Field and microanalytical isotopic investigation of ultradepleted in ^(18)O Paleoproterozoic "Slushball Earth" rocks from Karelia, Russia

Abstract

The 1.85 Ga Belomorian Belt, Karelia, Russia, hosts ultralow δ^(18)O and δD (as low as −27.3‰ and −235‰ standard mean ocean water [SMOW], respectively), high-Al gneisses and amphibolites that we attribute to the Paleoproterozoic "Slushball Earth" glaciation. They now occur in at least 11 localities spanning 450 km. To constrain distribution of ^(18)O-depleted rocks, we performed detailed field mapping in Khitostrov, where δ^(18)O values are the lowest. Using 430 new and previously published laser fluorination isotope analyses, we show that the elongated, concentrically zoned area of δ^(18)O depletion is greater than 6 × 2 km in areal extent, ∼10 times larger than previously thought. Relationships between δ^(17)O versus δ^(18)O strictly adhere to the equilibrium terrestrial mass-dependent fractionation with a slope of 0.527. We also report the results of ion microprobe U-Pb geochronology of zircons coupled with co-registered oxygen isotope spot analyses for mafic intrusions and host gneisses in six localities. The 2.9–2.7 Ga gneiss zircon cores are normal in δ^(18)O (5‰–7‰). They show truncated oscillatory cathodoluminescence (CL) patterns and rounded shape indicative of original igneous crystallization with subsequent detrital overprinting. A younger 2.6–2.55 Ga metamorphic zircon domain with normal δ^(18)O, low Th/U, dark cathodoluminescence, and also with rounded crystal morphology is commonly preserved. Cores are surrounded by ubiquitous rims highly depleted in δ^(18)O (re-)crystallized with Svecofennian (1.85–1.89 Ga) ages. Rims are interpreted as metamorphic due to bright and uniform CL and Th/U <0.05. Mafic intrusions preserve few igneous zircon crystals between ca. 2.23 and 2.4 Ga in age, but neoblastic zircon in these intrusions originated mostly during 1.85 Ga Svecofennian metamorphism. The δ^(18)O-age relationship for metamorphic rims in zircon and corundum grains suggests that δ^(18)O values of fluids were subtly increasing with time during metamorphism. Large metamorphic corundum grains have ∼3‰ intracrystalline δ^(18)O isotope zonation from –24 to –21‰, which likely developed during interaction with metamorphic fluids. The Zr-in-rutile geothermometer temperatures are in the range of 760 to 720 °C, in accordance with mineral assemblages and amphibolite metamorphic grade. High and irregular rare-earth element (REE) abundance in cores and rims of many zircons correlates with high phosphorus content and is explained by nanometer-scale xenotime and monazite inclusions, likely in metamict zones during 1.85 Ga Svecofennian metamorphism. A survey of oxygen isotopes in ultrahigh-pressure diamond and coesite-bearing metamorphic terrains around the world reveals normal to high-δ^(18)O values, suggesting that the low δ^(18)O in metamorphic rocks of Dabie Shan, Kokchetav, and in Karelia, are genetically unrelated to metamorphism. We discuss alternative ways to achieve extreme δ^(18)O depletion by kinetic, Rayleigh, and thermal diffusion processes, and by metamorphism. We prefer an interpretation where the low-δ^(18)O and high-Al signature of the rocks predates metamorphism, and is caused by shallow hydrothermal alteration and partial dissolution of the protolith surrounding shallow mafic intrusions by glacial meltwaters during pan-global Paleoproterozoic "Slushball Earth" glaciations between ca. 2.4 and ca. 2.23 Ga.

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