An in situ Technique for U-Th-Pb-He Chronometry with Applications to Diamond Exploration

Noreen Evans1, Brent McInnes2, Brad McDonald1, Martin Danišík3, Michael Shelley4 and Pieter Vermeesch5, (1)Curtin University, Applied Geology, Perth, Australia, (2)Curtin University, Applied Physics, Perth, Australia, (3)Curtin University, John de Laeter Centre, Perth, Australia, (4)Laurin Technic Pty. Ltd., Canberra, Australia, (5)University College London, London, United Kingdom

Contact First Author: Noreen Evans; noreen.evans@curtin.edu.au

Previously Published Material: Submitted for publication in Geochimica et Cosmochimica Acta in December 2014. Under review.

Abstract ID#: 34741

 

English Abstract:
Zircon U-Pb and (U-Th-Sm)/He ‘double dating’ can distinguish deep mantle source lithologies from upper crustal source lithologies and can be used in geochemical exploration for diamonds (Evans et al., 2013). However, improved data production efficiency was required before it could be considered a practicable tool for industry. We present a new laser-based technique for rapid, quantitative and automated in situ microanalysis of U, Th, Sm, Pb and He for applications in geochronology, thermochronometry and geochemistry. This novel capability permits a detailed interrogation of the time-temperature history of rocks containing apatite, zircon and other accessory phases by providing both (U-Th-Sm)/He and U-Pb ages (+trace element analysis) on single crystals. In situlaser microanalysis offers several advantages over conventional bulk crystal methods in terms of safety, cost, productivity and spatial resolution.

We developed and integrated a suite of analytical instruments including a 193 nm ArF excimer laser system (RESOlution M-50A-LR), a quadrupole ICP-MS (Agilent 7700s), a quadrupole helium mass spectrometry system (ALPHAchron) and swappable flow-through and ultra-high vacuum analytical chambers. The analytical protocols for zircon require the following steps: mounting/polishing in PFA Teflon using methods similar to those adopted for fission track etching (e.g., Zaun and Wagner, 1985); laser He extraction and analysis using a 2 s ablation at 5 Hz and 2-3 J/cm2fluence; He pit volume measurement using confocal laser scanning microscopy or atomic force microscopy, and U-Th-Sm-Pb (plus optional trace element) analysis using traditional laser ablation methods. A freeware application has been developed for determining (U-Th-Sm)/He ages from the raw analytical data and Iolite software was used for U-Pb age and trace element determination.

In situdouble dating has successfully replicated conventional U-Pb and (U-Th)/He age variations in xenocrystic zircon from the diamondiferous Ellendale lamproite pipe, Western Australia and increased zircon analytical throughput by a factor of 50 over conventional methods.

References:

Zaun, P. and Wagner, G.A. 1985. Nucl. Tracks Radiat. Meas.10, 303-307

Evans, N.J., et al., 2013. Mineralium Deposita, 48, 413-421.

Back to: Origin of Cratonic Mantle Lithosphere, Diamonds, and Deeply Sourced Volatile-Rich Melts: Processes and Timescales II Posters