| 1. |
- Brengman, L., et al.
(författare)
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Micro-scale silicon isotope heterogeneity observed in hydrothermal quartz precipitates from the >3.7 Ga Isua Greenstone Belt, SW Greenland.
- 2016
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Ingår i: Terra Nova. - : Wiley. - 0954-4879 .- 1365-3121. ; 28, s. 70-75
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Tidskriftsartikel (refereegranskat)abstract
- Pillow basalt and chert form integral lithologies comprising many Archean greenstone belt packages. To investigate details of these lithologies in the >3.7 Ga Isua Greenstone Belt, SW Greenland, we measured silicon isotope compositions of quartz crystals, by secondary ion mass spectrometry, from a quartz-cemented, quartz-amygdaloidal basaltic pillow breccia, recrystallized chert and chert clasts thought to represent silica precipitation under hydrothermal conditions. The recrystallized chert, chert clasts and quartz cement have overlapping δ30Si values, while the δ30Si values of the quartz amygdules span nearly the entire range of previously published values for quartz precipitates of any age, despite amphibolite facies metamorphism. We suggest that the heterogeneity is derived from kinetic isotope fractionation during quartz precipitation under disequilibrium conditions in a hydrothermal setting, consistent with the pillow breccia origin. On the basis of the present data, we conclude that the geological context of each sample must be carefully evaluated when interpreting δ30Si values of quartz.
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| 2. |
- Whitehouse, Martin J., 1962-
(författare)
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Does a Heavy Fe-Isotope Composition of Akilia Quartz-Amphibole-Pyroxene Rocks Necessitate a BIF Origin?
- 2015
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Ingår i: Astrobiology. - 1531-1074 .- 1557-8070. ; 15, s. 816-824
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Tidskriftsartikel (refereegranskat)abstract
- The age and origin of the quartz-amphibole-pyroxene (qap) gneiss from the island of Akilia, southern West Greenland, has been the subject of intense debate since the light C-isotope composition of graphite inclusions in apatite was interpreted to indicate the presence of Earth’s earliest biological activity. Although this claim for biogenic relicts has been vigourously challenged, the possibility that the rocks might represent some of the Earth’s earliest water lain sediments and, hence, a suitable repository for life, remains an open question. While some workers have suggested that the entire sequence represents an originally mafic-ultramafic igneous precursor subsequently modified by metasomatism, quartz injection, high-grade metamorphism and extreme ductile deformation, others maintain that at least a small part of the sequence retains geochemical characteristics indicative of a chemical sedimentary origin. Fractionated Fe isotopes with d56Fe values similar to those observed in Isua BIF reported from high-SiO2 units of qap have been used to support a chemical sedimentary protolith for the qap unit. Here we present new Fe isotope data from all lithological variants in the quartz-amphibole-pyroxene gneiss on Akilia, including layers of undisputed ultramafic igneous origin. Since the latter require introdution of fractionated Fe into at least part of the qap unit, we argue that Fe isotopes must therefore be treated with considerable caution when used to infer BIF for part or all of the qap protolith.
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