Highly siderophile element and Re-Os isotope systematics of the Neoproterozoic Urucum Iron Formation, Brazil
The Urucum iron- and manganese formation (IF and Mn-F) in the Neoproterozoic Santa Cruz Formation, Brazil, has proven to be a reliable and robust archive for the late Neoproterozoic period, allowing unique insights into the composition of seawater during that time-period. Here we present isotope dilution-generated highly siderophile element (HSE) data and Re-Os isotope data of selected Urucum IF and Mn-F samples to evaluate the depositional setting and elemental sources that affected the Neoproterozoic seawater from which the Urucum IF-Mn-F precipitated. Concentrations of selected HSEs vary from ~0.03 to ~3.45 ppb for Re, from ~0.03 to ~4.38 ppb for Os, from ~0.04 to ~0.17 ppb for Ir, from ~0.12 to ~2.20 ppb for Pt, from ~0.04 to ~5.80 ppb for Ru and from ~0.31 to ~2.30 ppb for Pd. Iron formation samples show diverse 187Re/188Os ratios ranging from 0.89 to 1007 and from 0.14 to 2.20 for 187Os/188Os ratios. Manganese formation samples exhibit 187Re/188Os ratios from 0.22 to 198 and 187Os/188Os ratios between 0.11 and 2.10. While concentrations of HSEs are comparable to the modern crust, initial 187Os/188Os ratios resemble typical mantle values. In a Re-Os isochron diagram, selected authigenic Urucum samples (including IF and Mn-F samples) define a regression-line corresponding to an age of 599 ± 69 Ma (MSWD = 0.16) and an initial 187Os/188Os ratio of 0.111 ± 0.007. This age, although overlaps with previous published age data of the Urucum BIF, probably relating the Urucum BIF deposition to the Gaskiers glaciation. In an alternative scenario, Re-Os isotope data could also be interpreted to reflect mixing of three source components. Although none of the scenarios can be ruled out, we consider the isochron scenario as more plausible. The initial 187Os/188Os ratio of 0.111 ± 0.007 supports enhanced influence of mantle-derived sources to the Urucum seawater, but differs from the established 187Os/188Os isotopic seawater evolution curve for the late Neoproterozoic. We consider this discrepancy as being caused by a partly to fully closed Urucum depositional basin, which is more susceptible to local variations and influences.