Wed 25th September@ noon, Rm 312.222
Abstract:
In this talk I will present two case studies that show the insights novel stable isotopes can provide us when studying the evolution of planet Earth.
The first will use neodymium stable isotopes to discuss planetary formation. As a geologist there is no more fundamental question than what the Earth is made from? About 15 years ago pioneering work using 142Nd showed that the Earth is distinct from chondritic meteorites the supposed building blocks of planet Earth and threw into question the long-standing chondritic Earth hypothesis. A possible solution to this offset is the so called sulfide matte. Here I will discuss using the newly developed Nd stable isotope system to investigate this solution.
In the second example I will use molybdenum stable isotopes to constrain the volume of crust on the early Earth. Estimates of the volume of the earliest crust based on zircon ages and radiogenic isotopes remain equivocal. Stable isotope systems, such as molybdenum, have the potential to provide further constraints but remain underused, due to the lack of complementarity between mantle and crustal reservoirs. Mo isotope data for Archean komatiites and Phanerozoic komatiites and picrites and demonstrate that their mantle sources all possess sub-chondritic signatures complementary to the super-chondritic continental crust. These results confirm that the present-day degree of mantle depletion was achieved by 3.5 billion years ago and that the Earth has been in a steady state with respect to molybdenum recycling. Mass balance modelling shows that this early mantle depletion requires the extraction of a far greater volume of mafic-dominated proto-crust than previous thought, more than twice the volume of the continental crust today, implying rapid crustal growth and destruction in the first billion years of Earth’s history.
Short bio:
Alex McCoy-West studied geology (to MSc) at Victoria University of Wellington (New Zealand) and then completed a PhD at the Australian National University. These projects focused on using a combination of igneous petrology and radiogenic isotope geochemistry. Following this he moved to Durham University (UK) to undertake postdoctoral research developing novel stable isotope systems. Alex is now based at Monash University where he is focused on understanding the early Earth. His main research interests centre around using novel-stable isotopes to constrain various aspects of planetary evolution.
