Stories set in stones: metaconglomerate
The Earth formed 4,567 million years ago, give or take – but it was not the Earth we know. That alien place was most likely an ocean of hot glowing magma with no land in sight. About 100 million years later, a Mars-sized planet collided with our proto-Earth, which threw a bunch of material into its orbit. The two planetary bodies fused together and the debris accreted to form the Moon.
Welcome to the Hadean Eon which lasted until about 4,000 million years ago. It was named after Hades, the Greek god of the underworld – so far, this hot and desolate time seems to live up to its name. Emphasis on seems to because we don’t really know what the early Earth looked like. There simply aren't rocks old enough that we could use to unravel the mystery. But scattered around the world are small outcrops that offer us a tiny glimpse into the Hadean…
If we want to see the oldest terrestrial material, we need to venture to the west coast of Australia. Nestled within the Jack Hills Range is Erawondoo Hill: it may not look like much from the distance but this hill is home to zircons that have been dated up to a whopping 4,404 million years ago (although most are around the 4,000 million year mark). They are found as detrital grains in a metamorphosed conglomerate that is thought to have formed at some point between 3,100 and 2,700 million years ago. While still really old, this is not the rock that the zircons originally formed in. These teeny minerals crystallised from magma…
A great thing about zircons is that they are really difficult to destroy by geological processes: they are hard and resistant, so even heating them up won’t do much. Unlike diamonds, zircons are indeed forever. Works for us. These ancient crystals have persevered through deep time and still hold important information about where and how they formed. If we take a closer look at their ages, oxygen isotopes and rare earth elements, we'll find that they are varied. Some are pretty consistent with those found in granites formed by partially melting pre-existing continental crust, while others are more similar to igneous rocks of mafic and intermediate composition. What does that mean for the history of our planet? There was most likely some sort of crust on Earth before 4,404 million years ago. It wasn't just an ocean of magma. In fact, the oxygen isotopes also suggest that the zircons formed in a rock that interacted with surface water. There was an actual ocean of water on parts of the Earth's surface. Great news for any life forms that may or may not have been emerging around that time. Suddenly, the Hadean appears to be much less of a hellscape.
Later, the zircon-bearing rocks were uplifted (hello plate tectonics!) and began to erode away. The small but mighty zircons resisted everything that was thrown at them and settled down in between pebbles of quartz, probably in a basin. This sediment was buried by more sediment and slowly solidified into a conglomerate. So far, the zircons have been on quite a journey but that was still not the end… The conglomerate was buried even deeper into the Earth where the conditions became unbearable (even for a rock). The intense temperature and pressure stretched and squished the quartz pebbles and the matrix between them which completely changed the rock’s texture. And yet, the zircons remained unbothered. After some time of being baked inside the Earth, the metaconglomerate got uplifted to the surface where the cycle of erosion started yet again. Poor zircons get no rest.
Hungry for more? Here are some scientific articles that describe it in more detail:
Cavosie, A. J., Valley, J. W., & Wilde, S. A. (2007). Chapter 2.5: The Oldest Terrestrial Mineral Record: A Review of 4400 to 4000 Ma Detrital Zircons from Jack Hills, Western Australia. In Developments in Precambrian Geology (Vol. 15, pp. 91–111). Elsevier B.V.
Crowley, J. L., Myers, J. S., Sylvester, P. J., & Cox, R. A. (2005). Detrital zircon from the Jack Hills and Mount Narryer, Western Australia; evidence for diverse >4.0 Ga source rocks. The Journal of Geology, 113(3), 239–263.
International Commission on Geoheritage. (2022). Archean zircons of Erawondoo Hill. The First 100 IUGS Geoheritage Sites. Available at: https://iugs-geoheritage.org/geoheritage_sites/archean-zircons-of-erawondoo-hill/
Kinny, P. D., Clark, C., Kirkland, C. L., Hartnady, M., Gillespie, J., Johnson, T. E., & McDonald, B. (2022). How old are the Jack Hills metasediments really?; the case for contamination of bedrock by zircon grains in transported regolith. Geology (Boulder), 50(6), 721–725.
Spaggiari, C. V., Pidgeon, R. T., & Wilde, S. A. (2007). The Jack Hills greenstone belt, Western Australia; Part 2, Lithological relationships and implications for the deposition of > or =4.0 Ga detrital zircons. Precambrian Research, 155(3–4), 261–286.
Valley, J. W., Cavosie, A. J., Ushikubo, T., Reinhard, D. A., Lawrence, D. F., Larson, D. J., Clifton, P. H., Kelly, T. F., Wilde, S. A., Moser, D. E., & Spicuzza, M. J. (2014). Hadean age for a post-magma-ocean zircon confirmed by atom-probe tomography. Nature Geoscience, 7(3), 219–223.
Wilde, S. A., Valley, J. W., Peck, W. H., & Graham, C. M. (2001). Evidence from detrital zircons for the existence of continental crust and oceans on the Earth 4.4 Gyr ago. Nature (London), 409(6817), 175–178.
