Zircon study urges re-look at early Earth and Moon cataclysm dates

A study of terrestrial zircons at the Vredefort impact structure in South Africa by a research team led by Aaron Cavosie, a professor of geoscience at the University of Wisconsin-Madison, has questioned the accuracy of the currently used method to date lunar zircons, and the dates of some of the earliest and most massive collisions in our Solar System.

Zircons are neosilicate minerals, currently considered to be the oldest minerals on Earth, which are commonly found in the Earth's crust. They also have trace amounts of uranium and thorium which makes them suitable candidates for radiometric dating. 

Zircons have been traditionally used to date some of the earliest and most massive collisions in our Solar System. Two prominent examples of such events include the super-collision which caused a massive ejection of matter from the Earth leading to the formation of the Moon, approximately 50 million years after the formation of Earth, and the Late Heavy Bombardment (LBH), a series of meteor impacts which may have created an extremely craterous surface on Earth around 4 billion years ago.

Even though the above mentioned events are widely accepted by geologists, actual physical proof is hard to come by. The dating of these events is mainly based on the radiometric dating of lunar zircons which were retrieved during the Apollo missions in the 1970s.

The deformed lunar zircon at center was brought from the moon by Apollo astronauts. The fractures characteristic of meteorite impact are not seen in most lunar zircons, so the ages they record probably reflect heating by molten rock, not impact. Credit: Apollo 17/Nicholas E. Timms.

Aaron Cavosie, a visiting professor of geoscience and member of the NASA Astrobiology Institute at the University of Wisconsin-Madison has published a study of zircons from a gigantic meteorite impact in South Africa in the journal Geology on October 1, 2015. The study claims that zircons retrieved from the lunar surface do not allow for accurate dating of lunar impacts as they are "ex situ" – removed from the rock in which they originated. This makes it difficult for geologists to accurately calculate the date of impact.

"While zircon is one of the best isotopic clocks for dating many geological processes," Cavosie says, "our results show that it is very challenging to use ex situ zircon to date a large impact of known age."

Although many of their zircons show evidence of shock, "once separated from host rocks, ex situ shocked zircons lose critical contextual information," Cavosie says.

Radiometric dating of zircon is based on the lead isotopes which accumulate in the zircon during the radioactive decay of its uranium content. Geologists use precise measurements of the isotopes and uranium's half life to arrive at a definite date which marks the beginning of the lead accumulation. A hypothetical asteroid impact would remove the existing lead and the whole process would repeat. Thus the accumulated lead isotopes should point to the exact date when impact occurred. Studies of lunar zircons using this method put the Late Heavy Bombardment (LBH) between 4.3 billion to 3.9 billion years ago.

This highly shocked zircon, from the Vredefort Dome in South Africa, shows thin, red bands that are a hallmark of meteorite impact. Uranium-lead dating from this zircon matched the age of the rocks exposed at Vredefort, not the more recent age of impact (2 billion years). Credit: Aaron Cavosie 

To assess the accuracy of this method, a team of researchers led by Cavosie collected zircons from the 2 billion year old Vredefort impact structure in South Africa. Cavosie described the structure as deeply eroded and approximately 90 kilometers (56 miles) across. "The original size, estimated at 300 kilometers (186 miles) diameter, is modeled to result from an impactor 14 kilometers in diameter," he added.

The researchers examined the zircons for tell tale signs of impact. The zircons from South Africa are "out of place grains that contain definitive evidence of shock deformation from the Vredefort impact," Cavosie says. "However, most of the shocked grains do not record the age of the impact but rather the age of the rocks they formed in, which are about 1 billion years older."

According to zircon expert John Valley, a professor of geoscience at UW-Madison, "Most zircons on Earth are found in granite, and they formed in the same process that formed the granite. This has led people to assume that all the zircons were reset by impact, so the ages they get from the Moon are impact ages. Aaron is saying to know that, you have to apply strict criteria, and that's not what people have been doing."

Our understanding of Earth's early history is based on the supposed infallibility of zircon dating. Events like the Late Heavy Bombardment (LBH) are linked to the very beginning of life on our planet and incorrect dating can adversely affect our conception of evolution and other sciences.

"The question of what resets the zircon clock has always been very complicated. For a long time people have been saying if zircon is really involved in a major impact shock, its age will be reset, so you can date the impact. Aaron has been saying, 'Yes, sometimes, but often what people see as a reset age may not really be reset.' Zircons are the gift that keep on giving, and this will not change that, but we need to be a lot more careful in analyzing what that gift is telling us."

References:

• "A terrestrial perspective on using ex situ shocked zircons to date lunar impacts" – Aaron J. Cavosie, Timmons M. Erickson, Nicholas E. Timms, Steven M. Reddy, Cristina Talavera, Stephanie D. Montalvo, Maya R. Pincus, Ryan J. Gibbon and Desmond Moser​ – Geology, first published on October 1, 2015 – doi:10.1130/G37059.1
• "Study questions dates for cataclysms on early moon, Earth​" – an article published at UW-Madison on October 16, 2015.

Featured image credit:  Apollo 17/Nicholas E. Timms