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

Abstract

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 ^(87)Sr/^(86)Sr 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, ^(87)Sr/^(86)Sr and Rb/Sr ratios of leachates mostly define a binary mixing relationship and the leaching-sequence-minimum, Rb-corrected ^(87)Sr/^(86)Sr 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 ^(87)Sr/^(86)Sr ratio is ~ 0.7092 on average (± 0.0004, 1σ). We interpret these minimum ^(87)Sr/^(86)Sr 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 ^(87)Sr/^(86)Sr variation during Marinoan deglaciation. In contrast, elevated ^(87)Sr/^(86)Sr 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 ^(87)Sr/^(86)Sr ratios and δ^(18)O 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 ^(87)Sr/^(86)Sr ratios. Alternatively, the elevated ^(87)Sr/^(86)Sr ratios may record the values of 'glacial lake Harland'.

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