Friday, August 19, 2016

Evidence of a Suducting Crustal Slab From EoArchean Sulfur Isotopes


Siedenberg et al


The Archean sulfur cycle was different from the present-day cycle, as the emission of volcanogenic sulfurous gases was the dominant process in the anoxic environment of the early Earth.This emitted sulfur exhibits mass-independently fractionated sulfur isotopes (MIF-S), resulting from photochemical reactions in the atmosphere, and it differs substantially from unfractionated sulfur in the mantle. So far, the main focus of multiple sulfur analyses (32S32S, 33S33S, 34S34S and 36S36S) was placed on the sedimentary part of the Archean sulfur cycle. In order to constrain the magmatic part of the sulfur cycle, we analyzed the sulfur isotopic composition of oceanic crustal rocks from the ca. 3.7–3.8 Ga Isua Supracrustal Belt (ISB). Differently altered samples were taken from two units:(1) the Undifferentiated Amphibolites (UA) and (2) the younger Amphibolites with Boninitic affinity (AB). The mean values are:δ34SCRS=+0.01±0.65‰δ34SCRS=+0.01±0.65‰ (values range from −0.87 to 1.37‰; CRS = chromium-reducible sulfur),Δ33SCRS=+0.02±0.12‰Δ33SCRS=+0.02±0.12‰ (values range from −0.17 to 0.26‰), Δ36SCRS=-0.47±0.06‰Δ36SCRS=-0.47±0.06‰ (values range from −0.56 to −0.38‰). Thus, the mean isotope values support the assumption that the sulfur isotopic signature reflects the expected near-zero signature of their mantle origin. However, differences in Δ33SCRSΔ33SCRS values are discernible and non-zero suggesting that different sources are contributing to the isotopic signature. An influence of alteration is excluded for all samples as different alteration-sensitive geochemical parameters do not show any correlation with the multiple sulfur isotope signatures. Further, it is unlikely that the small magnitudes in Δ33SCRSΔ33SCRS are generated by microbial mass-dependent processes because of the narrow range of δ34SCRSδ34SCRS values. Possible sources contributing an atmospheric MIF-S signature include seawater sulfate (negative Δ33SCRSΔ33SCRS values) through hydrothermal circulation, the assimilation of ocean floor sediments during the ascent of the melt and/or a mantle source contamination by subducted oceanic slab.

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