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Solid source lead isotope studies with application to rock samples from the Superior geological province Ozard, John Malcolm

Abstract

Data of good quality is necessary to further the study of rock lead isotopic interpretations. An internal standard (double-spike) was employed to correct for fractionation accompanying the single filament analyses, and lead isotope ratios with a standard deviation of 0.15% were obtained. Lead sulphide mounted on tantalum was employed for the lead analyses. Discrimination in the analyses using tantalum filaments was consistent with the theoretical fractionation laws. This was not found to be the case for rhenium. Lead and uranium analyses from the Vogt-Hobbs area near Lake Timagami, Ontario and from the Rice Lake-Beresford Lake Area, Manitoba revealed variations from a simple two stage model that could not be accounted for by experimental error. Interpretation of the data yields a three stage history for both of these regions, marked by major events at approximately 3400 and 2600 my ago. This earlier crustal history is supported by ore lead data from the vicinity. Both of the suites have a lower average value of (U238/Pb204) for the second stage, of their three stage history, than is characteristic of the source of single stage leads. Variations from closed system behaviour for the Ontario samples is attributed to lead remobilization at the time of the Grenville event. Remobilization of the lead tends to mask the 2600 my old event. The Vogt-Hobbs area is characterized by more acidic igneous rocks and more radiogenic lead than the Rice Lake-Beresford Lake area. These differences are thought to represent the environment 2600 my ago or earlier. The Manitouwadge lead analysed by Ostic resembles the lead - 2600 my ago in the Rice Lake-Beresford Lake rock samples. The Rice Lake-Beresford Lake rock samples are shown according to the interpretation presented, to have had a crustal history prior to 2600 my ago. This lends weight to arguments that exclude Manitouwadge from the class of single stage leads used in calculations of the age of the earth.

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