CONFIRMATION OF MASS-INDEPENDENT ISOTOPE EFFECTS IN ARCHEAN (2.5–3.8 GA) SEDIMENTARY SULFIDES AS DETERMINED BY ION MICROPROBE ANALYSIS

Sulfur isotope (³⁴S/³²S)/(³³S/³²S) anomalies consistent with mass-independent fractionations (D ³³S=d³³S x 0.515 d³⁴S, MIF when D³³S) reported from bulk analyses of sulfates and sulfides in a diverse suite of sedimentary rocks potentially place constraints on the nature of ancient atmospheric chemistries. A secondary ion mass spectrometer (SIMS; ion microprobe) multicollector technique previously developed to measure microdomain D³³S values in martian meteorites obtains precise (³⁴S /³²S) and (³³S/³²S) ratios. Our data reveal large non-mass-dependent D³³S anomalies, some of which form separate linear d³³S/d³⁴S arrays in sulfides from Archean sediments. The D³³S data reported here corroborate prior bulk measurements of Precambrian sulfides and most likely reflect atmospheric MIF sulfur transferred to the oceans in the Archean, implanted into marine sediments, and subsequently preserved within sedimentary sulfides. Our ion microprobe data provide additional support for a postulated low pO2 and photochemically active atmosphere on the Earth before ca. 2.2 Ga, as well as extends the record of atmosphere-hydrosphere interactions revealed by sulfur isotopes to ca. 3.83 Ga. This technique offers new opportunities for exploring ancient sulfur metabolisms preserved in the rock record. The presence of MIF sulfur in sulfides from a ~3.8 Ga metaquartzite/banded iron formation from Akilia island, West Greenland, supports models for a marine sedimentary origin for this rock.