Unravelling atmospheric photolysis and ocean redox chemistry from Paleoarchean pyrite: a multiple sulfur and iron stable isotope study

Abstract

This thesis brought together four individual studies on the geochemistry of three recently obtained drill cores from the Barberton Greenstone Belt, South Africa. These data were used to provide a synthesis of the iron and sulfur cycles in the Paleoarchean, with a particular focus on the nature and origin of sulfur species in the (near) surface environment. The first part of the thesis examined multiple sulfur and Fe stable isotope variability in pyrites from the BARB4 drill core in the 3.2 Ga lower Mapepe Formation that represents a relatively deep-water sedimentary basin environment. The pyrite from this setting closely records the composition of atmospheric elemental sulfur, derived from the UV photolysis of volcanic SO2. Combined Fe and S isotopes were used to argue that conventional models of photolysis, derived from studies across the Archean, and recorded in the Archean reference array (ARA), are an accurate representation of inputs into the Paleoarchean sulfur cycle. The data are consistent with microbial processing of the elemental sulfur, most likely through elemental sulfur disproportionation that was buffered by the presence of sedimentary iron oxide minerals. Further, I examined the geochemistry of the BARB5 drill core, drilled from a slightly higher part of the stratigraphy in the middle Mapepe formation. These rocks represent a nearshore, deep- to shallow-water environment containing terrigenous and volcaniclastic as well as some chemical sediments. The rocks are complex and have a significant detrital input, which under anoxic conditions included the input of reworked sulfide minerals. I show that the rocks were derived from weathering of predominantly basaltic to komatiitic source material, with more minor input from intermediate to felsic volcanics. Penecontemporaneous volcaniclastic rocks were deposited onto the shallow water platform where chert, barite and carbonate were deposited. Much of the carbonate was reworked from original deposits that were in equilibrium with seawater or which show an input from freshwater. The third part of the thesis is a detailed isotopic study of pyrite from both the BARB4 and ETC-1975 drill cores from Barite Valley in the central part of the Barberton Greenstone Belt. The results were compared with recent published work from the ETC-1975 (Roerdink et al., 2013) and BBDP (Philippot et al., 2012) cores in Barite Valley. I provided the first Fe isotope and combined Fe-multiple S isotope data on pyrite from this region. It is essential to distinguishing different groups of pyrite in these highly heterogeneous rocks in order to correctly interpret the sources and redox cycling of sulfur and iron. Two different interpretations of the early S cycle were examined and provided new interpretations for deviations seen in the photolytic source from the Archean reference array. The final results chapter examined a hydrothermally-influenced deeper water basinal setting at 3.4 Ga at Buck Reef in the Onverwacht Group of the Barberton Greenstone Belt where the multiple S and Fe isotopes in a range of different pyrite morphologies found in different types of chert are interpreted in light of prevalently magmatic influenced S sources in a low sulfate environment.