​The next Trinity College Dublin Department of Geology seminar will be presented by Sami Mikhail (Distinguished Lecturer / Mineralogical Society University of St Andrews) on how 'Diamonds illuminate the nature of Earth's deep and dynamic carbon cycle'.

Abstract

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.

All welcome.