Weathering in a world without terrestrial life recorded in the Mesoproterozoic Velkerri Formation

Abstract

Today the terrestrial surface drives biogeochemical cycles on Earth through chemical weathering reactions mediated by the biological influence of soils. Prior to the expansion of life on to land, abiotic weathering may have resulted in different boundary conditions affecting the composition of the biosphere. Here we show a striking difference in weathering produced minerals preserved in the Mesoproterozoic Velkerri Formation. While the bulk chemistry and mineralogy is dominated by illite similar to many modern mudstones, application of a novel microbeam technology reveals that the initial detrital minerals were composed of mica (28%) and feldspar (45%) with only a trace amount (<2%) of typical soil formed clay minerals. The majority of illite and the high Al 2 O 3 fraction previously interpreted as a weathering signal, is present as a replacement of feldspar and mica. These sediments record physical erosion with limited pedogenic clay mineral formation implying fundamentally different weathering pathways.

Supplemental Material: Unraveling the histories of Proterozoic shales through in situ Rb-Sr dating and trace element laser ablation analysis

“…Therefore, we conclude that the Rb-Sr data collected from each spot analysis are derived predominantly from these illites. In addition, we also identify an absence of detrital clays, as has been reported quite widely for Proterozoic shales (Rafiei and Kennedy, 2019), and instead recognize quartz to be the main detrital component. Quartz has negligible Rb and Sr, so does not influence our results.…”

“…Together with the matrix supported nature of the sample and evidence for differential compaction of illite domains around siderite concretions, these observations argue for an early, reverse weathering origin for the dominant proportion of illite (Rafiei & Kennedy, 2019;Rafiei et al, 2020). Rare exceptions to include large flakes of illite, interpreted to be physical weathering products of mica, and equant domains of pure illite with sharply delineated outlines and of similar size to chlorite grains, interpreted to be of feldspar alteration origin.…”

Enhanced chemical weathering triggered an expansion of euxinic seawater in the aftermath of the Sturtian glaciation

“…Dynamically changing redox states after the Sturtian glaciation reflect transitions between euxinic and ferruginous conditions on the Yangtze margin, likely driven by continental chemical weathering. Previous studies have suggested widespread ferruginous conditions and only spatially limited euxinic conditions during the middle Proterozoic (Planavsky et al, 2011;Gilleaudeau et al, 2019), during which continental chemical weathering and marine primary productivity were significantly depressed (Crockford et al, 2018;Rafiei et al, 2019). In this view, transitions of local marine ferruginous and euxinic conditions in the aftermath of the Sturtian glaciation were more likely controlled by microbially mediated sulfate reduction linked to changes in continental weathering and marine primary productivity(e.g., Reinhard et al, 2009;Planavsky et al, 2011;Li et al, 2012;Guilbaud et al, 2015).…”

The Teena Zn-Pb Deposit (McArthur Basin, Australia). Part II: Carbonate Replacement Sulfide Mineralization During Burial Diagenesis—Implications for Mineral Exploration

“…Provided there is a source and sink for K and Si, illitization of smectite is complete by ~120°C and will then gradually evolve to phengite and eventually muscovite (Hower et al, 1976;Merriman and Peacor, 1998). The abundance of detrital clay minerals is thought to have been much lower during the Proterozoic due to lower levels of chemical weathering (Rafiei and Kennedy, 2019). Nevertheless, the immature volcaniclastic composition of siliciclastic input into the Teena subbasin means that smectite would either have been a major detrital phase or present as an alteration product of volcanic glass.…”