Friday, 23 October 2015

Reevaluating The Zircon Clock

A zircon crystal showing concentric growth rings
Zircons are loved by geochemists. They are resilient to most forms of geochemical and geological wear and tear, and as they grow, they can incorporate atoms and molecules from their surroundings into their chemical structure.

As a result they can be radiometrically dated and chemically and isotopically analysed to give insight into the environmental conditions in which they formed. Some of the oldest zircons have survived lunar cataclysms, giant impacts and the deaths of continents to yield valuable data. Yet a recent study has shown that we should take our zircons with a grain of salt.

'While zircon is one of the best isotopic clocks for dating many geological processes,' said Aaron Cavosie from the NASA Astrobiology Institute at the University of Wisconsin-Madison. 'Our results show that it is very challenging to use ex situ zircon to date a large impact of known age. Once separated from host rocks, ex situ shocked zircons lose critical contextual information.'

One of the zircons used in the study. The thick sub-vertical
lines were produced by an impact event 2 billion years ago 
Zircons have been used to date giant impact events. The logic being that the giant impact events drive off the lead decay products of uranium contained within zircons, resetting their radiometric clocks to the date of the impact event.

To test this assumption the researchers examined zircons which came from the Earth's largest impact crater, the 2 billion year old Vredefort structure in South Africa. Many zircons preserve evidence of having survived impact events. The Vredefort zircons have such traces, but their ages showed that their radiometric clock had not been reset during the event. Instead the dates were indicative of when they crystallised from magma.

'The question of what resets the zircon clock has always been very complicated' said John Valley, also from the University of Wisconsin-Madison. ' Zircons are the gift that keeps 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.'