Hyperventilation During the Paleoproterozoic – Evidence for Extensive Oxidation of Terrestrial Surfaces in Fennoscandia

English Abstract:

The ca. 2.06 Ga Kuetsjärvi Volcanic Formation (KVF) is a subaerially erupted volcanic sequence in Fennoscandia that displays an anomalously elevated ratio of oxidized to total iron (Fe³⁺/∑Fe) compared to those of typical volcanic rocks. To date, two hypotheses have been put forth to explain the oxidized nature of KVF: 1) eruption from a highly oxidized magma source region, or 2) oxidation during terrestrial exposure by oxygen-charged groundwaters in the wake of the proposed atmospheric oxygen ‘overshoot’ 2.2 Ga ago.

Petrographic observations from Fennoscandia Arctic Russia – Drilling Early Earth Project (FAR–DEEP) Cores 7A and 8B suggest that the KVF’s elevated Fe³⁺/∑Fe ratios result from post-crystallization hematization of the rocks. We propose that oxidation of the KVF was initiated at the erosional contact marking the top of the KVF, and progressed downward through interactions between the bedrock and oxidizing groundwaters exploiting the oldest fracture sets prior to regional metamorphism. This interpretation is supported by the strong correlation between the measured Fe³⁺/∑Fe ratios and the calculated chemical index of alteration of the altered rocks, and the presence of anomalously oxidized KVF clasts within the overlying conglomerate whose matrix does not exhibit any signs of pervasive oxidation. Detailed U–Th–Pb isotope work indicates that the precipitation of the secondary Fe–oxides occurred after the initial crystallization, but prior to regional metamorphism. Iron appears to have undergone limited mobility as indicated by our detailed Fe–isotope work. Conversely, our preliminary Cr–isotope data indicate that Cr may have been transported laterally during weathering. Altogether, these observations and data support that the oxidation of the KVF resulted from interaction with highly oxidizing groundwaters between ca. 2.06 and 1.75 Ga ago.