Geochemical constraints on the origin of Marinoan cap dolostones from Nuccaleena Formation, South Australia

- Step-leaching procedure
- seawater 87Sr/86Sr
- Alteration.
- glacial lake Harland

The geochemistry (including Sr isotope and trace element ratios) of “Snowball Earth” terminal deglacial cap-dolostone from the Nuccaleena Formation, South Australia, is studied. An incremental leaching approach was applied in order to extract the nearest-to-primary chemical composition. Based on the relationship between 87Sr/86Sr and Rb/Sr ratios of the leachates, the samples can be categorized into two groups: 1) Group-I dolostones are located 9 cm below to 1.1 m above, and again 4-6 m above a lithologic contact within Nuccaleena Formation marking the end of diamictite (dominantly siliciclastic sediments) and the onset of solid dolostone deposition (“meters above solid dolostones”, MASD). In each of these intervals, 87Sr/86Sr and Rb/Sr ratios of leachates mostly define a binary mixing relationship and the leaching-sequence-minimum, Rb-corrected 87Sr/86Sr ratio is ~ 0.7077 on average (± 0.0003, 1σ, with a minimum value of 0.7073 among all samples); 2) Group-II dolostones define an interval between 1.7 and 4.0 MASD, in which the leaching-sequence-minimum, Rb-corrected 87Sr/86Sr ratio is ~ 0.7092 on average (± 0.0004, 1σ). We interpret these minimum 87Sr/86Sr ratios in Group-I dolostones to represent Sr isotope compositions of penecontemporaneous seawater, similar to values previously reported for high-Sr limestones under- and over-lying “Marinoan” cap dolostones around the world and permitting an interpretation of no significant long-term seawater 87Sr/86Sr variation during Marinoan deglaciation. In contrast, elevated 87Sr/86Sr ratios of Group-II cap dolostones indicate later alteration events or formation of samples in a non-marine environment (e.g., the 'glacial lake Harland', Hoffman, 2011). Negative correlation between 87Sr/86Sr ratios and δ18O values can be explained by fluid-rock interaction, either with brine/saline groundwater, ice melt seeps, or hydrothermal fluids as possible alteration fluids, leading to the elevated 87Sr/86Sr ratios. Alternatively, the elevated 87Sr/86Sr ratios may record the values of 'glacial lake Harland'.