Dating the Paleoproterozoic snowball Earth glaciations using contemporaneous subglacial hydrothermal systems D.O. Zakharov1, I.N. Bindeman1, A.I. Slabunov2, M. Ovtcharova3, M.A. Coble4, N.S. Serebryakov5, and U. Schaltegger3 1Department of Earth Sciences, 1272 University of Oregon, Eugene, Oregon 97403, USA 2Institute of Geology, Karelian Research Centre, RAS, Pushkinskaya 11, Petrozavodsk 185910, Russia 3Department of Earth Sciences, University of Geneva, 13, Rue des Maraîchers, 1205 Geneva, Switzerland 4Geological Sciences Department, Stanford University, 367 Panama Street, Stanford, California 94305, USA 5Institute of Petrography, Mineralogy and Geology of Ore Deposits, RAS, Staromonetny per. 35, Moscow 119017, Russia ABSTRACT on the continent during the early Paleoproterozoic. As the Baltic Shield The presence of Paleoproterozoic glacial diamictites deposited was located at low latitudes (latitudes 20°–30°; Bindeman et al., 2010; at low latitudes on different continents indicates that three or four Salminen et al., 2014) when the low-δ18O hydrothermally altered rocks worldwide glaciations occurred between 2.45 and 2.22 Ga. During formed, reconstructed low δ18O (as low as −40‰) of original meteoric that time period, the first atmospheric oxygen rise, known as the water suggest low-latitude, snowball Earth glaciations, which is in line Great Oxidation Event (GOE), occurred, implying a potential con- with deposition of glacial diamictites at low latitudes (Evans et al., 1997). nection between these events. Herein we combine triple oxygen iso- Thus, by applying precise U-Pb geochronology to intrusions with low- tope systematics and in situ and high-precision U-Pb zircon ages of δ18O signatures from the Baltic Shield, we can directly date the presence mafic intrusions to date two episodes of snowball Earth glaciations. of glacial ice at low latitudes in the early Paleoproterozoic. Subglacial hydrothermal alteration was induced by intrusions of high- Mg and high-Fe gabbros during the early Paleoproterozoic rifting GEOLOGICAL SETTING on the Baltic Shield, which at the time was located at low latitudes. The Sumian large igneous provinces include numerous mafic intru- The low δ18O values of hydrothermally altered rocks associated with sions that formed on the Baltic Shield as the result of global mantle plume these intrusions are attributed to high-temperature isotopic exchange activity in early Paleoproterozoic (Sumian) time (Amelin et al., 1995; between hot rock and glacial meltwater, indicating the presence of Kulikov et al., 2010). The Belomorian belt, located in Karelia, Russia, glacial ice globally. The triple oxygen isotope approach is used here between the Karelian and Kola cratons (Fig. 1), was a zone of extensive to show that the δ18O of glacial meltwaters during the dated epi- rifting from 2.44 to 2.39 Ga. The rift zone formed during plume activity sodes of snowball Earth glaciation was approximately –40‰ VSMOW and hosts numerous high-Mg mafic intrusions varying in size from dikes (Vienna standard mean ocean water). High-Mg gabbro intrusions and to large layered intrusions (Lobach-Zhuchenko et al., 1998; Bibikova et associated low-δ18O hydrothermally altered rocks formed during the al., 2004; Slabunov et al., 2011). A younger, less-voluminous episode earliest episode of snowball Earth glaciation between 2.43 and 2.41 of magmatism produced high-Fe gabbro intrusions at ca. 2.3–2.1 Ga in Ga. High-Fe gabbro from the Khitoostrov locality (Karelia, Russia) the same region (Salminen et al., 2014; Stepanova et al., 2015). Extreme hosts a δ18O value of −27.3‰ and is dated here at 2291 ± 8 Ma. This oxygen isotope depletions are found in the Belomorian belt and are com- age is interpreted to reflect the interaction between the intrusion monly associated with these mafic intrusions (Fig. 1). The distribution and glacial meltwaters during the third Paleoproterozoic glaciation, of low δ18O values has been mapped out as halos (bullseyes) around the which occurred after the GOE. intrusions (Bindeman et al., 2014). This indicates that short-lived (103–104 yr) high-temperature meteoric hydrothermal systems were driven by the INTRODUCTION heat of cooling intrusions. The lowest δ18O measured in terrestrial silicates The occurrence of several glacial diamictites in Paleoproterozoic strati- and very low δD (as low as −27‰ and -233‰ respectively; Bindeman et graphic sections around the world indicate that three or four episodes of al., 2014) uniquely fingerprint high-temperature interaction between the global glaciation occurred between 2.45 and 2.22 Ga (Hoffman, 2013, rocks and glacial meltwaters. We report new low δ18O values from the and references therein). The ages of individual glaciations are currently southernmost locality within the Belomorian belt, Kiy Island, Karelia, constrained by the relative position of glacial diamictites with respect to which extends the occurrence of low-δ18O rocks to a 500-km-long zone dated tuffs, lavas, and intrusions, and disappearance of mass-independent that traces the elongation of the Paleoproterozoic rift that was supposedly fractionation of sulfur isotopes resulting in several cross-continental corre- operating under continental ice sheets. The Belomorian belt underwent lation schemes (Kirschvink et al., 2000; Hoffman, 2013; Rasmussen et al., regional metamorphism at 1.9 Ga, which did not alter the original low- 2013). Firmer time constraints are needed to correlate glaciations across δ18O signature of hydrothermally altered rocks. continents and to understand the cause-effect connection between global cooling and the Great Oxidation Event (GOE) bracketed between 2426 TRIPLE OXYGEN ISOTOPE GEOCHEMISTRY Ma and 2308 Ma (Bekker et al., 2004; Gumsley et al., 2017). In contrast Here we use low-δ18O rocks from the Belomorian belt to derive the to using relative ages, in this study we attempt to obtain absolute ages of δ18O of Paleoproterozoic meteoric water by employing Δ17O-δ18O sys- the glaciations using high-precision U-Pb geochronology of intrusions tematics of the hydrothermally altered rocks (Herwartz et al., 2015). The that induced subglacial hydrothermal alteration. We used conventional relationship between δ18O and δ17O in mantle, crustal rocks, and meteoric and triple oxygen isotope systematics of low-δ18O hydrothermally altered water is exponential, with the power varying between 0.528–0.529 (Luz rocks associated with these intrusions from the Baltic Shield to reconstruct and Barkan, 2010; Pack and Herwartz, 2014). The offset from a function the δ18O of original meteoric water and to show the presence of glacial ice with slope of 0.5305 that approximates high-temperature equilibrium can GEOLOGY, July 2017; v. 45; no. 7; p. 667–670 | Data Repository item 2017221 | doi:10.1130/G38759.1 | Published online 08 May 2017 ©GEOLOGY 2017 Geological | Volume Society 45 | ofNumber America. 7 For| www.gsapubs.org permission to copy, contact [email protected]. 667 36°E NORWAY 70°N A Kola craton BARENTS SEA +2 ‰ Baltic Sheild Belomorian Belomorian belt belt -3 ‰ WHITE 68°N -13 ‰ SEA -27 ‰ -2 ‰ A B E 18 Low δO C -1 ‰ -3 ‰ zone Karelian craton -20 ‰ low δO18 hosted RUSSIA in gneisses 0 ‰ FINLAND East EuropeanPlatform low δO18 hosted in or 62°N around mafic intrusions B high-Mg gabbro: A - Height 128 East European B - Mt Dyadina -2 ‰ WHITE Platform C - Varatskoe SEA D - Kiy Island -2 ‰ high-Fe gabbro: D E-Khitoostrov 60 km Figure 1. Location of hydrothermally altered low-δ18O rocks on Baltic Shield. Each locality is shown with circle and lowest δ18O value (in ‰) found at the locality (see Bindeman and Serebryakov [2011] and Bin- deman et al. [2014] for details). Mafic intrusions used here for dating episodes of snowball Earth are shown with letters (see legend). Vari- able δ18O of rocks is interpreted to represent alteration by glacial meltwaters at variable water-to-rock ratios. be expressed in form of Δ17O = δ17O − 0.5305 × δ18O using conventional notation of δ18O and δ17O (Pack and Herwartz, 2014). Earth’s mantle has Δ17O of approximately -0.1‰ (Pack and Herwartz, 2014) and δ18O of 5.5‰ relative to VSMOW (Vienna standard mean ocean water). Meteoric water has a range of Δ17O and δ18O that is defined by the extent of distillation Figure 2. Triple oxygen isotope plot for low-δ18O rocks from Baltic (Luz and Barkan, 2010; Fig. 2). Hydrothermally altered rocks represent Shield. Curved line shows oxygen isotope composition of meteoric water. Thick gray portion of curve shows glacial meltwaters. Regres- input from two reservoirs: unaltered silicate rock and meteoric water. sion line constructed through hydrothermally altered rocks intercepts A regression line drawn through an array of analyses of hydrothermally meteoric water curve at the δ18O of original meteoric water. Range of altered rocks represents a mixing line between unaltered rock and original δ18O of hydrothermally altered rocks is due to variable water-to-rock meteoric water (Fig. 2). Because metamorphism is largely an isochemi- ratio. Water-to-rock ratio is expressed with percentage of water that reacted with rock. 90% confidence envelopes on regression lines are cal process, triple oxygen isotopic abundances in the hydrothermally shown. δ18O of Paleoproterozoic oceanic water is assumed to have δ18O altered rocks are preserved. Using the Khitoostrov locality, Herwartz et of modern-day ocean (Muehlenbachs, 1998). A: High-Mg gabbros dated al. (2015) showed that using the δ18O-Δ17O relationship enables back- at ca. 2.43–2.41 Ga interacted with meteoric water with δ18O of -39‰ ± tracking through the effect of metamorphism to deduce the δ18O of original 7‰ (for location of samples, see Fig. 1). Two out of three samples from 17 meteoric water. Details on triple oxygen isotope fractionation are given Kiy Island, Karelia, Russia, yield elevated Δ O that can be explained by 1 involvement of oceanic water in formation of hydrothermally altered in the GSA Data Repository . rocks. B: Dated here to 2291 ± 9 Ma, Khitoostrov intrusion shows Here we analyzed hydrothermally altered rocks from the low-δ18O well-defined trend between mantle and meteoric water with δ18O of localities Khitoostrov, Height 128, Varatskoe, Mount Dyadina, and Kiy −40‰ ± 3‰.