Field and microanalytical isotopic investigation of ultradepleted in 18O Paleoproterozoic “Slushball Earth” rocks from Karelia, Russia

The 1.85 Ga Belomorian Belt, Karelia, Russia, hosts ultralow 18 O and D (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 fi eld mapping in Khitostrov, where 18 O values are the lowest. Using 430 new and previously published laser fl uorination isotope analyses, we show that the elongated, concentrically zoned area of 18 O depletion is greater than 6 2 km in areal extent, ~10 times larger than previously thought. Relationships between 17 O versus 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 mafi c intrusions and host gneisses in six localities. The 2.9-2.7 Ga gneiss zircon cores are normal in 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 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 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. Mafi c 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 18 O-age relationship for metamorphic rims in zircon and corundum grains suggests that 18 O values of fl uids were subtly increasing with time during metamorphism. Large metamorphic corundum grains have ~3 intracrystalline 18 O isotope zonation from -24 to -21, which likely developed during interaction with metamorphic fl uids. 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 coesitebearing metamorphic terrains around the world reveals normal to high- 18 O values, suggesting that the low 18 O in metamorphic rocks of Dabie Shan, Kokchetav, and in Karelia, are genetically unrelated to metamorphism. We discuss alternative ways to achieve extreme 18 O depletion by kinetic, Rayleigh, and thermal diffusion processes, and by metamorphism. We prefer an interpretation where the low- 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 mafi c intrusions by glacial meltwaters during pan-global Paleoproterozoic "Slushball Earth" glaciations between ca. 2.4 and ca. 2.23 Ga.