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

Abstract

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 (δ 37 Cl) 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 δ 37 Cl SMOC 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 δ 37 Cl values cannot uniquely document primary signatures of apatite crystals, but the results are nonetheless helpful for assessing the extent of secondary overprint.Minerals 2020, 10, 27 2 of 25 of evidence, such as putative microbial fabrics and carbon isotope signatures, arguing for traces of biological activity in the Isua rocks [6][7][8][9][10][11][12][13][14][15][16]. The recent study by Dodd et al. [17], in which variably 13 C-depleted graphitic carbon was identified in world-wide occurring banded iron formations, suggests Minerals 2020, 10, 27 21 of 25 materials by GS-IRMS. We would like to thank the editorial team and the reviewers for their suggestions and comments, which helped us to improve the quality of the manuscript. Conflicts of Interest:The authors declare no conflict of interest.

Inter‐laboratory Characterisation of Apatite Reference Materials for Chlorine Isotope Analysis

“…Application of such an approach improved quality and validated the SIMS data in the previous study by Wudarska et al . (2020), in which the data reduction was conducted using four RMs.…”

“…2015a or from −0.8 to +1.6, Wudarska et al . 2020) compared with those recorded in the extra‐terrestrial materials.…”