Questions regarding Precambrian sulfur isotope fractionation

Revised delta34S reference values for IAEA sulfur isotope reference materials S-2 and S-3

Revised delta(34)S reference values with associated expanded uncertainties (95% confidence interval (C.I.)) are presented for the sulfur isotope reference materials IAEA-S-2 (22.62 +/- 0.16 per thousand) and IAEA-S-3 (-32.49 +/- 0.16 per thousand). These revised values are determined using two relative-difference measurement techniques, gas source isotope ratio mass spectrometry (GIRMS) and double-spike multi-collector thermal ionization mass spectrometry (MC-TIMS). Gas analyses have traditionally been considered the most robust for relative isotopic difference measurements of sulfur. The double-spike MC-TIMS technique provides an independent method for value-assignment validation and produces revised values that are both unbiased and more precise than previous value assignments. Unbiased delta(34)S values are required to anchor the positive and negative end members of the sulfur delta (delta) scale because they are the basis for reporting both delta(34)S values and the derived mass-independent Delta(33)S and Delta(36)S values.

Simultaneous determination of delta33S(V-CDT) and delta34S(V-CDT) using masses 48, 49 and 50 on a continuous flow isotope ratio mass spectrometer

A new, fast, continuous flow technique is described for the simultaneous determination of delta33S and delta34S using SO masses 48, 49 and 50. Analysis time is approximately 5 min/sample with measurement precision and accuracy better than +/-0.3 per thousand. This technique, which has been set up using IAEA Ag2S standards S-1, S-2 and S-3, allows for the fast determination of mass-dependent or mass-independent fractionation (MIF) effects in sulfide, organic sulfur samples and possibly sulfate. Small sample sizes can be analysed directly, without chemical pre-treatment. Robustness of the technique for natural versus artificial standards was demonstrated by analysis of a Canon Diablo troilite, which gave a delta33S of 0.04 per thousand and a delta34S of -0.06 per thousand compared to the values obtained for S-1 of 0.07 per thousand and -0.20 per thousand, respectively. Two pyrite samples from a banded-iron formation from the 3710 Ma Isua Greenstone Belt were analysed using this technique and yielded MIF (Delta33S of 2.45 and 3.31 per thousand) comparable to pyrite previously analysed by secondary ion probe.