Testing models of pre-GOE environmental oxidation: A Paleoproterozoic marine signal in platform dolomites of the Tongwane Formation (South Africa)

Abstract

A B S T R A C TCarbonate facies of the upper Tongwane Formation preserve a largely overlooked record of pre-GOE Paleoproterozoic seawater. This inventory has survived despite a complex paragenetic history involving both diagenetic and contact metamorphic processes. BIF mineral assemblages are dominated by a medium-grade grunerite overprint and the formation of prograde and retrograde riebeckite. The massive dolomite member, a platform top carbonate which caps the succession, is characterized by marine REY SN patterns lacking significant negative Ce SN anomalies (< 10%) and δ 13 C carb values of between +1.83 and +2.95 ‰ VPDB that are slightly elevated above Paleoproterozoic 'normal marine' values. Systematically decreasing δ 13C carb values measured in monotonously recrystallized, talc bearing, slope carbonates reflect interaction with devolatilization fluids during contact metamorphism caused by the intrusion of the Bushveld Igneous Complex; slope carbonates offer no insight into marine paleoredox conditions. Thus, despite contact metamorphism, Paleoproterozoic marine signals are retained in platform-top dolomites, however they do not support claims for widespread oxygen accumulation in shallow depositional environments of the Transvaal Basin on the eve of the GOE.

Decimeter-scale mapping of carbonate-controlled trace element distribution in Neoarchean cuspate stromatolites

“…Non-REY accuracy was better than 6 % and RSD lower than 6 %, with the exception of Al which at standard concentrations of 1 ppb had RSD values as high as 14 %. Samples were digested using previously published methods (Warke et al, 2018), further details are provided in the supplementary information. REY concentrations were normalized to Post Archean Australian Shale (PAAS;Taylor and McLennan, 1985) and are denoted as REY SN .…”

Neoarchean‐Palaeoproterozoic sedimentary basins in the Sarmatian Craton: Global correlations and connections

“…Geochemical characteristics and carbon isotopic composition of the carbonate deposits of the Tongwane (the Transvaal Basin; Warke, Schröder, & Strauss, 2018), the Gandarella (the Minas Gerais Basin; Nogueira et al, 2019), and the Rogovoe and Tim (the Tim‐Kryvyi Rih Basin) (Savko et al, 2020) formations…”

A copper isotope investigation of methane cycling in Late Archaean sediments

“…In addition to causing pronounced and coupled lowering of δ 18 Ocarb and δ 13 Ccarb values, as discussed above, they can strongly alter carbonate-bound, shale-normalised, Rare Earth Element and Yttrium (REYSN) trends. Hydrothermal alteration of this manner leads to characteristic REYSN arrays that are depleted in the heavier lanthanides, lack Y/Ho anomalies, and carry a strong positive Eu anomaly(Tostevin et al, 2016;Warke et al, 2018). Dolomitic facies in the upper Nauga Formation, however, show no evidence of such disturbance and preserve robust seawater REYSN arrays(Eroglu et al, 2017;Warke et al, 2019) albeit with the small, positive, shale-normalised Eu anomaly (EuSN) that is characteristic of Neoarchean seawater which we discuss below.…”

“…Rare earth element and yttrium values (REYSN) were normalized to Post-Archean Australian Shale (PAAS; Taylor and McLennan, 1985). REYSN anomalies (namely EuSN) were calculated using the equations of Lawrence et al (2006) which are commonly used to assess the preservation and redox history of Archean and Proterozoic carbonates (Tostevin et al, 2016;Bellefroid et al, 2018;Warke et al, 2018;…”