How continents form: insights from minute mineral inclusions

The Earth has a bimodal surface elevation reflecting the contrasting chemical and mechanical properties of the continental and oceanic crust. The oceanic crust is dense, unstable, and recycled back into the mantle within 200 Myr through plate tectonics, whereas the continental crust – due to its lower density – tends to remain at the Earth’s surface, recording each step of our planet's evolution. For billions of years the continental crust has evolved to form the environment we live in and the resources we depend on, and yet how and when it formed remain a great matter of debate.

A feature of the rocks of the continental crust is that most of them were derived from rocks that were already in the crust. Thus the challenge has been to see back through to the processes that made new continental crust. At present ~80% of the continental crust is generated along subduction zones, but there is increasing evidence that earlier in Earth's history continents were formed in environments that were either away from plate margins or in an environment which pre-dates plate tectonics and the presence of subduction zones. The timing for the onset of plate tectonics remains a key step in understanding the evolution of our planet, and this study will bring new insights into when plate tectonics started and how continents formed and evolved through time. 

A cross section of the Earth where the two main locus of continental crust formation, subduction zones and intraplate domains, are highlighted. About 80% of the crust is formed through subduction today, but things may have been fairly different in the past. Image credit : Chris Hawkesworth.

To understand continent-scale processes that happened millions to billions of years ago, this project moves to a different scale analytically, to the scale of a few micrometres in mineral inclusions trapped within zircons. Zircon is a zirconium silicate (ZrSiO4) mineral ubiquitous in crustal rocks and sediments. It is difficult to destroy, and since it can be dated precisely it has been used for decades to determine the ages of geological events.

Apatite and K-feldspar crystals trapped as tiny inclusions within zircon will be used because the variation in the abundance of radiogenic isotopes in these minerals (Sr isotopes in apatite and lead isotopes in K-feldspar) offer new constraints into the composition and the tectonic setting of formation of the new continental crust. The isotopes will be measured in situ, and for the first time in mineral inclusions within zircons, using the state-of-the-art laser ablation system and mass spectrometers at the University of Bristol. 

Dr Bruno Dhuime
University of Bristol
Research Project Grant