The deposition and significance of an Ediacaran non‐glacial iron formation

Abstract

Most Neoproterozoic iron formations (NIF) are closely associated with global or nearglobal "Snowball Earth" glaciations. Increasingly, however, studies indicate that some NIFs show no robust evidence of glacial association. Many aspects of non-glacial NIF genesis, including the paleo-environmental setting, Fe(II) source, and oxidation mechanisms, are poorly understood. Here, we present a detailed case study of the Jiapigou NIF, a major non-glacial NIF within a Neoproterozoic volcano-sedimentary sequence in North Qilian, northwestern China. New U-Pb geochronological data place the depositional age of the Jiapigou NIF at ~600 Ma. Petrographic and geochemical evidence supports its identification as a primary chemical sediment with significant detrital input. Major and trace element concentrations, REE + Y systematics, and ε Nd (t) values indicate that iron was sourced from mixed seawater and hydrothermal fluids. Iron isotopic values (δ 56 Fe = −0.04‰-1.43‰) are indicative of partial oxidation of an Fe(II) reservoir. We infer that the Jiapigou NIF was deposited in a redox stratified water column, where hydrothermally sourced Fe(II)-rich fluids underwent oxidation under suboxic conditions. Lastly, the Jiapigou NIF has strong phosphorous enrichments, which in other iron formations are typically interpreted as signals for high marine phosphate concentrations. This suggests that oceanic phosphorus concentrations could have been enriched throughout the Neoproterozoic, as opposed to simply during glacial intervals.

A phosphate-rich marine reservoir in the redox stratified Ediacaran ocean

“…Energy dispersive spectroscopy (EDS) analyses (Fig. 2h) indicate that the apatite grains are carbonate uorapatite [CFA, Ca 5 (PO 4 , CO 3 ) 3 F], which is consistent with previous electron probe micro-analyser (EPMA) studies 24 . Most CFA grains are less than 5 μm in diameter (Figs.…”

“…The above observations suggest that the primary source of P to the North Qilian IFs was due to drawdown in association with Fe minerals. In the North Qilian IFs, the dominance of 'primary' hematite implies a hydrous ferric oxide (e.g., ferrihydrite) precursor that likely precipitated in the water column during the oxidation of upwelling ferruginous waters 23,24 . During this process, co-precipitation and adsorption would have delivered P to the sediment, wherein the early diagenetic transfer of ferrihydrite to more stable hematite (which has a much lower sorptive capacity 34 ) would have released P to the porewaters.…”

“…Many new Ediacaran IFs have recently been identi ed in the lower Zhulongguan Group of the Qilian Orogenic Belt (Supplementary Figs. 1 and 2), including the Kawa, Jiapigou and Xiaoliugou IFs, which are ~600 Ma in age 23,24 (see Supplementary Information for further details of the geological setting). These IFs mainly comprise hematite, microquartz (jasper) and minor clay minerals, with rare magnetite, carbonate minerals and apatite (Figs.…”

“…600 Ma) IFs in North Qilian, NW China. These IFs have only been subjected to very low-grade metamorphism, and are mainly composed of primary hematite and microquartz (jasper) 23,24 , thus offering a valuable window into contemporaneous Fe and P cycling. To achieve this, we conducted high-resolution petrographic, mineralogical and geochemical studies on the North Qilian IFs, which we subsequently place in the context of other global data-sets, allowing a reconstruction of P bioavailability across a critical interval of Earth surface and biological evolution.…”

Cryogenian and Ediacaran integrative stratigraphy, biotas, and paleogeographical evolution of the Qinghai-Tibetan Plateau and its surrounding areas

“…Recently, Xu et al (2015) suggested a formation age between 600 and 580 Ma for the basalts in the Zhulongguan Group. Similar age constraints on the corresponding strata were recently provided by Yang X et al ( , 2023. These results confirm an Ediacaran age for the Zhulongguan Group and overlying units.…”