Correlating trace element compositions, petrology, and Raman spectroscopy data in the ∼3.46 Ga Apex chert, Pilbara Craton, Australia

Abstract

The Apex chert unit (∼3.46 Ga, Pilbara Craton, Australia) constitutes one of the oldest sedimentary units on Earth in which putative carbonaceous microfossils have been reported. The source of carbonaceous matter (CM) in this unit, however, is hotly debated. Hydrothermal fluids have circulated through the underlying crust and up into the bedded unit; these fluids could have remobilized sedimentary microbial biomass, generated abiological hydrocarbons, or harbored in situ chemolithoautotrophic microbial communities. Are there parts of the unit where microfossils might be best preserved?A potential fossil microbiota – if present – would probably be best preserved in the stratiform portion of the unit where hydrothermal influence seems to have been lowest. In order to shed light on the history of hydrothermal overprinting and the source of carbonaceous fractions in the Apex chert, we correlate here at a high spatial resolution petrographic observations and trace element analyses over a transect from the dyke where putative microfossils were found to the stratiform part where remnants of microbial mats were found. The layered, stratiform part of the unit has positive La anomalies up to 1.7, and Light Rare Earth Element depletions, indicating a seawater source. However, as far as 300 m from the dyke, the stratiform part also shows hydrothermal brecciation, high Eu anomalies (2–12; µ = 4.2) and chondritic Y/Ho ratios (24.3–30.3; µ = 27.0), indicating that hydrothermal fluids have laterally infiltrated over large distances. Overall, the pervasive influence of hydrothermal fluids throughout the entire unit and the presence of carbonaceous matter both in the sedimentary part and the hydrothermal dyke is consistent with a ‘hydrothermal pump’ model that was earlier proposed for the nearby Dresser Formation. In this model, organic matter from surface environments is circulated along with hydrothermal fluids and redistributed in the crust and overlying sediments, therefore complicating paleobiological interpretations. Raman measurements show that most of the CM experienced temperatures of ∼350 °C, while some samples contain CM with a variable, but markedly lower maturity (temperature ranging from 200 to 350 °C). Correlation to texture points out a potential mixing of pre-metamorphic CM with post-metamorphic CM during late hydrothermal events.