Diagenetic Fe-carbonates in Paleoarchean felsic sedimentary rocks (Hooggenoeg Formation, Barberton greenstone belt, South Africa): Implications for CO2 sequestration and the chemical budget of seawater

摘要

In order to evaluate the potential of felsic sediments as a CO2-sink in the Archean, we studied felsic volcaniclastic/epiclastic sedimentary rocks of the 3.45 Ga Hooggenoeg Formation, Barberton greenstone belt, which were affected by metasomatic processes during seafloor alteration and diagenesis. Water-rock interactions leading to K-, Si- and CO2-metasomatism were quantified. The precursor rock, a K-metasomatized dacite, was leached of Ca, Mg, Fe, Na, Sr, and Ba and enriched in K, Rb and Si prior to erosion and deposition. The formation of K-mica and quartz (and minor K-feldspar) in the dacites suggests low pH metasomatic conditions, likely produced by hydrothermally induced circulation of hot, acidic and reduced Archean seawater in equilibrium with a high P-CO2 atmosphere. Erosion and transport of K-metasomatized dacitic detritus away from the felsic volcanic centers resulted in the deposition of conglomerate, sandstone and shale by mass flow processes. Early diagenetic silicification affected mainly the fine-grained sediments with higher silica sorption capacity, forming impermeable layers,while sand-rich sediments were partly silicified and remained permeable. Trapped fluids precipitated two generations of Fe-rich dolomites and finally calcite. Up to 30 vol.% of siliciclastic coarse-grained sediment was replaced by carbonates in a shallow-burial, high heat-flow diagenetic regime (depth: similar to 750 m, temperature: 80-160 degrees C), and likely throughout deposition of overlying volcano-sedimentary units. The carbon isotopic composition of Fe-rich dolomites (delta(13) C-PDB = +1.9 to +2.4 parts per thousand) and the strong Fe-Ca-Mg leaching of the Paleoarchean volcanic formations support the influence of seawater-derived fluids throughout CO2-metasomatism. For each gram of eroded dacite, the overall chemical exchange involved by K-Si-CO2-metasomatism was characterized by a mass transfer of Fe (1.2 mmol/g), Na (2.1 mmol/g) and O2- (2.0 mmol/g) to seawater. In contrast, seawater was depleted in Si (10 mmol/g), Ca (0.51 mmol/g), Mg (0.43 mmol/g), K (1.5 mmol/g) and H (0.93 mmol/g) during incorporation of these elements in the volcanic and sedimentary rocks. The average CO2 Uptake by the sedimentary rocks studied here is estimated to be 1.8 mmol/g, in the same order of magnitude as previous estimates for the Paleoarchean basaltic crust. Although mafic-ultramafic rocks are the most abundant rocks in Paleoarchean greenstones belts, and represent the most important atmospheric CO2-sink upon seafloor alteration in the Paleoarchean, coarse felsic sedimentary rocks provide a non-negligible contribution to the build-up of the continental CO2 reservoir.