Geochemistry of the Krivoy Rog Banded Iron Formation, Ukraine, and the impact of peak episodes of increased global magmatic activity on the trace element composition of Precambrian seawater

Iron formations: A global record of Neoarchaean to Palaeoproterozoic environmental history

“…The Y/Ho ratio in chemical precipitates is also typically depressed by clastic input because crustal rocks have lower Y/Ho ratios (Kamber and Webb, 2001;Bolhar et al, 2004). Similarly, Viehmann et al (2015) noted the depression of the La anomaly in IF with increased detrital contamination; La anomalies are strongly expressed in pure IF (La SN /La SN * ratios between 1.32 and 1.80), whereas they are small or absent in impure IF (between 0.85 and 1.52). Detrital contamination would also act to decrease the slope of the REE+Y pattern.…”

“…5). The magnitude of positive Eu anomalies steadily decreases from the Eoarchaean, which reflects both high hydrothermal input and low dissolved oxygen required to stabilise Eu 2+ for transport before the deep oceans became oxygenated (Viehmann et al, 2015). The very high peak 14 between 2.7 and 2.5 Ga likely corresponds to a very high mantle input of hydrothermally-derived iron at that time.…”

“…Recently, greenalite has also been suggested as a primary precipitate that formed in the water column (Rasmussen et al, 2015(Rasmussen et al, , 2016 Europium anomalies in REE+Y patterns have been central in tracing Fe sources. Europium enrichment in chemical sedimentary rocks that precipitated from seawater indicates a strong influence of high-temperature hydrothermal fluids on the seawater dissolved REE+Y load (e.g., Klinkhammer et al, 1983;Derry andJacobsen, 1988, 1990;Bau and Dulski, 1996;Viehmann et al, 2015). Positive Eu anomalies in hydrothermal fluids are linked to the breakdown of plagioclase minerals within the volcanic rocks underlying hydrothermal vents, as plagioclase is intrinsically enriched in Eu relative to other REE+Y during igneous crystallisation (Schnetzler and Philpotts, 1970;Graf, 1977Graf, , 1978Fryer et al, 1979).…”

“…However, Eu concentrations can also be elevated by the erosion of Eu-enriched Archaean continental crust (Taylor and McLennan, 1985;Bohlar et al, 2004), and anomalies generated in vent systems may be diluted by mixing of seawater and hydrothermal fluids (Olivarez and Owen, 1991). Nonetheless, secular trends in the magnitude of Eu anomalies in large Superior-type IFs have historically been assumed to record variations in hydrothermal flux (e.g., Fryer, 1977a;Derry and Jacobsen, 1990;Danielson et al, 1992;Bau and Möller, 1993;Sreenivas and Murakami, 2005;Viehmann et al, 2015). This trend is clearly demonstrated through a compilation of chondrite-normalised Eu anomaly data (Eu CN /Eu CN * ) in IF spanning the Precambrian (Fig.…”

“…Sources of REE+Y and the fidelity of their signals have been investigated using the 138 Ce and 147 Nd isotope systems (e.g., Tanaka et al, 1982;Derry and Jacobsen, 1990;Shimizu et al, 1991;Hayashi et al, 2004;Viehmann et al, 2015;Haugaard et al, 2016). Both Ce and Nd have short residence times in the modern ocean, 90-165 and 1000-1500 years, respectively (Amakawa et al, 1996), and in the Archaean oceans these elements likely had a similar residence times on the order of 10 3 years, suggesting that Ce and Nd isotope compositions would be spatially heterogeneous.…”