Extreme sulfur isotope heterogeneity in individual Ediacaran pyrite grains revealed by NanoSIMS analysis

Abstract

Pyrite sulfur isotopic composition (δ³⁴S_py) is a crucial proxy for reconstructing ancient Ediacaran marine environments. However, recent in situ isotopic analyses of sedimentary pyrite have revealed distinct δ³⁴S_py signatures among different pyrite morphologies, indicating that secular changes in bulk δ³⁴S_py may reflect variations in proportions of different pyrite morphologies rather than environmental signals. Up to now, intragrain isotopic patterns within individual pyrite grains have not yet been extensively investigated for Ediacaran samples. The absence of this specific data set has hindered our ability to understand current complexities of bulk δ³⁴S_py in reconstruction paleoenvironment. This study aims to elucidate δ³⁴S_py patterns by conducting in situ isotopic analysis on pyrite grains from Ediacaran drill-core samples. We employed scanning electron microscopy (SEM), energy dispersive spectrometer (EDS), Raman spectroscopy, and nanoscale secondary ion mass spectrometry (NanoSIMS) to examine the crystal texture, element atomic ratios (S/Fe), mineral composition, and in situ isotopic composition within single pyrite grains. SEM-EDS observations reveal that the euhedral/subhedral pyrite crystals generally exhibit uniform mineral texture, although some grains show varying degrees of dissolution edges and surface cavities. Raman spectroscopy demonstrates the possible presence of pyrrhotite mineral within pyrite grains at some specific sites and points. In situ isotopic analysis reveals significant intragrain δ³⁴S heterogeneity, with difference reaching up to 69.3‰ on a micrometer scale. Distinct deposition depths and burial rates may contribute to the varying value ranges and degrees of isotopic heterogeneity amongst pyrite grains. A model suggesting rapid precipitation from numerous nucleation sites simultaneously could account for the observed intragrain heterogeneity of in situ δ³⁴S values. Pyrite grains exhibit general atomic S/Fe ratios of > 2, potentially due to the presence of trace elements incorporated into the pyrite structure. The occurrence of pyrrhotite leads to a slightly positive correlation between in situ δ³⁴S_py values and S/Fe ratios, yet the ³²S-enriched pyrrhotite plays less substantial in generating intragrain δ³⁴S heterogeneity of pyrite grains. Our findings reveal clear isotopic heterogeneities within individual pyrite grains, in addition to the notable δ³⁴S_py differences among pyrite of various morphologies. These results highlight significant microenvironmental heterogeneity and dynamic sulfur pool mixing on rapid short-term timescale during pyrite growth.