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Open AccessArticle

Chlorine Isotope Composition of Apatite from the >3.7 Ga Isua Supracrustal Belt, SW Greenland

1
Institute of Geological Sciences, Polish Academy of Sciences, Research Centre in Warsaw, Twarda 51/55, 00-818 Warsaw, Poland
2
GFZ German Research Centre for Geosciences, Telegrafenberg, 14473 Potsdam, Germany
3
TU Bergakademie Freiberg, Institute of Mineralogy, Brennhausgasse 14, 09596 Freiberg, Germany
4
Geological Survey of Norway, Leiv Eirikssons vei 39, 7491 Trondheim, Norway
5
Department of Geology, University of Tartu, 50411 Tartu, Estonia
6
Institute of Geology, Tallinn University of Technology, 19086 Tallinn, Estonia
7
Institute of Geological Sciences, Polish Academy of Sciences, Research Centre in Cracow, Senacka 1, 31-002 Cracow, Poland
*
Author to whom correspondence should be addressed.
Minerals 2020, 10(1), 27; https://doi.org/10.3390/min10010027
Received: 20 November 2019 / Revised: 20 December 2019 / Accepted: 23 December 2019 / Published: 28 December 2019
(This article belongs to the Special Issue Feature Papers in Mineral Geochemistry and Geochronology 2019)
The study of the oldest surviving rock suites is crucial for understanding the processes that shaped the early Earth and formed an environment suitable for life. The metasedimentary and metavolcanic rocks of the early Archean Isua supracrustal belt contain abundant apatite, the geochemical signatures of which may help decipher ancient environmental conditions. However, previous research has shown that secondary processes, including amphibolite-facies metamorphism, have reset the original hydrogen isotope composition (δD) of apatite from the Isua belt; therefore, δD values are not indicative of primary conditions in the Archean. Here, we report the first in situ chlorine isotope (δ37Cl) analyses by Secondary Ion Mass Spectrometry (SIMS) from Isua apatite, which we combine with information from transmission electron microscopy, cathodoluminescence imaging, and spectroscopy, documenting the micron-scale internal features of apatite crystals. The determined δ37ClSMOC values (chlorine isotope ratios vs. standard mean ocean chloride) fall within a range from −0.8‰ to 1.6‰, with the most extreme values recorded by two banded iron formation samples. Our results show that δ37Cl values cannot uniquely document primary signatures of apatite crystals, but the results are nonetheless helpful for assessing the extent of secondary overprint. View Full-Text
Keywords: chlorine isotopes; apatite; Isua; Archean; SIMS; TEM; cathodoluminescence; geochemistry chlorine isotopes; apatite; Isua; Archean; SIMS; TEM; cathodoluminescence; geochemistry
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Wudarska, A.; Słaby, E.; Wiedenbeck, M.; Birski, Ł.; Wirth, R.; Götze, J.; Lepland, A.; Kusebauch, C.; Kocjan, I. Chlorine Isotope Composition of Apatite from the >3.7 Ga Isua Supracrustal Belt, SW Greenland. Minerals 2020, 10, 27.

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