere change?
Sadly, air doesn’t preserve its own history so well (it doesn’t precipitate many solids, for example). However, the history of Earth’s atmosphere can be illuminated by studying the fundamental science of the components which make up air. To this effect, the geochemistry of nitrogen is important because it is a major element in all planetary atmospheres (Venus, Earth, Mars) and makes up most of Titan’s atmosphere (the only moon with a significant atmosphere).
Earth’s atmospheric mass is sensitive to temporal variations in the external (biosphere and atmosphere) and internal (crust and mantle) nitrogen cycles. Nitrogen is a redox sensitive element, and we know Earth’s surface environment and atmosphere have shifted from relatively reducing to very oxidising. Importantly, the different oxidation states manifest as different compounds with strikingly different properties and behaviour in geological systems. For example, molecular nitrogen behaves like a noble gas and ammonic nitrogen can behave like an alkali metal. Interestingly, the conditions where nitrogen shifts from molecular to ammonic in Earth’s mantle occur within the range of redox states observed in mantle silicates. Therefore, we have three potential scenarios: two require the mass of nitrogen in the atmosphere to changed unidirectionally (increased or decreased) and one in which a dramatic deflection occurred following the Great Oxidation Event. However, despite decades of data acquisition it is very challenging to discriminate between these scenarios using the currently available datasets.
My work combines experimental petrology, theoretical geochemistry, and the study of the continental crust and the mantle (diamond) to predict and trace the behaviour of nitrogen in silicate-dominated geological systems. I will present evidence to support the notion that the nitrogen cycle is decoupled from the cycling of the noble gases, carbon, and hydrogen. I will present predictive models supported by empirical data which strongly suggest that nitrogen is heterogeneously distributed within Earth’s interior and that the mass of nitrogen in Earth’s atmosphere has grown and shrunk, over time.
Prof. Caroline Peacock
Lecture C: Mud mud glorious mud – mineralogical controls on Earth’s climate
Trace metals are essential for life and the concentration of trace metals in the surface oceans regulates photosynthetic activity, which in turn regulates the drawdown of atmospheric carbon dioxide and ultimately climate. The processes governing the concentration of trace metals in the oceans are complex but in most cases the ocean sediments provide an important sink for these bio-essential elements, locking up trace metals with sediment minerals. The iron and manganese minerals are particularly reactive towards trace metals and can exert a primary control on trace metal concentrations in the sediments and overlying water column. Investigating the processes by which trace metals become locked into these minerals, and whether trace metals are retained or released by these minerals as they age and change with time, is critical to understanding the role of marine sediments in the carbon cycle and climate. This talk will introduce the importance of trace metals in the ocean system, the importance and peculiarities of marine sediment minerals, the processes by which trace metals are scavenged by these intriguing phases, and what happens to trace metals as these phases change during sediment diagenesis. We will see that the global scale cycling of trace metals in the oceans is governed by molecular scale interactions between trace metals and minerals.
Lecture D: Secrets of the deep – shining light on nutrient cycling at the sediment-seawater interface
Synchrotron microscopy and spectroscopy, together with electron microscopy, are powerful tools for investigating mineral formation and transformation in sediments, and can be tailored to experimental and natural samples in order to investigate how minerals interact with, and potentially control, elemental fluxes across the sediment-seawater interface. This talk will compare and contrast different spectroscopy and microscopy studies of elemental cycling in the marine environment, and highlight how these approaches shed new light on the processes that control elemental abundance and distribution in seawater and marine sediments, and how these sediments might be used and interpreted as archives of the chemical history of seawater through time.