Roberta Rudnick: Evolution of the continental crust as sampled by wind and ice
An ongoing problem for geologists is that we are very small and our planet is very large. This means that we are all busily looking at little pieces of a much larger picture, and try as we might, some of the pieces have been lost or have proved to be very difficult to find. In fact, even if we limit ourselves to the continental crust (our home and primary source of economic resources) representing less than a measly 0.5% of the bulk silicate Earth by mass, we still struggle to sample it representatively. The situation is made worse when we want to play detective and figure out how Earth has evolved on timescales that our minds can scarcely conceive. This might seem like an insurmountable problem, but in Tuesday’s Goldschmidt 2019 Plenary Lecture, Professor Roberta Rudnick enlightened us as to how we can enlist the help of giant glaciers and their associated sedimentary processes to help us more efficiently sample the upper continental crust: An idea first forwarded by the father of geochemistry, Goldschmidt himself.
Rather than tirelessly sampling the Earth’s crust, Rudnick sits back and lets the forces of air and ice conveniently sample the continental crust for her. The wind leaves its collections for her in loess deposits, and glaciers leave them behind in diamictites. The idea is that the composition of these sediments average out the composition of their source rocks (the upper continental crust), at least in terms of all the elements that are insoluble. Established correlations between the preserved insoluble elements and soluble elements then allow us to infer the latter’s concentration. Glaciers are particularly generous servants to geologists because they have been taking samples of the upper continental crust for us at multiple periods in time. This allows Rudnick and her colleagues to put together a historical record of the upper continental crust’s composition without having to: (a) develop a time machine and (b) systematically sample the continents at multiple points in Earth’s rather long history.
The record held in diamictites has resulted in a number of exciting insights into our Earth’s history. Highlights include:
- A record of the oxygenation of the atmosphere. This is marked by a decrease in the concentration of certain redox sensitive elements like Mo, V, and U in the sediment, implying that these elements became fluid-mobile and were washed away from the rocks once they were oxidized.
- A record of the changing silica content of the upper continental crust — a key insight into the modern crust’s formation history. This is revealed by a change in the Cr/Ni ratio of the upper continental crust in time, hinting at a change in the mafic vs felsic character of the Earth’s crust.
- Concurrently, the older, more mafic character of the early continental crust is complemented by evidence for an old-school style of tectonics (the days before plate tectonics became so trendy) dominated by intra-plate crust formation. This is supported by increased Cu in the Archean sediments, suggesting an upper crust comprising at least 50% basalt, a rock we associate with oceanic crust today.
Rudnick ended her lecture by echoing ideas raised the day before by Bethany Ehlmann, pointing out that our celestial neighbor Venus might hold the secrets behind the mechanics of early intra-plate crust formation.