Pb isotopic compositions of Archean komatiites and sulfides

Abstract

Pb isotopic compositions were measured in komatiites and other volcanic rocks from several Archean greenstone belts, and in magmatic and exhalative sulfides associated with these volcanics. Thin komatiite flows generally yielded PbPb isochron ages consistent with other geochronological or geological information, but thicker basaltic flows gave ages that are too young. These results suggest that the thin komatiite flows were hydrothermally altered during eruption and that this event is recorded by the PbPb age. Thicker basaltic flows resisted alteration during eruption but were affected by later events: their PbPb ages probably correspond to the time of metamorphism, and not to the time of emplacement. Magmatic and exhalative sulfides from the Abitibi Belt in Canada fall on the same trend in the 207Pb/ 204Pb 206Pb/ 204Pb diagram as komatiites, basalts, sedimentary rocks and K-feldspars from granites. The calculated μ 1-value is 7.8±0.1. A similar suite of rocks and minerals from the Kambalda area in Australia has distinctly higher 207 Pb 204 Pb and a higher μ 1 of 8.3. Nd isotopic data demonstrate that some Kambalda rocks are highly contaminated with older continental crust. The differences in initial Pb isotopic compositions are attributed to the presence or absence, and the age, of underlying granitoid basement. Kambalda greenstones are underlain and contaminated by old (> 3.0 Ga) granitoid crust but significantly older granitoid basement appears absent from the Abitibi Belt. The Pb isotopic compositions of Kambalda greenstones are dominated by the crustal component and do not represent the composition of the mantle source of these rocks. The Abitibi greenstones are not highly influenced by crustal contamination and their Pb isotopic compositions may reflect those of Archean mantle.