Questioning the biogenicity of Neoproterozoic superheavy pyrite by SIMS

Abstract

The Neoproterozoic sulfur isotope (δ 34 S) record is characterized by anomalously high δ 34 S pyrite values. Many δ 34 S pyrite values are higher than the contemporaneous δ 34 S sulfate (i.e., δ 34 S pyrite > δ 34 S sulfate ), showing reversed fractionation. This phenomenon has been reported from the Neoproterozoic post-glacial strata globally and is called "Neoproterozoic superheavy pyrite." The commonly assumed biogenic genesis of superheavy pyrite conflicts with current understanding of the marine sulfur cycle. Various models have been proposed to interpret this phenomenon, including extremely low concentrations of sulfate in seawaters or pore waters, or the existence of a geographically isolated and geochemically stratified ocean. Implicit and fundamental in all these published models is the assumption of a biogenic origin for pyrite genesis, which hypothesizes that the superheavy pyrite is syngenetic (in the water column) or early diagenetic (in shallow marine sediments) in origin and formed via microbial sulfate reduction (MSR). In this study, the Cryogenian Datangpo Formation in South China, which preserves some of the highest δ 34 S pyrite values up to +70‰, is studied by secondary ion mass spectrometry (SIMS) at unprecedented spatial resolutions (2 μm). Based on textures and the new sulfur isotope results, we propose that the Datangpo superheavy pyrite formed via thermochemical sulfate reduction (TSR) in hydrothermal fluids during late burial diagenesis and, therefore, lacks a biogeochemical connection to the Neoproterozoic sulfur cycle. Our study demonstrates that SEM-SIMS is an effective approach to assess the genesis of sedimentary pyrite using combined SEM petrography and micrometer-scale δ 34 S measurements by SIMS. The possibility that pervasive TSR has overprinted the primary δ 34 S pyrite signals during late diagenesis in other localities may necessitate the reappraisal of some of the δ 34 S pyrite profiles associated with superheavy pyrite throughout Earth's history.

Microbial metallogenesis of Cryogenian manganese ore deposits in South China

“…During diagenesis, the system became anoxic and framboidal pyrite formed through bacterial sulfate reduction (BSR) in the sulfate reduction zone. Although the pyrite framboids in the DFMnD range from 10 to 30 μm in size, previous research has revealed that the pyrite framboids in the DFMnD witnessed thermochemical sulfate reduction (TSR) and contain growth rims with superheavy δ 34 Spyrite (+50‰ to +70‰) and normal cores with biogenic δ 34 Spyrite values (+15‰ to +20‰) (Cui et al, 2018). The original diameters of the pyrite framboids in the DFMnD should have been 2-5 μm.…”

Shifting modes of iron sulfidization at the onset of OAE-2 drive regional shifts in pyrite δ34S records

“…δ SCRS values can reflect variable contributions by multiple generations of mineral growth (Cui et al, 2018) and can be further complicated by the presence of multiple mineralogies (e.g., pyrite and marcasite; Bryant et al, 2019). Sulfide minerals record an integrated value representing the  34 S of aqueous sulfide during their formation.…”

Sulfur isotope analysis of microcrystalline iron sulfides using secondary ion mass spectrometry imaging: Extracting local paleo‐environmental information from modern and ancient sediments

“…First, by rastering over grains, scanning ion imaging can generate a continuous record of isotope variations, one that can be interrogated at variable spatial resolution after data collection. Previously, most SIMS studies of pyrites relied on analyses of fairly large (≥10 μm‐diameter) spots within grains . This precludes analyses of many sedimentary pyrites (i.e., those with diameters <10 μm).…”

“…Progress has been made using spot analyses by secondary ion mass spectrometry (SIMS) 1,23,[28][29][30][31][32][33][34][35][36][37][38] or laser ablation (LA)-MC-ICP-MS, 20,39,40 to make spatially resolved δ 34 S measurements. Some have already attributed detectable intra-sample pyrite δ 34 S variability to temporal changes in the S-isotopic composition of the fluids from which the pyrites precipitated.…”

“…Previously, most SIMS studies of pyrites relied on analyses of fairly large (≥10 μm-diameter) spots within grains. 1,23,[28][29][30][31][32][33][34][35][36][37][38] This precludes analyses of many sedimentary pyrites (i.e., those with diameters <10 μm). Moreover, while spot analyses can be sufficient to determine the presence of inter-grain isotopic variability on larger grains, they are not able to discern intra-grain variability, [31][32][33][34][35][36][37] except for the case of unusually large (diameters >~100 μm) pyrites.…”