Low δ18O rocks in the Belomorian belt, NW Russia, and Scourie dikes, NW Scotland: A record of ancient meteoric water captured by the early Paleoproterozoic global mafic magmatism

Abstract

The 2.45–2.38 Ga intrusions and their host rocks in the Belomorian belt of NW Russia and the Scourie dikes of the Lewisian complex in NW Scotland are marked by extremely low δ18O values. Their isotope signatures reflect high-temperature exchange between rocks and low δ18O meteoric waters representing a record of active hydrologic cycle early in the Earth’s history. Here we explore triple oxygen and hydrogen isotope systematics (δD, δ18O and Δ17O) of these rocks to evaluate the isotope composition of meteoric water. The new occurrences from Belomorian belt range in δ18O from −14 to −2‰; combined with previously reported data, ranging between −27 and +3‰, the low δ18O rocks are traced across 500 km of the Belomorian belt. Based on spatial distribution of the values, hydrothermal alteration driven by the emplacement of the mafic intrusions is the most consistent explanation for the low δ18O values recorded in the Belomorian belt. The Δ17O systematics reveals contribution of distinct meteoric waters with δ18O of −38 and −9‰ recorded at different localities across the belt. We also present new data for the 2.41–2.38 Ga Scourie dikes, with the lowest δ18O value of −2.5‰ recorded in the amphiboles of Loch na h dike. Unlike the Belomorian belt, the low δ18O signature of Scourie dikes requires igneous incorporation of low δ18O hydrothermally altered rocks through contamination of mantle-derived melts. The Δ17O data analyzed at the Loch na h dike reveals that the incorporated material experienced water-rock interaction with meteoric water that had δ18O of −35 ± 10‰. The exact mechanism for incorporation of such low δ18O rocks in the mafic Scourie dike melts is equivocal and could involve subduction or assimilation of low δ18O hydrothermally altered mafic rocks. These reconstructed isotope compositions of meteoric waters are diverse and comparable to the modern-day precipitation of high-latitude regions (e.g. Greenland, δ18O = −35‰) and precipitation of mid-latitude regions (−9‰), possibly reflecting spatial δ18O gradients resulted from an active hydrologic cycle during the cold climate of the early Paleoproterozoic, as depicted by previous isotope-enabled global circulation models of snowball Earth state.