Dalton, Hayden1, Giuliani, Andrea 1,2, Hergt, Janet1, Maas, Roland1, Matchan, Erin1, O’Brien, Hugh3, Phillips, Professor David1, Woodhead, Jon1
1School of Earth Sciences, The University of Melbourne, Parkville, Australia, 2Institute of Geochemistry and Petrology, Department of Earth Sciences, Zurich, Switzerland, 3Geological Survey of Finland, Espoo, Finland
Kimberlites are enigmatic, volcanic rocks of both great economic and scientific importance due to acting as the primary host-rock to diamonds and being the deepest-derived continental magmas on Earth. Despite this significance, there remains debate concerning the sources of kimberlites and what triggers mantle melting to form these rocks. Robust determination of the timing of kimberlite eruption is a crucial prerequisite if we are to unravel the presence of any spatiotemporal relationships between kimberlite emplacement and large-scale tectonic processes, super-continental cycles or mantle plumes.
Despite the benefit of the remarkably high precision achieved with modern 40Ar/39Ar analytical techniques, and the presence of K-bearing groundmass phlogopite in many kimberlites, this technique has seldom been applied to kimberlites and related rocks. Early utilisation of this method revealed issues related to the presence of extraneous argon in mica macrocrysts and phenocrysts which yields anomalously old or maximum emplacement ages. Nonetheless, apparently reliable age results have been obtained on magmatic mica from kimberlites and related rocks. In this study we compare new, precise 40Ar/39Ar ages with other independent age constraints (e.g., Rb/Sr, U/Pb) on three clusters of kimberlites and related rocks from Finland to rigorously assess the instances where 40Ar/39Ar dating produces older apparent ages.
Our results indicate that sample selection and groundmass mica (phlogopite or kinoshitalite) separation needs to be extremely judicious prior to analysis. Where fresh mica phenocrysts are available for 40Ar/39Ar analyses we recommend that plateau results are interpreted with caution. Age spectra which are entirely flat, such that an age plateau includes 100% of the gas are likely the most accurate and precise reflection of the emplacement age of a kimberlite. In contrast, aliquots that yield younger apparent ages for heating steps preceding the plateau may reflect argon recoil redistribution resulting in anomalously older high-temperature/plateau ages when compared with independent age constraints. In cases where such discordance exists, we recommend that total-gas ages give a better approximation of the emplacement age and one which agrees more closely with ages from other geochronometers.
Hayden Dalton is presenting on behalf of the greater AuScope Geochemistry Laboratory Network. Hayden is a PhD researcher in the School of Earth Sciences at the University of Melbourne, his research focuses on the geochronology and geochemistry of kimberlites.