Thallium isotopes as a potential tracer for the origin of cratonic eclogites

Nielsen, Sune G. and Williams, Helen M. and Griffin, William L. and O'Reilly, Suzanne Y. and Pearson, Norman and Viljoen, Fanus (2009) Thallium isotopes as a potential tracer for the origin of cratonic eclogites. Geochimica Et Cosmochimica Acta, 73 (24). pp. 7387-7398. ISSN 0016-7037 DOI 10.1016/j.gca.2009.09.001

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Abstract

Cratonic eclogites are inferred to originate either from subducted ocean crust or mantle melts accreted onto the roots of continents. These models have different implications for the growth of continents, but it is currently difficult to determine the origin of individual eclogite suites. Upper ocean crust altered at low temperatures and marine sediments both display high thallium (Tl) concentrations and strongly fractionated Tl isotope signatures relative to the ambient upper mantle. In this study we carry out the first examination of the suitability of Tl isotopes as a tracer for an ocean-crust origin of cratonic eclogites. We have analysed the Tl isotope composition of clinopyroxene and garnet in six eclogites from the Kaalvallei and Bellsbank kimberlite pipes in South Africa. Minerals were pre-cleaned with an HCl leaching technique and the leachates display variably light Tl isotope ratios. These most likely reflect low-temperature hydrothermal alteration occurring after eruption of the kimberlite that carried the eclogites to the surface. The leached mineral pairs all display identical Tl isotope ratios, strongly suggesting that the source of the analysed Tl is identical for each mineral pair. It is, however, not possible to exclude the possibility that the analysed Tl originates from kimberlitic material that was not removed by the cleaning procedure. Only one of the six samples exhibits a Tl isotope composition different from ambient mantle. Assuming that the Tl isotope signatures indeed represent the eclogite minerals and not any form of contamination, the Tl isotope composition in this sample is consistent with containing a minor component (<3) of ocean crust altered at low temperatures. Thallium isotopes may become one of the most sensitive indicators for the presence of low-T altered ocean crust because of the stark contrast in Tl concentration and isotopic composition between the mantle and altered ocean crust. In fact, no other chemical or isotopic tracer could have provided an indication that any of the samples studied here had a subduction origin. However, much work is still required before it becomes clear if Tl isotope measurements are a viable means to establish the origin of cratonic eclogites. © 2009 Elsevier Ltd. All rights reserved.

Item Type: Article
Uncontrolled Keywords: NILAREP
Subjects: 05 - Petrology - Igneous, Metamorphic and Volcanic Studies
Divisions: 05 - Petrology - Igneous, Metamorphic and Volcanic Studies
Journal or Publication Title: Geochimica Et Cosmochimica Acta
Volume: 73
Page Range: pp. 7387-7398
Identification Number: 10.1016/j.gca.2009.09.001
Depositing User: Sarah Humbert
Date Deposited: 20 Jul 2016 14:10
Last Modified: 20 Jul 2016 14:10
URI: http://eprints.esc.cam.ac.uk/id/eprint/3695

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