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Archean crustal evolution constrained by strontium isotopes in apatite and uranium-lead geochronology and trace element geochemistry of zircon

University of British Columbia

Archean continents were the nuclei for crustal growth and large volumes of continental crust appear to have been produced during the Archean. Much of the preserved Archean crust is of tonalite-trondhjemite-granodiorite (TTG) composition and it is argued that this made up the bulk of Earth’s earliest crust. Other models involve a bulk mafic crust that was very different to the modern crust. New data are therefore needed to test and refine these models and determine how continents were first formed. The Rb-Sr isotopic system provides a potentially powerful proxy for crustal composition yet it has thus far been underutilized in studies on early crustal evolution due to its susceptibility to re-equilibration. Overcoming this issue requires new analytical approaches to micro-sample ancient Sr-rich minerals, such as apatite, that may retain primary 87Sr/86Sr signatures. In this thesis study, a novel method in laser-ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICPMS) was applied to apatite from TTG complexes of different Archean age. The first area of focus was the Acasta Gneiss Complex, Northwest Territories, which contains the oldest known terrestrial rocks. Apatite inclusions within ca. 3.7 Ga zircon host grains were subjected to Sr isotope analysis by LA-MC-ICPMS. The initial 87Sr/86Sr values of these inclusions are identical within error and are different from values obtained from altered matrix apatite. Combining the 87Sr/86Sr results with information on the protolith and source-extraction age yields estimates for the range of source Rb/Sr and suggests that an evolved Hadean source was involved in the formation of the Acasta Gneiss Complex. The Sr isotope LA-MC-ICPMS method was also applied to matrix apatite from TTG of the ca. 3.6 Ga Bastar Craton, India, and the 3.0-2.8 Ga Kvanefjord Block, Greenland. The radiogenic 87Sr/86Sr signatures from these apatite grains also require a high Rb/Sr crustal source. This suggests that enriched crustal vestiges played a role in the formation of TTG crust.

Text

2018-05-01

Attribution-NonCommercial-NoDerivatives 4.0 International

http://hdl.handle.net/2429/65728

Geological Sciences

University of British Columbia

UBCV

http://creativecommons.org/licenses/by-nc-nd/4.0/