Microscale oxygen isotope variations in 1.9 Ga Gunflint cherts: Assessments of diagenesis effects and implications for oceanic paleotemperature reconstructions

Abstract

Variations in the oxygen isotope composition (δ 18O) of five cherts from the 1.9 Ga Gunflint iron formation (Canada) were studied at the micrometer scale by ion microprobe to try to better understand the processes that control δ 18O values in cherts and to improve seawater paleotemperature reconstructions. Gunflint cherts show clearly different δ 18O values for different types of silica with for instance a difference of ≈15‰ between detrital quartz and microquartz. Microquartz in the five samples is characterized by large intra sample variations in δ 18O values, (δ 18O of quartz varies from 4.6‰ to 6.6‰ at the 20 μm scale and from ≈12‰ to 14‰ at 2 μm scale). Isotopic profiles in microquartz adjacent to hydrothermal quartz veins demonstrate that microquartz more than ≈200 μm away from the veins has preserved its original δ 18O value. At the micrometer spatial resolution of the ion probe, data reveal that microquartz has preserved a considerable δ 18O heterogeneity that must be regarded as a signature inherited from its diagenetic history. Modelling of the δ 18O variations produced during the diagenetic transformation of sedimentary amorphous silica precursors into microquartz allows us to calculate seawater temperature ( T sw at which the amorphous silica precipitated) and diagenesis temperature ( T diagenesis at which microquartz formed) that reproduce the δ 18O distributions (mean, range and shape) measured at micrometer scale in microquartz. The two critical parameters in this modelling are the δ 18O value and the mass fraction of the diagenetic fluid. Under these assumptions, the most likely ranges for T sw and T diagenesis are from 37 to 52 °C and from 130 to 170 °C, respectively.