Zirconian

The Zirconian is the second Era within the Hadean Eon in a proposed revision of the Precambrian time scale.[1] It lasted 373 million years from the end of the Chaotian Era 4,404 million years ago to the beginning of Eoarchean Era 4,031 million years ago. The Zirconian follows the Chaotian Era and its beginning is chronometrically set at 4.404 ± 0.008 Gya. This corresponds to the age of the first occurrence of Hadean zircons in the Jack Hills in Western Australia (Yilgarn craton). The end of the Zirconian Era and the transition to the Acastan Period (the earliest period of the Archean Eon and Eoarchean Era) occurred with the appearance of the oldest rock at 4.031 ± 0.003 Gya.

Zirconian Era 4404–4031 million years ago

Geologic time scale -4500 — – -4000 — – -3500 — – -3000 — – -2500 — – -2000 — – -1500 — – -1000 — – -500 — – 0 — Hadean Archean Proterozoic Phanerozoic Eo Paleo Meso Neo Paleo Meso Neo Paleo Meso Ceno     Scale: millions of years

Etymology

Zircon crystal with magmatic Anwachssäumen under the light microscope

The term Zirconian is derived from the mineral zircon, since it is the only substance that is preserved from this early period. An alternative proposal for the name of this era is "Jack Hillsian Era".

Meaning

The first 196 million years after the formation of Earth, left no solid trace, except for the formation of the Moon. This time is termed the Chaotian Era. The first datable solid Earth minerals, zircons, mark the start of the Zirconian Era. The oldest known crystal could be dated to 4,404 ± 8 Mya.[2] It was in metasedimentary rock on Erawandoo Hills in the Jack Hills of Narryer Gneiss Terrane in Western Australia. The 4.4 Gya is the limiting age, and zircons this old are very rare. But from 4.2 to 4.1 Gya they are already common. The bulk of the zircons in Narryer Gneiss Terrane date much later, from 3.75 to 3.5 Gya.

Occurrence

The zircons are limited only to Archean cratons. In the Yilgarn craton quartzites of Southern Cross Terrane also contains zircons with an age of 4.35-3.13 Gya).[3]

Possibly the Nuvvuagittuq Greenstone Belt of the Superior Craton is of Zirconian age, its proposed age of 4.28 Gya is however still controversial. The end of the Zirconian Era is at 4.031 Gya, marked by the Acasta Gneiss of Slave craton and it is likely that its protolith is older. In 2006, a single zircon crystal in Acasta Gneiss was dated to 4.2 Gya[4]

References

1. Gradstein, Felix M.; Ogg, James G.; Hilgen, Frits J. (1 March 2012). "On The Geologic Time Scale". Newsletters on Stratigraphy. Schweizerbart. 45 (2): 171–188. doi:10.1127/0078-0421/2012/0020. ISSN 0078-0421.
2. Wilde, SA; et al. (2001). "Evidence from detrital zircons for the existence of continental crust and oceans on the Earth 4.4 Gya ago". Nature. 409 (6817): 175–178.
3. Wyche, S.; et al. (2004). "4350-3130 Ma detrital zircons in the Southern Cross Granite-Greenstone Terrane, Western Australia: implications for the early evolution of the Yilgarn Craton". Australian Journal of Earth Sciences. Wiley. 51 (1): 31–45. doi:10.1046/j.1400-0952.2003.01042.x. ISSN 0812-0099.
4. Iizuka, Tsuyoshi; et al. (2006). "4.2 Ga zircon xenocryst in an Acasta gneiss from northwestern Canada: Evidence for early continental crust". Geology. Geological Society of America. 34 (4): 245. doi:10.1130/g22124.1. ISSN 0091-7613.