Multiple Sulfur Isotope Records of the 3.22 Ga Moodies Group, Barberton Greenstone Belt

Johanna Marin-Carbonne,Nicolas Olivier,Masafumi Saitoh,Yuichiro Ueno,Sami Nabhan,Christophe Thomazo,Jean-François Moyen

doi:10.3390/geosciences10040145

Abstract

The Moodies Group, the uppermost unit in the Barberton Greenstone Belt (BGB) in South Africa, is a ~3.7-km-thick coarse clastic succession accumulated on terrestrial-to-shallow marine settings at around 3.22 Ga. The multiple sulfur isotopic composition of pyrite of Moodies intervals was newly obtained to examine the influence of these depositional settings on the sulfur isotope record. Conglomerate and sandstone rocks were collected from three synclines north of the Inyoka Fault of the central BGB, namely, the Eureka, Dycedale, and Saddleback synclines. The sulfur isotopic composition of pyrite was analyzed by Secondary Ion Mass Spectrometry (SIMS) for 6 samples from the three synclines and by Isotope Ratio Mass Spectrometry (IR-MS) for 17 samples from a stratigraphic section in the Saddleback Syncline. The present results show a signal of mass-independent fractionation of sulfur isotopes (S-MIF), although t-tests statistically demonstrated that the Moodies S-MIF signals (mostly 0‰ < ∆33S < +0.5‰) are significantly small compared to the signal of the older Paleoarchean (3.6–3.2 Ga) records. These peculiar signatures might be related to initial deposition of detrital pyrite of juvenile origin from the surrounding intrusive (tonalite–trondhjemite–granodiorite; TTG) and felsic volcanic rocks, and/or to secondary addition of hydrothermal sulfur during late metasomatism. Moreover, fast accumulation (~0.1–1 mm/year) of the Moodies sediments might have led to a reduced accumulation of sulfur derived from an atmospheric source during their deposition. As a result, the sulfur isotopic composition of the sediments may have become susceptible to the secondary addition of metasomatic sulfur on a mass balance point of view. The sulfur isotopic composition of Moodies pyrite is similar to the composition of sulfides from nearby gold mines. It suggests that, after the Moodies deposition, metasomatic pyrite formation commonly occurred north of the Inyoka Fault in the central BGB at 3.1–3.0 Ga.

Highlights

The 3.22 Ga, ~3.7-km-thick Moodies Group forms the stratigraphically uppermost part of the Barberton Greenstone Belt (BGB) in South Africa (e.g., [1])
The sulfur isotopic composition of 17 samples was measured in bulk, and the composition of 48 pyrite grains in six samples from three synclines were analyzed by Secondary Ion Mass Spectrometry (SIMS)
6.1, we argue that metasomatic sulfur in a fluid was a major sulfur source of the Moodies detrital pyrite as discussed in Section 6.1, we argue that metasomatic sulfur in a fluid was a major euhedral/subhedral pyrite. euhedral/subhedral pyrite

Summary

Introduction

The 3.22 Ga, ~3.7-km-thick Moodies Group forms the stratigraphically uppermost part of the Barberton Greenstone Belt (BGB) in South Africa (e.g., [1]). The Moodies Group is suggested to be one of the oldest terrestrial-to-shallow marine transitional units in the geological record (e.g., [2,3]). This group is mainly composed of medium-to-coarse-grained sandstone [4,5]. The quartz-rich siliciclastic sands, conglomerates, and silts accumulated on terrestrial to shallow-marine settings, reflecting alluvial, coastal flood plain and intertidal to subtidal depositional environments [5]. A systematic difference in the carbon and nitrogen isotopic composition between the Moodies terrestrial and marine strata was observed and interpreted to reflect the oldest evidence for the co-existence of a distinctive land and marine biosphere. The sulfur cycle and the search for a possible sulfur isotopic biosignature in the Moodies rocks have not yet been addressed in previous studies [12,13]

Methods

Discussion

Conclusion