Australia: The Land Where Time Began

A biography of the Australian continent 

Jack Hills Detrital Zircons Internal Zoning & U-Th-Pb Chemistry – Mineral Record of Magmatism in the Early Archaean to Mesoproterozoic 4,348-1,576 Ma

According to Cavosie et al. for the first 500 million years following the accretion of the Earth magmatic process were important on the nascent Earth, but the causes and timing of this magmatism has remained largely unconstrained, with no surviving rocks from this period in Earth history having been found. The only direct geological evidence of this early magmatism are rare detrital zircons that have been preserved from >4,000 Ma in Western Australia. In this paper Cavosie et al. present the results of a study involving ion and electron microprobe and SEM of the age, Th-U chemistry, cathodoluminescence (CL) zoning patterns, and inclusions for a population of detrital zircons obtained from Jack Hills, Western Australia, with ²⁰⁷Pb/²⁰⁶Pb ages that ranged from 4,348-1,576 Ma. Primary growth features that are preserved in the majority of zircons that are discernible by CL imaging, such as oscillatory and sector zoning, have Th/U ratios that range from 0.1 to 1.0, with several containing granitic mineral inclusions. They are, therefore, apart from their age they are largely indistinguishable from zircons that are produced in common felsic magmas. Therefore the zircons from Jack Hills are remnants of igneous events that formed rocks predating the rock record by up to 400 million years. It is suggested by the ²⁰⁷Pb/²⁰⁶Pb age patterns of distribution for zircons older than 3,800 Ma from Western Australia that during the Archaean magmatism was punctuated in terms of high-frequency events as well as conspicuous gaps. In the Jack Hills the variable age distributions within different rock units demonstrate that zircons from the Early Archaean were derived from Multiple source rocks. Also, rims of younger than 3,700-3,400 Ma on zircons of 4,300-4,000 Ma age crust are evidence that survived long enough to take part in younger tectonic events in the Yilgarn Craton, western Australia. A quartzite 50 m from Eranondoo Hill contained igneous zircons that have been attributed to either sedimentation or tectonic interleaving of younger sediments at 1,576 Ma at most. The age of the Jack Hills sediments is called into to question by this Proterozoic (or younger) geologic history that was previously unrecognised and demonstrates the heterogeneous distribution of grains within this belt of >4,000 Ma.  

Sources & Further reading

1. Cavosie, A. J., S. A. Wilde, D. Liu, P. W. Weiblen and J. W. Valley (2004). "Internal zoning and U–Th–Pb chemistry of Jack Hills detrital zircons: a mineral record of early Archaean to Mesoproterozoic (4348–1576 Ma) magmatism." Precambrian Research 135(4): 251-279.