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Journal of Petrology Advance Access published online on June 2, 2006
Journal of Petrology, doi:10.1093/petrology/egl026
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© The Author 2006. Published by Oxford University Press. All rights reserved. For Permissions, please e-mail: journals.permissions@oxfordjournals.org
Received September 27, 2004
Accepted April 3, 2006

Article

Magmatic Evolution and Ascent History of the Aries Micaceous Kimberlite, Central Kimberley Basin, Western Australia: Evidence from Zoned Phlogopite Phenocrysts, and UV Laser 40Ar/39Ar Analysis of Phlogopite-Biotite

PETER J. DOWNES 1 *, JO-ANNE WARTHO 2, and BRENDAN J. GRIFFIN 3

1 WESTERN AUSTRALIAN MUSEUM, FRANCIS ST, PERTH, WESTERN AUSTRALIA, 6000, AUSTRALIA
2 JOHN DE LAETER CENTRE OF MASS SPECTROMETRY, DEPARTMENT OF APPLIED GEOLOGY, CURTIN UNIVERSITY OF TECHNOLOGY, HAYMAN ROAD, BENTLEY, WESTERN AUSTRALIA, 6102, AUSTRALIA
3 CENTRE FOR MICROSCOPY AND MICROANALYSIS, UNIVERSITY OF WESTERN AUSTRALIA, CRAWLEY, WESTERN AUSTRALIA, 6009, AUSTRALIA

* To whom correspondence should be addressed.
PETER J. DOWNES, E-mail: peter.downes{at}museum.wa.gov.au


  Abstract

The Neoproterozoic Aries kimberlite was emplaced in the central Kimberley Basin, Western Australia, as a N-NNE-trending series of three diatremes infilled by lithic-rich kimberlite breccias. The breccias are intruded by hypabyssal macrocrystic phlogopite kimberlite dykes that exhibit differentiation to a minor, high-Na-Si, olivine-phlogopite-richterite kimberlite, and late-stage macrocrystic serpentine-diopside ultramafic dykes. Mineralogical and geochemical evidence suggests that the high-Na-Si, olivine-phlogopite-richterite kimberlite was derived from the macrocrystic phlogopite kimberlite as a residual liquid following extended phlogopite crystallization and the assimilation of country rock sandstone, and that the macrocrystic serpentine-diopside ultramafic dykes formed as mafic cumulates from a macrocrystic phlogopite kimberlite. Chemical zonation of phlogopite-biotite phenocrysts indicates a complex magmatic history for the Aries kimberlite, with the early inheritance of a range of high-Ti phlogopite-biotite xenocrysts from metasomatized mantle lithologies, followed by the crystallization of a population of high-Cr phlogopite phenocrysts within the spinel facies lithospheric mantle. A further one to two phlogopite-biotite overgrowth rims of distinct composition formed on the phlogopite phenocrysts at higher levels during ascent to the surface. Ultra-violet laser 40Ar/39Ar dating of mica grain rims yielded a kimberlite eruption age of 815·4 ± 4·3 Ma (95% confidence). 40Ar/39Ar laser profiling of one high-Ti phlogopite-biotite macrocryst revealed a radiogenic 40Ar diffusive loss profile, from which a kimberlite magma ascent duration from the spinel facies lithospheric mantle was estimated (assuming an average kimberlite magma temperature of 1000°C), yielding a value of ~0·23-2·32 days for the north extension lobe of the Aries kimberlite.

Keywords: 40Ar/39Ar; diamond; kimberlite; mantle metasomatism; phlogopite-biotite.
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