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Project Progress

Progress in stable isotope analysis at WiscSIMS was summarized by Kita et al. (2011). The effects of sample relief, the location of standards within sample mounts, and crystal orientation can be of critical importance for attaining the highest possible accuracy. New procedures are reported to mitigate each of these problems. In addition, Peres et al. (2012) report the development and testing of a new sample holder for CAMECA ion microprobes that triples the area of a sample mount that can be analyzed at high accuracy from <1 to ~3 cm3. Heck et al. (2011) report new silicon isotope data on quartz in standards and BIF. Our new quartz standard UWQ-1 is shown to be more homogeneous that the commonly used NIST-28.

High silicon contents (1-3 wt. %) have been found in low grade BIFs from many localities, worldwide by Huberty et al. (2012) who studied samples from the Dales Gorge Fm in detail. Silician magnetite can be seen in back-scattered electron images by SEM if contrast is adjusted to make silicates and carbonates appear black (figure, left). TEM and XRD show that Si is substituted with in the crystal structure of magnetite rather than as inclusions. Si-magnetite forms as overgrowths on low-silicon magnetite. Electron backscatter diffraction (EBSD) and forescatter imaging show that individual magnetite grains grow across this boundary and that Si-magnetites grew during evolving burial and metamorphic conditions rather than in late alteration. Huberty et al. show report coupled substitutions and suggest that Si is stabilized in magnetite by low oxygen fugacity requiring the former presence of reduced organic matter and thus that Si-magnetite is a biosignature in BIF.

The secular change of oxygen isotope ratio of cherts in BIF and other lithologies (lower δ¹⁸O(quartz) in the Archean) has long been controversial leading to conflicting proposals that Archean oceans were hotter than 75°C; that the Archean ocean was lower in δ¹⁸O (< -10‰); or that Archean cherts are systematically more altered than younger rocks. Several recent studies have attempted to resolve pristine vs. altered quartz using in situ SIMS analysis (Robert and Chaussidon 2006, Huberty et al. 2010, Marin et al. 2010). Recently, we selected quartz cements from non-compacted granular ion formation in L. Superior-style BIFs (fig., left) for SIMS analysis and have shown that values of δ¹⁸O(quartz) are uniformly at the high end of reported values (fig., right, Cunningham et al., 2012). These results show that lower values represent altered domains and that the δ¹⁸O value of the earliest cements in these samples was 24-26‰. While warmer oceans with lower δ¹⁸O are not excluded, Cunningham et al. propose that its more likely that the L. Superior BIFs formed in a restricted basin and that the water conditions were been representative of worldwide oceans at 1.85 Ga.