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• Extensive new Ba isotopic survey of modern barites. • First Proterozoic Ba isotopic results. • Provides genetic model for enigmatic post-Marinoan barite formation. • Provides evidence for stratified post-Marinoan ocean. • Provides arguments for why barite horizons are confined to Marinoan and not Sturtian or Paleoproterozoic strata. Measurements of triple oxygen isotope ratios in barite horizons within post-Marinoan cap carbonates have provided some of the most compelling evidence that the Marinoan glaciation was a Snowball Earth event. However, the origin of these barite horizons remains unresolved. To constrain the Ba sources, and thus formation mechanisms of these horizons, we analyzed the Ba isotope composition of post-Marinoan barite deposits from Northwest Canada, Northern Norway, Brazil and South China. We augment these analyses with a Ba isotope survey of almost 100 modern and ancient additional barite measurements, including samples from pelagic (or 'marine'), hydrothermal, terrestrial, Proterozoic stratiform and cold seep environments. Unlike modern cold seep or terrestrial barites, we find that globally-distributed post-Marinoan barites exhibit a relatively narrow isotopic range, suggesting a well-mixed, effectively limitless Ba source. Moreover, post-Marinoan deposits exhibit a similar mean Ba isotope composition to modern marine barites, which we interpret as evidence of a marine Ba source. Considered alongside existing geochemical, geological, and new Ba isotope survey data, we conclude that Ba in barite horizons was sourced from a well-mixed, Ba-replete but SO 4 -poor reservoir that accumulated during the Marinoan Snowball Earth interval. This deep Ba reservoir was then transported upward—either by ocean circulation or dolomitization of underlying cap carbonates—and was brought into contact with continental weathering-derived sulfate in a post-glacial meltwater surface layer. Thus, in addition to providing a plausible mechanism for generating globally-synchronous deposition of post-Marinoan barite horizons that reconciles all existing geochemical and geological data, our results demonstrate the utility of Ba isotopes to interrogate the origin of enigmatic barite deposits throughout the sedimentary record. [ABSTRACT FROM AUTHOR]

Unique deposits that formed in the Precambrian oceans are the Banded Iron-Formations (BIFs) which are chemical sediments comprised of alternating layers of iron- and silica-dominated precipitates. The origin of this enigmatic banding is still debated, with most favoring a primary sedimentary and a few others arguing for a secondary diagenetic origin. We here use a high-resolution integrated multi-proxy approach and report on the Ge-Si-Fe and Th-U systematics and on the isotopic composition of Cr in exceptionally pure adjacent magnetite and metachert bands from the ∼2.7 billion years old Temagami BIF, Ontario, Canada. While each of the geochemical proxies alone may be controversial, our multi-proxy approach sheds new light on their applicability, as the two types of adjacent BIF bands are each characterised by a distinct geochemical signature. The magnetite bands show super-crustal (i.e. fractionated) Ge/Si ratios approaching those of modern marine high-temperature hydrothermal fluids, unfractionated Th/U ratios similar to those of crustal rocks, and unfractionated Cr isotope ratios similar to "Bulk Silicate Earth". Adjacent metachert bands, however, show lower (i.e. unfractionated) Ge/Si ratios close to those of modern seawater, sub-crustal (i.e. fractionated) Th/U ratios, and fractionated Cr isotope ratios above those of Bulk Silicate Earth. The composition of the magnetite bands suggests that the original iron-oxyhydroxide precipitated during periods when the dissolved Ge, Th, U, and Cr inventory of ambient seawater was dominated by input from mafic crust via anoxic high-temperature hydrothermal fluids. In contrast, the composition of the metachert bands indicates that these formed during times when ambient seawater was dominated by low-temperature riverine input from an evolved landmass that despite its Neoarchean age had provided environments for oxidative Cr and U mobilization during terrestrial weathering. This suggests that habitats with oxygenic photosynthesis existed on land at least 200 million years before the Great Oxidation Event. The systematic and internally consistent difference between the geochemical signatures shown by the two types of adjacent BIF bands also demonstrates that the prominent banding of BIFs is a primary depositional feature reflecting precipitation from different watermasses and is not the result of post-depositional separation of an initially homogeneous chemical sediment. • Metachert bands in Temagami BIF provide information about Neoarchean landmasses. • Temagami BIF's metacherts show unfractionated Ge/Si and fractionated δ 53 Cr and Th/U. • Fractionated δ 53 Cr and Th/U reveal oxidative weathering on land 200 my before the GOE. • Temagami magnetite bands show fractionated Ge/Si and unfractionated δ 53 Cr and Th/U. • Internally consistent signals from various proxies show primary origin of BIF banding. [ABSTRACT FROM AUTHOR]

Abstract: Lu–Hf, Sm–Nd and in-situ clinopyroxene Sr and Pb isotope systematics, and mineral major and in-situ trace element compositions were obtained for a suite of non-diamond and diamond-bearing eclogites from the Diavik kimberlites (A154; 55 Ma old), Slave craton (Canada). Temperature estimates of last equilibration in the lithosphere for the non-diamond-bearing Diavik eclogites define two groups; low-temperature (800–1050 °C) and high-temperature eclogites (1100–1300 °C). Most diamond–eclogites indicate temperatures similar to those of the high-temperature eclogites. Isotopic and major and trace element systematics for the non-diamond- and diamond-bearing eclogites indicate overlapping chemical compositions suggesting similar rock formational histories. Calculated whole rock major and trace element abundances using chemical and modal abundances for constituent minerals exhibit broad similarities with mafic cumulates from ophiolite sequences. Most importantly the calculated whole rock eclogite compositions display positive Sr and Eu anomalies, typically interpreted as the result of plagioclase accumulation in cumulate rocks of oceanic crust sequences. Initial whole rock Hf isotopic values and in-situ Sr isotope data from clinopyroxene grains provide evidence that the eclogites were derived from precursor rocks with depleted mantle isotope characteristics. These combined results support the interpretation that the eclogites from Diavik represent remnants of subducted oceanic crust. Lu–Hf isotope systematics indicate that the oceanic protolith for the eclogites formed in the Paleoproterozoic at ∼2.1 Ga, which is in agreement with the in-situ Pb isotope data from clinopyroxene. This result also corroborates the ∼2.1 Ga Lu–Hf model ages recorded by mantle zircons from eclogite found within the Jericho kimberlite in the northern Slave Province (∼200 km northwest of Diavik). The results from both studies indicate a link between eclogite formation and Paleoproterozoic subduction of oceanic lithosphere along the present-day western margin of the Archean Slave craton. [Copyright &y& Elsevier]

Investigations on the distribution of thorium isotopes in the water column have been utilized as powerful tracers to investigate the geochemical pathways of particle-reactive material in the ocean. We measured Th (228Th, 230Th, 232Th and 234Th) isotopes in samples collected along a horizontal profile from Barrow, AK to the Canada Basin as well as two vertical profiles in the Canada Basin. The suspended particle concentrations were also measured to investigate the role of suspended particulate matter (SPM) in Th scavenging.The SPM inventory is more than an order of magnitude higher than that reported for the Alpha Ridge ice station, indicating that the SPM inventories vary over an order of magnitude in the deep Arctic Basins. The conditional partition coefficient of Th isotopes between suspended particulate matter and dissolved phase generally appears to be higher than those in non-polar regions, and this is attributed to more reactive suspended particulate matter. The residence times of total 230Th are significantly lower than those calculated for the Alpha Ridge station, while higher than those found in the Eurasian Basin and Beaufort Sea in the Canadian Basin. The calculated residence time in all the deep basins of the Arctic Ocean varied between 13±1 and 45±1 years. A reversible scavenging model applied to the distribution of Th isotopes (230Th–228Th in the deep waters and 234Th–228Th in the upper 100 m) indicates that the forward removal rate constant is higher than the value reported for the Alpha Ridge while the reverse rate constant is comparable to the Alpha Ridge station suggesting faster removal onto particulate matter and slower desorption of Th from the particulate matter. [Copyright &y& Elsevier]

• We present LASS U–Pb and Sm–Nd isotope data from the Acasta Gneiss Complex, Canada. • Sm–Nd redistribution occurred at 3.3 Ga and again during the 1.89 Ga Wopmay orogen. • Sm–Nd redistribution is a common feature of ancient poly-metamorphic terranes. • Sm–Nd mobility may compromise the initial Nd isotope composition of ancient rocks. • Analysis of Nd-rich minerals proves useful for assessing bulk-rock Nd isotope data. The whole rock 143Nd isotope record of the Acasta Gneiss Complex (AGC) has been used to argue for the development of both depleted mantle and enriched crustal reservoirs during the Hadean. The 147Sm–143Nd isotope compositions of rocks from AGC, however, fall on an array with an errorchron date of ∼3.3 Ga, despite the majority of samples yielding substantially older zircon U–Pb ages. This has led to the suggestion of a major Sm–Nd "resetting" event at this time in the AGC. To better understand the Sm–Nd bulk-rock systematics of the AGC, we investigate the U–Pb age and Sm–Nd isotope compositions of apatite and titanite from the AGC, using the laser ablation split stream (LASS) method. Apatite from all samples yield broadly similar U–Pb dates of ∼1.89 to 1.83 Ga, in agreement with resetting during the 1.9 Ga Wopmay orogen. The Sm–Nd isotope compositions of the apatite, however, lie off the WR errorchron and show a high degree of initial Nd isotope heterogeneity. Conversely, titanite Sm–Nd systematics are consistent with the ∼3.3 Ga whole rock errorchron further supporting Sm–Nd modification at ∼3.3 Ga. Taken together, these show that the oldest rocks in the AGC underwent at least two post-crystallization events (at 3.3 and 1.9 Ga) that modified the Sm–Nd isotope system. These results also demonstrate the relative mobility of the Sm–Nd isotope system during orogenic events, which may compromise the ability to determine robust bulk-rock 143Nd isotope signatures. [ABSTRACT FROM AUTHOR]