Chlorine Isotope Composition of Apatite from the &gt;3.7 Ga Isua Supracrustal Belt, SW Greenland

Alicja Wudarska,Łukasz Birski,Richard Wirth,Izabela Kocjan,Michael Wiedenbeck,Christof Kusebauch,Aivo Lepland,Jens Götze,Ewa Słaby

doi:10.3390/min10010027

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 (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.

Highlights

The Isua supracrustal belt, located in SW Greenland, is among the oldest surviving rock suites (>3.7 Ga [1,2,3]) formed at or near the Earth’s surface
Experimental work by Higashi et al [30] showed that hydrogen diffusion in apatite is much faster than OH diffusion, which in turn is similar to F and Cl diffusion [31]. This suggests that chlorine should be more resistant to exchange processes than hydrogen, leading us to this current study in which we investigated whether the chlorine isotope ratio in apatite reflects the degree of alteration processes and whether the Cl isotope ratio can be used for assessing the origin of the host apatite crystals
The δ37 Cl signature of apatite from banded iron formations (BIFs) AL13 (~0.5%), which is close to the δ37 Cl values determined for the apatite crystals from metacarbonates AL8-1 and AL17 (~0.3%), suggests an extensive overprint of this BIF rock sample and confirms the earlier classification of AL13, which was made based on rare earth elements (REE)

Summary

Introduction

The Isua supracrustal belt, located in SW Greenland, is among the oldest surviving rock suites (>3.7 Ga [1,2,3]) formed at or near the Earth’s surface. There is an ongoing debate on whether carbonaceous matter found in the Isua rocks might represent some of the earliest traces of life. This debate was initiated by Schidlowski et al [4] and followed by Mojzsis et al [5], who described graphite inclusions within apatite crystals from the Isua belt and from the Akilia island that were interpreted as evidence for life prior to 3.8 Ga. Subsequent research has shown inconsistencies with apatite-graphite biogenicity interpretation while providing other lines. The recent study by Dodd et al [17], in which variably

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Conclusion