A microanalytical isotopic study of archean anorthosites

Souders, Amanda Kate (2011) A microanalytical isotopic study of archean anorthosites. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

This thesis focuses on developing and applying in-situ microanalytical methods to study Archean mantle evolution using radiogenic isotope tracers. The timing and mechanism of Archean crust-mantle differentiation is uncertain. Much of our knowledge of the geochemical composition of Archean mantle is through whole rock analysis of mantle-derived magmatic rocks or analysis of minerals separated from these rocks. Pervasive metamorphism and secondary alteration have affected almost all Archean rocks making bulk rock analyses unreliable. Because of this, isotopic analysis of preserved igneous domains of minerals is preferred. -- In this work, isotopic analyses of Lu-Hf in zircon and Pb-Pb in plagioclase are made on coarse-grained, Archean anorthosites. An advantage of these two minerals is both have low parent-daughter ratios and therefore analyses require minimal correction for radiogenic in-growth. LA-MC-ICPMS methods for measuring Lu-Hf in zircon are well established but in-situ measurement of Pb isotopes in Pb-poor minerals required significant analytical development work. Chapters two and three document LA-MC-ICPMS methodology of Pb isotope measurements in Pb-poor silicate glasses, feldspars and sulfides and evaluate any potential biases due to sample matrix. Results demonstrate accurate and precise Pb isotope ratios can be obtained using silicate glass reference materials as calibration standards, despite differences in physical and chemical properties between samples and standards. Average accuracies for Pb-poor feldspars are within 0.40% of the preferred values with external precisions better than 0.60% (RSD, 1σ). Chapter four applies the in-situ Pb isotope method to igneous plagioclase megacrysts from Fiskenæsset and Nunataarsuk anorthosite complexes of south West Greenland. Lead isotope data from each anorthosite complex are used to determine timing of crust extraction from the mantle. This information is coupled with Lu-Hf LA-MC-ICPMS analysis of zircon, from the same rocks, to constrain the ¹⁷⁶Lu/¹⁷⁷ Hf and characterize the composition of the crustal end member. Results show the ∼ 2936 Ma Fiskenæsset and ∼ 2914 Ma Nunataarsuk anorthosite complexes crystallized from mantle-derived melts that had interacted with ancient (Eoarchean to Hadean) mafic crust, which could still be present in south West Greenland today.

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