Did Catastrophic Melting Events Control the Evolution of the Atmosphere-Ocean-Earth System?

There are a number of rocky bodies in our Solar System, including our own planet and moon. Largely they have similar internal structures. At the heart is an iron core, surrounded by a solid silicate mantle, which itself is covered by a thin silicate crust. This crust was (and still is) formed when the mantle partially melted and the less dense magmas rose to the surface. The crust of all planets is mostly made of a rock type called basalt. However, the Earth appears to be unique in the Solar System in that the basaltic crust only covers two-thirds of the surface (mostly covered by the world's oceans). The other one third is made of material that is more silica-rich and significantly thicker than the basaltic crust. These are the continents we live on. Exactly when this peculiar feature of our planet formed is one of the central, longest-lived mysteries in the study of the Earth. Using radioactive elements, the age of crustal rocks can be dated. The age of thousands of samples has been measured, and what is clear is that the continental crust is not all the same age and some parts of it are very, very old. The Earth itself is 4.55 billion years old and some crustal material is as old as 4.4 billion years. In fact, there are four ages of continental crust that appear over and over again: 1.2, 1.9, 2.7 and 3.3 billion years. Some have interpreted this repetition to mean that these were times of accelerated continental growth, pulses of magmatism. However, it may be that the record of ages is largely incomplete, and the paucity of continental crust at other ages may not mean that growth rates were low at those times. It might just be that crust of those ages was destroyed by erosion, a process we can see happening today. The data is fundamentally ambiguous. It is analogous to looking through someone's diary and finding missing dates. Did nothing happen on those days? Or were the pages torn out? Our research aims to understand the formation of the continental crust, not by looking at the crust itself, but by looking at the mantle. Radioactive elements in the mantle should have recorded the time when the melts that formed the crust were extracted. If the same pulses of magmatism that are seen in the continents are also found in the mantle, it would confirm the idea of pulsed continental growth. Further, it would suggest that during these events, there was massive magmatism on the planet, far greater than at any time since. If the peaks are not found in the mantle, then it is likely the crustal age peaks were produced primarily by erosion. Our research focuses on the isotope 187Os, because osmium has unique chemical properties that makes it a more robust recorder of melting ages than other isotopes. The study will take advantage of recent advances in analytical technology and most of the analyses will be done by ablating samples with a laser. This will allow a large amount of data to be acquired in a short time. While understanding the formation of the continents is a worthy topic in itself, understanding its growth may have broader implications. A growing set of observations suggests that the Earth's atmosphere and oceans have undergone radical changes in the planet's ancient past, including the abrupt rise of oxygen in the atmosphere that is essential to all animal life. These changes had major effects on the course of biologic evolution. What caused these changes is not clear. Intriguingly, a number of the atmospheric/evolutionary shifts seem to correspond in age to the apparent crustal growth pulses. If the pulses were times of massive, global magmatism, it is likely they would have had a profound effect on the composition of the atmosphere and oceans, as even single volcanic eruptions have been observed to change the global climate. This raises the interesting possibility that the large-scale pattern of life's evolution was set by catastrophic events in the Earth's interior.