Variability of sulfur isotopes and trace metals in pyrites from the upper oceanic crust of the South China Sea basin, implications for sulfur and trace metal cycling in subsurface

Abstract

The circulation of seawater within the oceanic crust promotes cycling of metals and sulfur, as well as development of the subsurface biosphere. Characterization of the full spectrum of sulfide geochemistry in the altered oceanic crust is critical for a complete understanding of global cycling of sulfur and metals, while those in off-axis hydrothermal circulation have received less attention. This study applied a combination of in-situ sulfur isotopic and trace metal analyses to study the pyrite geochemical features in altered basalts of the South China Sea basin. This contribution unravels a detailed picture of the cycling of sulfur and metals in the subsurface. Such cycling is likely to be co-controlled by the hydrothermal and microbial activities when oceanic crusts transit from on-axis to off-axis hydrothermal systems.A total of five groups of pyrites associated with distinct mineral assemblages, morphologies, trace metal enrichment and sulfur isotopic values were observed in altered basalts. Pyrite disseminated (type 1) within the background hydrothermal alteration is characterized by δ34S values close to zero, with relative enrichment of trace metals (> 500 ppm), such as Ni, Co, and Cu. This type of pyrite is most likely to precipitate from upwelling high-temperature hydrothermal fluids when they ascended towards the seafloor. Sulfur was mainly derived from magmatic sulfides. Linear regression analyses of trace metal contents in disseminated pyrite show various strong positive correlations between trace metals reflecting a series of metal mobility processes, including incorporation into pyrite and leaching directly from sulfides or silicate minerals. Additionally, a population of pyrite has δ34S values ranging from +4 to +8 ‰ and low trace element contents, which reveals precipitation conditions where the ascending high-temperature hydrothermal fluids have mixed with significant amounts of seawater-sourced fluids. This group of pyrites is either formed by the replacement of silicates as patchy texture (type 2) or are exclusively associated with epidote‑carbonate veins (type 3). Furthermore, strongly positive δ34S signatures are observed within framboidal pyrite (type 4) (δ34S up to +57 ‰) and within overgrown euhedral pyrite (type 5) (δ34S up to +24 ‰). These unprecedented isotopically-heavy framboidal pyrite in modern oceanic crust presents a well-preserved organic matter framework, which is interpreted as a consequence of microbial sulfate reduction activities based on current observations. The extremely positive values of the sulfur isotopes could be produced by Rayleigh isotopic fractionation when ~77% sulfate (assumed with an initial seawater sulfate δ34S of 21%) is consumed by microorganisms in closed systems, in this case, the cavities in the altered basalt. The microorganisms meanwhile may enhance the sequestration of trace metals into pyrite, possibly through microbial metabolisms or detoxification processes. The positive δ34S values of overgrown euhedral pyrite (type 5) are interpreted to be mainly introduced by the involvement of seawater-derived sulfur or inherited from the framboidal pyrite.The various isotopic and elemental signatures in these five types of pyrites could be linked to either the hydrothermal or microbial processes occurring within the upper oceanic crust in the South China Sea basin. Both processes are critical to the metal and sulfur cycling in subsurface and should be considered jointly in future studies. Additionally, our results exemplify the potential roles of microorganisms, not only in accumulating trace metals in sulfides in the subsurface but also as a potential biosignature to reveal microbial activities in similar environments in the early Earth.