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CET Quarterly News CET Quarterly News ISSUE 28 JUNE 2014 The ongoing search for Hadean crust on Earth A progress report on collaborative research ongoing between Tony Kemp (UWA ARC Future Fellow), GSWA, Simon Wilde (Curtin University), Vickie Bennett (ANU), Jeff Vervoort (Washington State University) and Martin Whitehouse (Stockholm University). Tony Kemp – The Centre for Exploration Targeting Introduction The familiar concentric ‘layer cake’ structure of the Earth depicted in textbooks formed within a few hundred million years of the accretion of the planet. This is when the metallic core segregated, the oceans condensed, the atmosphere outgassed and the first rocky crust formed. Remarkably, the isotopic legacy of this initial crustal separation still endures in the continents today. Attempts at a more complete understanding of how and exactly when these planetary differentiation processes occurred, are, however, confounded by the apparent absence of a rock record of the earliest part of Earth evolution. This mysterious ‘geological dark age’, extending from the established age of the Earth’s formation at ~4.5 Ga to the oldest preserved rocks known Figure 1: The Pilbara Craton, looking north from near Marble Bar. The ridges comprise at ~4.0 Ga (the Acasta Gneisses in uplifted volcanic successions, and granitic complexes underlie the flat ground. Hadean northwestern Canada), is commonly components may lurk undiscovered in either. referred to as the ‘Hadean’ Eon. A host Recent studies (e.g., Kemp et al. extraordinary preservation (in the case of questions about this critical formative 2010; Tessalina et al. 2010) raise the of the Pilbara) and vast mineral wealth. period remain unanswered. How much exciting possibility that Hadean crust This project is designed to address this crustal material existed on the Hadean lies undiscovered in the Archaean deficiency through piecing together the Earth? What was its composition and Yilgarn and Pilbara Cratons of growth history of the oldest building under what geodynamic conditions Western Australia, in outcrop, and/ blocks to the Australian continent – and did it form? Further progress on these or as part of the cryptic deeper crust. ultimately, it is hoped, testing for the fundamental issues would be greatly Unfortunately, the details of the earliest presence of Hadean crust. expedited by the discovery of actual evolution of these ancient terranes Hadean rocks. remain incomplete, despite their size, continued on page 4 Photo (top): An exploration drill rig on Lake Lefroy, south of Kambalda. This photograph is reproduced with the kind permission of BHP Billiton Mineral Exploration, from the Western Mining Corporation Holdings Limited 1990 Annual Report to Shareholders. CET NEWSLETTER ISSUE 28 JUNE 2014 continued from page 1 Background Figure 2: Narryer Terrane, Yilgarn Craton. Rubbly The ancient cratonic nuclei of Western quartzofeldspathic gneiss Australia (Figure 1) have long been on the slopes of Mount regarded as highly prospective for Murchison occupies the discovery of Hadean rocks. This the foreground. The is largely due to the well-publicised silhouette of Mount Narryer is on the horizon occurrence of ancient water-worn zircon to the north. (ZrSiO4) crystals with ages of up to 4.4 billion years, hosted by much younger sedimentary rocks of the Jack Hills and Mt Narryer supracrustal belts in the northwestern Yilgarn Craton. The Jack Hills and Mt Narryer detrital zircons, as well as zircons from a handful of other localities around the world, are the only can be recognized through field and Preliminary Results surviving relics from the Hadean period structural relationships. The second Initial efforts in the Yilgarn Craton have on Earth. involves interrogating the radiogenic focused in the Narryer Terrane at the isotope memory of the gneisses in these Zircon typically crystallizes from felsic northwestern edge of the craton, areas to deduce and map the age of (granitic) magmas and so these bisected by the sandy course of the the lithosphere from which the original grains may be the ‘smoking guns’ Murchison River (Figure 2). Boasting magmas were derived. This may help demonstrating the occurrence of large a bustling feral population of camels, signpost the most ancient parts of the Hadean continental masses. The actual pigs, donkeys, goats and scruffy cratons. parent rocks from which these long- geologists, this is also home to the oldest orphaned crystals were eroded, however, The analytical program involves U-Pb known rocks in Australia - the ‘Narryer have yet to be discovered; these may geochronology, measurement of isotope Gneisses’, components of which range now no longer exist, or may be located tracers in robust mineral archives to as old as ~3.73 Ga (Kinny et al., thousands of kilometres from the current (oxygen and hafnium isotopes in zircon, 1988). These constitute a critical, but resting place of the zircons. Moreover, neodymium isotopes in titanite) by under-studied piece of the global puzzle much of the geochemical and isotopic both in situ and conventional solution concerning the growth and evolution of information gleaned from these tiny techniques, and measurement of several the earliest continents. detrital mineral repositories is ambiguous different isotope systems from the A large number of samples have been and contestable, and a continental origin corresponding whole rocks. The power collected from several traverses across is far from assured. of long-lived radiogenic isotope systems the terrane and, via temporary residence like 147Sm-143Nd (with a half life of ~106 But the story doesn’t end there. on my office are being processed for billion years) is that they can constrain Tantalising, albeit indirect, support for geochemistry and geochronology. It the time at which crustal rocks separated Hadean crust in Western Australia is is clear from this, and from previous from the mantle. Ultra-high precision also provided by neodymium isotope studies, that the Narryer Terrane has measurements of the much shorter-lived studies of the oldest basalts in the Pilbara enjoyed a long and tumultuous geological 146Sm-142Nd system are also planned. Craton, around 800 km north of the Jack history, recording the imprint of major This decay system is now ‘extinct’, with Hills (Tessalina et al. 2010). These data thermal events at 3.73 Ga, 3.68 Ga, 3.62 naturally occurring 146Sm now absent could be interpreted to indicate eruption Ga, 3.5 Ga, 3.3 Ga (regional granulite from the terrestrial system due to its short of the Pilbara basaltic-komatiitic lavas facies metamorphism) and 2.65 Ga half life – a trifling 103 Ma. Deviations of onto crystalline Hadean basement. Again (regional granulite facies metamorphism sample 142Nd/144Nd ratio from that of the though, tangible pieces of the putative and granite intrusion). The oldest gneissic terrestrial standard due to excesses or >4 Ga-old substrate have not yet been components (> 3.5 Ga) occur as small deficiency of the radiogenic 142Nd can found. (1 cm to 20-30 m), dismembered flakes thus only have arisen prior to 4.2 Ga, Seeking the Ancient Crust strewn throughout a sea of younger before the parent 146Sm had decayed migmatitic gneiss and Late Archean Where to start in the quest to locate the away – and thus belie a Hadean ancestry. granite, itself locally strongly gneissic elusive Hadean crust? Short of an all-out This miniscule but unmistakable signature (Figure 3). brute force effort of trying to date every represents another way to detect the outcrop – which would likely take several participation of Hadean crustal materials Combine this extreme lithological lifetimes of an average Okinawan resident in younger magmatic episodes. The complexity with the discontinuous, and the GDP of a small (and wealthy) project will utilize the world-class mass bull-dust obscured outcrop, and it must country to accomplish – there are two spectrometry facilities established at be conceded that the task of winning approaches being adopted in this project the University of Western Australia (the the Hadean lottery is, to say the least, to identify promising targets. Advanced Geochemical Facility in SEE somewhat ambitious. Sampling to date and the ion microprobes in the Centre has therefore preferentially targeted The first is to narrow the search through for Microscopy, Characterisation, and relatively ‘simple’ rocks, since the original detailed field mapping and targeted Analysis (CMCA)), and at The Australian parentage of these gneisses is the reconnaissance sampling. The hope National University in Canberra. easiest to reconstruct. here is that potentially older rocks 4 ARTICLES Alas, the mooted Hadean crust has not significantly different from that of yet to come to light. On the plus side, chondritic meteorites, our best guess meta-gabbroic rocks with relict cumulate of the bulk Earth composition. These igneous textures have been mapped that data therefore provide no evidence for yield zircon ages clustered around 3.75- the re-melting of older (i.e. > 3.7 Ga) 3.76 Ga. These venerable remnants are crustal sources or for the existence of thus the oldest intact rocks in Australia, strongly depleted mantle from which and amongst the oldest in the world. large volumes of crust may have been Early indications are, however, that extracted, in apparent contradiction of gneisses with igneous precursor ages claims for a Hadean basement. This greater than 3.5 Ga occupy considerably interim conclusion must, however, be less real estate than previously thought. tempered by the fairly small number of samples that have been analysed to The geological framework of the exposed date, which has barely scratched the Pilbara Craton has been well established spinifex-clad surface of this large craton. through concerted mapping and geochronological programs conducted Where to from here…? over several decades by GSWA. The Back to the field areas, and don’t spare currently oldest dated part of the volcanic the horses! A return foray to the Pilbara stratigraphy is ~ 3.52 Ga, although Craton is on the agenda to resample layered gneissic enclaves with zircons some of the granitic complexes in view up to 3.66 Ga old occur in one of the of the initial results.
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