Low δ 18 O 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

• Published in: Precambrian Research
• Main Authors: Zakharov, D.O., Bindeman, I.N., Serebryakov, N.S., Prave, A.R., Azimov, P.Ya., Babarina, I.I.
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2019
• Subjects: Greenland
• Online Access: https://research-portal.st-andrews.ac.uk/en/researchoutput/low-18o-rocks-in-the-belomorian-belt-nw-russia-and-scourie-dikes-nw-scotland(c21d7bbd-2be8-4e8e-81f3-168219412ea7).html; https://doi.org/10.1016/j.precamres.2019.105431

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 δ 18 O values. Their isotope signatures reflect high-temperature exchange between rocks and low δ 18 O 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, δ 18 O and Δ 17 O) of these rocks to evaluate the isotopic composition of meteoric water. The new occurrences from Belomorian belt range in δ 18 O from -14 to -2 ‰; combined with previously reported data, ranging between -27 and +3 ‰, the low δ 18 O 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 δ 18 O values recorded in the Belomorian belt. The Δ 17 O systematics reveals contribution of distinct meteoric waters with δ 18 O 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 δ 18 O value of -2.5 ‰ recorded in the amphiboles of Loch na h dike. Unlike the Belomorian belt, the low δ 18 O signature of Scourie dikes requires igneous incorporation of low δ 18 O hydrothermally altered rocks through contamination of mantle-derived melts. The Δ 17 O data analyzed at the Loch na h dike reveals that the incorporated material experienced water-rock interaction with meteoric water that had δ 18 O of −35 ± 10 ‰. The exact mechanism for incorporation of such low δ 18 O rocks in the mafic Scourie dike melts is equivocal and could involve subduction or assimilation of low δ 18 O hydrothermally altered mafic rocks. These reconstructed isotopic compositions of meteoric waters are diverse and comparable to the modern-day precipitation of high-latitude regions (e.g. Greenland, δ 18 O = −35 ‰) and precipitation of mid-latitude regions (−9 ‰), ...