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Emplacement age and thermal footprint of the diamondiferous Ellendale E9 lamproite pipe, Western Australia |
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| Authors: | Noreen J Evans Brent I A McInnes Brad McDonald Martin Dani?ík Fred Jourdan Celia Mayers Eric Thern Dudley Corbett |
| Institution: | 1. CSIRO Earth Science and Resource Engineering, 26 Dick Perry Ave, Kensington, WA, 6151, Australia 2. John De Laeter Center for Isotope Research, Department of Applied Geology, Curtin University, Perth, WA, 6945, Australia 3. Department of Earth & Oceanic Sciences, The University of Waikato, Private Bag 3105, Hamilton, New Zealand 4. Department of Imaging and Applied Physics, Curtin University, GPO Box U1987, Perth, WA, 6001, Australia 5. Kimberley Diamond Company, Ellendale Mine, WA, Australia |
| Abstract: | The diamondiferous Ellendale 9 (E9) pipe is a funnel-shaped maar-diatreme volcano consisting of inward-dipping tuff sequences intruded by lamproite plugs and dykes. The host rocks for the E9 pipe are Permian sandstones. The multiple lithological contacts exposed within the mined maar volcano provide a natural laboratory in which to study the effect of volcanic processes on U–Th–Pb–He systematics. Zircon from the regional sandstone and E9 lamproite display a bimodal distribution of ages on (U–Th)/He–U/Pb plots. The zircon U/Pb ages for the E9 pipe (n?=?52) range from 440 to 2,725 Ma, while the cluster of (U–Th)/He ages for the lamproite dyke zircon indicate that dyke emplacement occurred at 20.6?±?2.8 Ma, concordant with a maximum emplacement age of about ≤22 Ma from phlogopite 40Ar/39Ar. These ages indicate a xenocrystic origin for the zircon entrained in the E9 dyke. The U/Pb ages of detrital zircon from the regional sandstone host (373–3,248 Ma; n?=?41) are indistinguishable from those of the lamproite zircon xenocrysts, whereas the detrital zircon in the host sandstone yield (U–Th)/He ages from 260 to 1,500 Ma. A thermochronology traverse across the E9 lamproite dyke reveals that the zircon (U–Th)/He ages in the host sandstone have not been significantly thermally reset during dyke emplacement, even at the contact. The capability of the zircon (U–Th)/He method to distinguish deep, mantle source lithologies from upper crustal source lithologies could be used in geochemical exploration for diamonds. Pre-screening of detrital samples using etching and helium assay methods will improve the efficiency and decrease the cost of greenfields exploration. |
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