Implications of 3.2 Ga deep seawater from sulfur isotopic analysis of barite crystals in Pilbara, Western Australia.

Sulfur isotopic (δ34S) analysis is used as one of the methods of Precambrian environmental reconstruction. It has been pointed out that δ34S fluctuations of sulfate and sulfide have close relationship with rise of oxygen level and increase in biological activity of sulfate reducing bacteria. For example, the difference of δ34S between sulfate and sulfide is small in Archean while it gets larger after evolution of oxygen level and biological activity (e.g. Canfield and Farquhar, 2009). 　However, evidence of δ34S difference between sulfate and sulfide in Archean is scarce. In this study, we focused on barite and pyrite occurred at the layer in the 3.2 Ga Dixon Island Formation in coastal Pilbara terrane, Western Australia. 　We found pyrites in from the bottom of the Black Chert Member to the Varicolored Chert Member of the Dixon Island Formation. Particularly, we can see pyrite layers of a few millimeters thick which make an alternate layers with black chert layers in the Varicolored Chert Member. The bulk δ34S values of these layers are -10.1~+26.8‰ (Sakamoto, 2010MS) and micro-meter scale heterogeneity of δ34S can be seen in minute spherical shell pyrite which was formed at early stage of diagenesis (Miki, 2015MS). 　On the other hand, barite layers are remained in the lower part of the Black Chert Member in the Dixon Island Formation. In these layers, columnar quartz crystals were representative which are considered to be a pseudomorph of barite. Such equigranular occurrences of barite are typical character in submarine hydrothermal system (Kiyokawa et al., 2006). There exist small crystals of barite (less than 200 um in diameter) which are expected to be remnants of original barite. We performed microscale sulfur isotope analyses using a NanoSIMS. 　As a preliminary result, we obtained δ34S value of +3.4~+9.1‰ (n=11). These values are similar to the reported values of barite which are considered to be a hydrothermal origin in 3.47 Ga North Pole, Pilbara (+3.2~+8.7‰, Shen et al. 2001). Our data reflects that of seawater at that time, and is lower than that of pyrite in the same place (Sakamoto, MS2010). This indicates that there was an event to make sulfide heavier than seawater sulfate.