Löcht, Julia van de
(2019).
Geochemistry and petrology of ~ 3.8 Ga mafic - ultramafic enclaves in the Itsaq Gneiss Complex, SW Greenland.
PhD thesis, Universität zu Köln.
Abstract
The 3.6-3.9 Ga Itsaq Gneiss Complex (IGC) of southern West Greenland hosts the most promising localities of potential well-preserved Eoarchean mantle remnants. In order to gain new insights into early Earth´s mantle evolution and mantle-crust interaction, a new, carefully collected sample set including mafic and ultramafic rocks from three of such localities were investigated. This study combines new petrological observations, comprehensive major and trace element data for mafic and ultramafic rocks as well as Lu-Hf, Sm-Nd and Rb-Sr isotope compositions for the ultramafic rocks. Osmium, Ir, Ru, Pt and Pd concentrations, and Re-Os isotope signatures of some peridotites were obtained. Moreover, Lu, Hf, Sm, Nd, Rb and Sr concentrations and isotope data for olivine, orthopyroxene and amphibole separates of selected peridotites.
This study provides evidences that remnants of > 3.8 Ga mantle peridotites occur in the IGC.
The Os, Ir, and Ru abundance of the investigated Eoarchean mantle peridotites are similar to present-day mantle rocks. Hence, the abundances of Os, Ir, and Ru combined with Os isotope compositions of the mantle peridotites indicate that primitive late accreted material appears to have been efficiently mixed into the sampled mantle domains by Eoarchean time.
Importantly, the Lu-Hf isotope inventory as well as most major elements, MREE, HREE, and HFSE were only slightly disturbed during metamorphic overprint. Lutetium-Hf whole rock age regression lines mainly yield Eoarchean ages of ca. 3.9 to 3.8 Ga, in good agreement with minimum ages inferred from field relationships (≥ 3.8 Ga). Yet, the Sm-Nd and especially the Rb-Sr isotope patterns exhibit larger scatter, indicating subsequent disturbance of the Sm-Nd and Rb-Sr isotope systems by late post-emplacement tectonothermal events.
The Major element compositions and Th-REE-HFSE characteristics obtained for the mantle peridotites indicate re-enrichment of initially depleted peridotite by melt-like subduction components. The Amphibolites also show geochemical characteristics similar to modern subduction suite related mafic rocks.
Thus, our findings, including geochemical data for mafic and ultramafic rocks, and simple evolution-models, support that subduction-like processes were in operation since at least the Eoarchean.
The 3.6-3.9 Ga Itsaq Gneiss Complex (IGC) of southern West Greenland hosts the most promising localities of potential well-preserved Eoarchean mantle remnants. In order to gain new insights into early Earth´s mantle evolution and mantle-crust interaction, a new, carefully collected sample set including mafic and ultramafic rocks from three of such localities were investigated. This study combines new petrological observations, comprehensive major and trace element data for mafic and ultramafic rocks as well as Lu-Hf, Sm-Nd and Rb-Sr isotope compositions for the ultramafic rocks. Osmium, Ir, Ru, Pt and Pd concentrations, and Re-Os isotope signatures of some peridotites were obtained. Moreover, Lu, Hf, Sm, Nd, Rb and Sr concentrations and isotope data for olivine, orthopyroxene and amphibole separates of selected peridotites.
This study provides evidences that remnants of > 3.8 Ga mantle peridotites occur in the IGC.
The Os, Ir, and Ru abundance of the investigated Eoarchean mantle peridotites are similar to present-day mantle rocks. Hence, the abundances of Os, Ir, and Ru combined with Os isotope compositions of the mantle peridotites indicate that primitive late accreted material appears to have been efficiently mixed into the sampled mantle domains by Eoarchean time.
Importantly, the Lu-Hf isotope inventory as well as most major elements, MREE, HREE, and HFSE were only slightly disturbed during metamorphic overprint. Lutetium-Hf whole rock age regression lines mainly yield Eoarchean ages of ca. 3.9 to 3.8 Ga, in good agreement with minimum ages inferred from field relationships (≥ 3.8 Ga). Yet, the Sm-Nd and especially the Rb-Sr isotope patterns exhibit larger scatter, indicating subsequent disturbance of the Sm-Nd and Rb-Sr isotope systems by late post-emplacement tectonothermal events.
The Major element compositions and Th-REE-HFSE characteristics obtained for the mantle peridotites indicate re-enrichment of initially depleted peridotite by melt-like subduction components. The Amphibolites also show geochemical characteristics similar to modern subduction suite related mafic rocks.
Thus, our findings, including geochemical data for mafic and ultramafic rocks, and simple evolution-models, support that subduction-like processes were in operation since at least the Eoarchean.
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