Archean-Proterozoic Boundary Research Articles

A proposed chronostratigraphic Archean–Proterozoic boundary: Insights from the Australian stratigraphic record

The Mount Bruce Supergroup (MBS) in Western Australia has been central to debates surrounding Precambrian stratigraphic subdivision and is one of the few relatively conformable successions that records both the chronometric Archean-Proterozoic boundary (APB) and the Great Oxidation Event/Episode (GOE) which has previously been proposed as the most practical basis for a chronostratigraphic boundary. Current understanding of the lithostratigraphy, chemostratigraphy, and geochronology of the MBS supports the placement of a chronostratigraphic APB in the vicinity of the contact between the Hamersley and Turee Creek Groups, although a precise boundary awaits the generation and integration of more detailed chemostratigraphic records of the GOE with the physical record. The chronometric APB as presently defined at 2500 Ma will suffice until consensus is reached regarding a revised definition based on the GOE, and reconciliation of the physical and proxy records of this event. However, the Australian stratigraphic record suggests placement of the APB either at the base of the first glacial diamictite, or at the top of the last banded iron formation (BIF) below the GOE. The first option would exclude some BIF from the Siderian, which in both cases would be the terminal period of the Archean, whereas the second option places all diamictite in a new period at the start of the Paleoproterozoic. It is further proposed that this new period be named to primarily reflect this glacial influence, rather than for the influence of the GOE which relies on less obvious laboratory-based proxies for its identification.

Transpression or polyphase deformation along craton margins: Insights from the Archean–Proterozoic boundary near Sudbury, Canada

Continental margins vary in composition and configuration, resulting in complex structural geometries during deformation. One example is the Archean–Proterozoic boundary near Sudbury, Canada, where the Creighton-Victoria Deformation Zone (CVDZ) contains both reverse dip-slip and dextral strike-slip kinematic indicators. To resolve if these structures formed during a single transpression event or multiple overprinting events, the orientation, relative timing and style of the CVDZ are compared to those of adjacent regional structures. Regional fold axes and their axial planar foliation are subparallel to the steeply-plunging stretching lineation and steeply-dipping foliation, respectively, along the CVDZ. Phase equilibria modelling of syn-tectonic staurolite-chloritoid assemblages indicates that the regional folds and foliation formed at 480–565 °C, similar to temperatures estimated for the formation of dip-slip quartz crystallographic preferred orientations (CPO) along the CVDZ. This suggests that the CVDZ, regional folds and foliation formed during the same regional folding and reverse shearing event. Major lateral lithological changes and inflections in the orientation of the craton margin controlled the formation of structures, and evidence for regional transpression, such as strike-slip kinematic indicators, steeply-plunging stretching lineation, and obliquity of regional folds with respect to major shear zones, resulted from polyphase deformation and transcurrent reactivation of the Superior craton margin.

Correction to: A Comparison of Oxygen Fugacities of Strongly Peraluminous Granites across the Archean-Proterozoic Boundary and Strongly Peraluminous Granites across the Archean-Proterozoic Transition

Correction to: A Comparison of Oxygen Fugacities of Strongly Peraluminous Granites across the Archean-Proterozoic Boundary and Strongly Peraluminous Granites across the Archean-Proterozoic Transition

Genesis of the Longwanggou iron deposit in the Yudongzi Complex, South China: Implications for the redox state of seawater at the onset of the Great oxidation Event

Banded iron formations (BIFs) deposited during the onset of the Great Oxidation Event (GOE) are crucial to understanding fundamental changes in the redox conditions of Earth’s surface environments, however, they are poorly understood in South China. Here, we investigate the petrographic, geochronological, and geochemical characteristics of a set of continuous magnetite quartzite samples from a drill core through the Longwanggou iron deposit in the Yudongzi Complex, South China. These magnetite quartzite samples exhibit diagnostic features of BIFs — 1) they comprise alternating magnetite and quartz layers at the centimeter–millimeter scales, 2) they are dominantly composed of SiO2 + Fe2O3T (87.8–95.8 wt%, average = 91.4 wt%), and 3) they are characterized by shale-normalized rare earth elements plus yttrium patterns similar to seawater. The maximum depositional age of the Longwanggou BIF (i.e., magnetite quartzite) is constrained to ca. 2.49 Ga based on U–Pb dating of detrital zircon grains from chlorite schist that is intercalated with the BIF. The metamorphic complex of the Yudongzi Complex that hosts the BIF was intruded by ca. 2.5–2.45 Ga granitoid gneisses, indicating that the BIF must be older than 2.45 Gyr. Accordingly, the depositional age of the Longwanggou BIF is constrained to ca. 2.49–2.45 Ga, coincident with the onset of the GOE. The positive Eu anomalies (1.3–4.2; average = 2.2) that occur throughout BIF samples suggest a continuous flux of submarine, high-temperature hydrothermal fluids to the ocean at the Archean–Proterozoic boundary. Based on elemental mass balance, the proportion of submarine, high-temperature hydrothermal fluids that contributed to the deposition of the BIF was less than 0.1%. Unlike BIFs deposited during the GOE, the Longwanggou BIF samples lack negative Ce anomalies, suggesting that the water column in which the BIF was deposited was at least locally anoxic during the onset of the GOE. Identification of the Longwanggou BIF provides new constraints for the Precambrian evolution of South China and lays the foundation for further study of the environmental changes at the Archean–Proterozoic boundary.

Trend and Phasing of Carbonate Accumulation in the History of the Earth

In the geological history of the Earth, there has been a significant change in the composition and areas of carbonate accumulation, which reflects the general evolution of the formation of carbonate deposits, paleogeographic and geochemical settings, and different stages of this evolution. At the Archean–Proterozoic boundary, acidic conditions in water reservoirs changed to moderately acidic, which resulted in the appearance of iron carbonates. The second milestone is the Siderian–Rhyacian boundary, when intensive assimilation of CO2 led to the development of alkaline conditions in water reservoirs and the accumulation of Mg carbonates. Carbonate accumulation itself occurred in the inland water reservoirs. The third milestone is the Vendian–Cambrian boundary, when skeletal fauna appeared, and carbonate accumulation became largely biogenic. Along with the basins of the continental block, there appeared new oceanic areas of carbonate accumulation, where the skeletons of nekton organisms became a significant supplier of material. The fourth milestone—the Paleozoic–Mesozoic—was the beginning of the accumulation of the proper oceanic deep-water carbonate sediments. The fifth milestone is associated with the Cretaceous period, when oceanic carbonate accumulation sharply increased due to accumulation of planktonic organisms.

DIRECTIONALITY AND PHASING OF CABONATE ACCUMULATION IN THE EARTH HISTORY

In the geological history of the Earth, there has been a significant change in the composition and areas of formation of carbonate deposits, which reflects the general evolution of methods, paleogeographic and geochemical settings during this time, and different stages in this evolution. At the Archean-Proterozoic boundary, acidic conditions in impounded bodies changed to moderately acidic, which is seen in the appearance of iron carbonates. The second frontier is the Siderian-Rhyacian boundary, when intensive assimilation of CO2 led to the formation of alkaline conditions in impounded bodies and the accumulation of magnesian carbonates. Carbonate accumulation itself occurred in the impounded bodies of continental block. The third milestone is the Vendian-Cambrian boundary, when skeletal fauna appeared, and carbonate accumulation became largely biogenic. Along with the basins of the continental block, there were new oceanic areas of formation of carbonate sediments, where the skeletons of nekton organisms became a significant supplier of material. The fourth boundary – Paleozoic-Mesozoic one – was the beginning of the accumulation of the proper oceanic deep-water carbonate sediments. The fifth milestone is associated with the Cretaceous period, when oceanic carbonate accumulation sharply increased due to planktonic organisms.

Mesoproterozoic surface oxygenation accompanied major sedimentary manganese deposition at 1.4 and 1.1Ga.

Manganese (Mn) oxidation in marine environments requires oxygen (O2 ) or other reactive oxygen species in the water column, and widespread Mn oxide deposition in ancient sedimentary rocks has long been used as a proxy for oxidation. The oxygenation of Earth's atmosphere and oceans across the Archean-Proterozoic boundary are associated with massive Mn deposits, whereas the interval from 1.8-1.0Ga is generally believed to be a time of low atmospheric oxygen with an apparent hiatus in sedimentary Mn deposition. Here, we report geochemical and mineralogical analyses from 1.1Ga manganiferous marine-shelf siltstones from the Bangemall Supergroup, Western Australia, which underlie recently discovered economically significant manganese deposits. Layers bearing Mn carbonate microspheres, comparable with major global Mn deposits, reveal that intense periods of sedimentary Mn deposition occurred in the late Mesoproterozoic. Iron geochemical data suggest anoxic-ferruginous seafloor conditions at the onset of Mn deposition, followed by oxic conditions in the water column as Mn deposition persisted and eventually ceased. These data imply there was spatially widespread surface oxygenation ~1.1Ga with sufficiently oxic conditions in shelf environments to oxidize marine Mn(II). Comparable large stratiform Mn carbonate deposits also occur in ~1.4Ga marine siltstones hosted in underlying sedimentary units. These deposits are greater or at least commensurate in scale (tonnage) to those that followed the major oxygenation transitions from the Neoproterozoic. Such a period of sedimentary manganogenesis is inconsistent with a model of persistently low O2 throughout the entirety of the Mesoproterozoic and provides robust evidence for dynamic redox changes in the mid to late Mesoproterozoic.

Evidence that the GOE was a prolonged event with a peak around 1900 Ma

The great oxygenation event (GOE), the first of two major rises in atmospheric oxygen in Earth history, was initially placed near the Archean-Proterozoic boundary (∼2500 Ma). More recently, the position of the GOE has been moved to between 2500 and 2300 Ma so as to coincide with the loss of the MIF sulfur isotope signal due to the creation of the Earth's ozone layer at that time. Here we present a revised interpretation of the history of atmospheric oxygen in the Archean and Proterozoic, based on a multi-geochemical proxy approach. Integration of a large database of analyses of redox sensitive elements in sedimentary pyrite (Se, Co, Mo), the matrix of black shales (U, Mo), and the temporal evolution of redox sensitive minerals, suggest Earth's first oxygenation was prolonged, reaching a peak between 2000 and 1700 Ma. Rather than a relatively short-lived event from 2500 to 2300 Ma, we suggest an alternative profile for Earth's atmospheric O2, i.e., an unsteady start to the rise in oxygen around 2700 Ma that undulated for approximately a billion years, reaching a peak around 1900 Ma before a plunge in the Proterozoic Eon after 1700 Ma.

Earth's new tectonic regime at the dawn of the Paleoproterozoic: Hf isotope evidence for efficient crustal growth and reworking in the São Francisco craton, Brazil

Abstract A zircon Hf isotope data set from Archean and Paleoproterozoic magmatic and metasedimentary rocks of the southern São Francisco craton (Brazil) is interpreted as evidence of accretionary and collisional plate tectonics since at least the Archean-Proterozoic boundary. During the Phanerozoic, accretionary and collisional orogenies are considered the end members of different plate tectonic settings, both involving preexisting stable continental lithosphere and consumption of oceanic crust. However, mechanisms for the formation of continental crust during the Archean and Paleoproterozoic are still debated, with the addition of magmatic rocks to the crust being explained by different geodynamic models. Hf isotopes can be used to quantify the proportion of magmatic addition into the crust: positive εHf values are usually interpreted as indications of magmatic input from the mantle, whereas crust-derived rocks show more negative εHf. We show that the crust of the amalgamated Paleoproterozoic tectonostratigraphic terranes that make up the southern São Francisco craton were generated from different proportions of mantle and crustal isotopic reservoirs. Plate tectonic processes are implied by a consistent sequence of events involving (1) the generation of juvenile subduction-related magmatic arc rocks, followed by (2) collisional orogenesis and remelting of older crust, and (3) post-collisional bimodal magmatism.

Geochemistry of metasedimentary clastic rocks from Dhanjori and Badampahar Groups, Singhbhum Craton, Eastern India: implications for tectonic setting and Archean–Proterozoic boundary

Geochemistry of metasedimentary clastic rocks from Dhanjori and Badampahar Groups, Singhbhum Craton, Eastern India: implications for tectonic setting and Archean–Proterozoic boundary

Petrogenesis and tectonic implications of the ca. 2.5 Ga granitoid assemblage in the eastern Zhongtiao region, southern North China Craton

The key to unraveling the crustal evolution and extensive magmatism of the North China Craton (NCC) at the Archean–Proterozoic boundary (ca. 2.5 Ga) is to investigate the petrogenesis of the volumetrically dominant granitoids. In this contribution, we report new petrological, mineralogical, and geochemical data for the Huping and Luojiahe trondhjemites, which are ca. 2.5 Ga granitoid assemblages from the eastern Zhongtiao region of the southern NCC. The Huping trondhjemites have high SiO2 and Na2O contents and high (La/Yb)N and Sr/Y values, which are consistent with most Tonalite–trondhjemite–granodiorite (TTG) rocks worldwide. Evolved whole-rock Nd and Hf isotopic compositions indicate that the Huping trondhjemites were derived from partial melting of preexisting ca. 2.7 Ga mafic crust, and high MgO, Cr, and Ni contents suggest the possible involvement of depleted mantle in their source region. Integrated petrological, mineralogical, and geochemical data indicate that the Huping trondhjemites were likely formed by partial melting of delaminated lower crust. In contrast, the Luojiahe trondhjemites are Na- and Si-rich with negative Eu and Sr anomalies. Their geochemical characteristics suggest that they were likely formed by partial melting of low-K mafic crust that was shallower than the source of the Huping trondhjemites. The higher LREE content of allanite from the Luojiahe trondhjemites relative to that from the Huping trondhjemites indicates the Luojiahe trondhjemites formed at a higher melting temperature. The occurrence of the two types of coeval granitoids indicates that ca. 2.5 Ga crustal anatexis occurred at different depths. Furthermore, a strong heat flux at the base of the southern NCC at ca. 2.5 Ga is evident, which might have been introduced by mafic magma underplating. Based on our new data and the records of coeval magmatism in the region, this study suggests that lithospheric delamination and large-scale mantle-derived magma underplating beneath the thickened crust could have played important roles in the extensive magmatism of the southern NCC at the Archean–Proterozoic boundary.

Isotopic Traces of Atmospheric O2 in Rocks, Minerals, and Melts

Research Article| January 01, 2021 Isotopic Traces of Atmospheric O2 in Rocks, Minerals, and Melts Andreas Pack Andreas Pack Georg-August-Universität Göttingen, Geowissenschaftliches Zentrum, Goldschmidtstraße 1 37077 Göttingen, Germany * E-mail: apack@uni-goettingen.de Search for other works by this author on: GSW Google Scholar Reviews in Mineralogy and Geochemistry (2021) 86 (1): 217–240. https://doi.org/10.2138/rmg.2021.86.07 Article history first online: 04 Jan 2021 Cite View This Citation Add to Citation Manager Share Icon Share Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation Andreas Pack; Isotopic Traces of Atmospheric O2 in Rocks, Minerals, and Melts. Reviews in Mineralogy and Geochemistry 2021;; 86 (1): 217–240. doi: https://doi.org/10.2138/rmg.2021.86.07 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyReviews in Mineralogy and Geochemistry Search Advanced Search In this chapter I review some of the knowledge about the oxygen isotope exchange between air O2, rocks, minerals, melts, including some technical products. The rise of free atmospheric molecular oxygen since the great oxygenation event at the Archean–Proterozoic boundary 2.3 Ga ago (Farquhar et al. 2000; Luo et al. 2016) was one of the major events of the Earth environment. The triple oxygen isotope composition of atmospheric O2 provides exciting insights into the Earth atmospheric composition and biosphere productivity (Bender et al. 1994; Luz et al. 1999, 2014; Luz... You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Petrogenesis of the Kanker Granites From the Bastar Craton: Implications for Crustal Growth and Evolution During the Archean-Proterozoic Transition

Archean-Proterozoic boundary represents a significant transitional phase in the Earth’s history. Bastar Craton is one of the major Archean cratons in the Indian subcontinent with voluminous granites, supracrustal rocks and tectonic belts. Malanjkhand, Dongargarh, and Kanker are the three major granitic plutons emplaced during the Archean-Proterozoic transition in the Bastar Craton, and this study is confined to the granites of Kanker pluton. Based on geochemical systematics, the Kanker granites are classified into sanukitoids, biotite and two-mica granites and hybrid granites. The compositional diversity of the Kanker granites is attributed to two end-member sources, i.e., the enriched mantle and an older felsic crust, and the interactions between them. The sanukitoids were derived from an enriched mantle source that was metasomatized by the subducted sediments. Heat supplied by the sanukitoid magmas induced the crustal melting to form the biotite and two-mica granites. The interaction between these two mutually end-member sources, i.e., the enriched mantle and an older felsic crust, resulted in the formation of hybrid granites. The evolution of the Kanker granites can be accounted for a transitional geodynamic environment, involving subduction and collisional tectonics during the Archean-Proterozoic transition.

The age and origin of cratonic lithospheric mantle: Archean dunites vs. Paleoproterozoic harzburgites from the Udachnaya kimberlite, Siberian craton

Cratonic lithospheric mantle is believed to have been formed in the Archean, but kimberlite-hosted coarse peridotites from Udachnaya in the central Siberian craton typically yield Paleoproterozoic Re-depletion Os isotope ages (TRD). By comparison, olivine megacrysts from Udachnaya, sometimes called “megacrystalline peridotites”, often yield Archean TRD ages, but the nature of these rare materials remains enigmatic. We provide whole-rock (WR) Re-Os isotope and PGE analyses for 24 olivine-rich xenoliths from Udachnaya as well as modal and petrographic data, WR and mineral major and trace element compositions. The samples were selected based on (a) high olivine abundances in hand specimens and (b) sufficient freshness and size to yield representative WR powders. They comprise medium- to coarse-grained (olivine < 1 cm) dunites, a megacrystalline (olivine > 1 cm) dunite, olivine megacrysts and low-orthopyroxene (11–21% opx) harzburgites equilibrated at 783–1154 °C and 3.9–6.5 GPa; coarse dunites have not been previously reported from Udachnaya; two xenoliths contain ilmenite. The harzburgites and dunites have similar WR variation ranges of Ca, Al, Fe, Cr and Mg# (0.917–0.934) typical of refractory cratonic peridotites, but the dunites tend to have higher MgO, NiO and Mg/Si. Mineral abundances and those of Ca and Al are not correlated with Mg#WR; they are not due to differences in melting degrees but are linked to metasomatism. Several samples with high 187Re/188Os show a positive linear correlation with 187Os/188Os with an apparent age of 0.37 Ga, same as eruption age of host kimberlite. Robust TRD ages were obtained for 16 xenoliths with low 187Re/188Os (0.02–0.13). TRD ages for low-opx harzburgites (1.9–2.1 Ga; average 2.0 ± 0.1 Ga, 1 σ) are manifestly lower than for dunites and megacrysts (2.4–3.1 Ga); the latter define two subsets with average TRD of 2.6 ± 0.1 Ga and 3.0 ± 0.1 Ga, and TMA of 3.0 ± 0.2 Ga and 3.3 ± 0.1 Ga, respectively. Differences in olivine grain size (coarse vs. megacrystalline) are not related to age. The age relations suggest that the dunites and megacrysts could not be produced by re-melting of harzburgites, e.g. in arc settings, nor be melt channel materials in harzburgites. Instead, they are relict fragments of lithospheric mantle formed in the Archean (likely in two events at or after 2.6 Ga and 3.0 Ga) that were incorporated into cratonic lithosphere during the final assembly of the Siberian craton in the Paleoproterozoic. A multi-stage formation of the Siberian lithospheric mantle is consistent with crustal basement ages from U-Pb dating of zircons from crustal xenoliths at Udachnaya and detrital zircons from the northern Siberian craton (1.8–2.0, 2.4–2.8 and 3.0–3.4 Ga). The new data from the Siberian and other cratons suggest that the formation of strongly melt-depleted cratonic lithosphere (e.g. Mg# ≥0.92) did not stop at the Archean-Proterozoic boundary as is commonly thought, but continued in the Paleoproterozoic. The same may be valid for the transition from the ‘Archean’ (4–2.5 Ga) to modern tectonic regimes.

Geochemical Environments of Precambrian Sedimentation

Distribution of carbonate sediments of different composition in the Precambrian section is reviewed. It is shown that calcium carbonates prevailed in the Archean–initial Early Proterozoic, and iron carbonates dominated in the banded iron formations. Dolomites and magnesites were more widespread in the Proterozoic. Such distribution suggests an acid reductive geochemical environment in the Archean–Early Proterozoic, which determined the corresponding character of continental weathering: appearance of dissolved forms of Fe and silica that were transported to ancient oceans and deposited, resulting in the formation of ferruginous–quartzites or jaspilites (banded iron formations). Large-scale and almost explosive development of photosynthesizing cyanobacteria at the Archean–Proterozoic boundary stipulated, first, the utilization of carbon dioxide and, correspondingly, the change of acid environment into alkaline, leading to the formation of magnesian carbonates; and, second, to the appearance of free oxygen.

Corrigendum to: ‘A Comparison of Oxygen Fugacities of Strongly Peraluminous Granites across the Archean–Proterozoic Boundary’

Corrigendum to: ‘A Comparison of Oxygen Fugacities of Strongly Peraluminous Granites across the Archean–Proterozoic Boundary’

Strongly Peraluminous Granites across the Archean–Proterozoic Transition

AbstractStrongly peraluminous granites (SPGs) form through the partial melting of metasedimentary rocks and therefore represent archives of the influence of assimilation of sedimentary rocks on the petrology and chemistry of igneous rocks. With the aim of understanding how variations in sedimentary rock characteristics across the Archean–Proterozoic transition might have influenced the igneous rock record, we compiled and compared whole-rock chemistry, mineral chemistry, and isotope data from Archean and Paleo- to Mesoproterozoic SPGs. This time period was chosen as the Archean–Proterozoic transition broadly coincides with the stabilization of continents, the rise of subaerial weathering, and the Great Oxidation Event (GOE), all of which left an imprint on the sedimentary rock record. Our compilation of SPGs is founded on a detailed literature review of the regional geology, geochronology, and inferred origins of the SPGs, which suggest derivation from metasedimentary source material. Although Archean and Proterozoic SPGs are similar in terms of mineralogy or major-element composition owing to their compositions as near-minimum melts in the peraluminous haplogranite system, we discuss several features of their mineral and whole-rock chemistry. First, we review a previous analysis of Archean and Proterozoic SPGs biotite and whole-rock compositions indicating that Archean SPGs, on average, are more reduced than Proterozoic SPGs. This observation suggests that Proterozoic SPGs were derived from metasedimentary sources that on average had more oxidized bulk redox states relative to their Archean counterparts, which could reflect an increase in atmospheric O2 levels and more oxidized sedimentary source rocks after the GOE. Second, based on an analysis of Al2O3/TiO2 whole-rock ratios and zircon saturation temperatures, we conclude that Archean and Proterozoic SPGs formed through partial melting of metasedimentary rocks over a similar range of melting temperatures, with both ‘high-’ and ‘low-’temperature SPGs being observed across all ages. This observation suggests that the thermo-tectonic processes resulting in the heating and melting of metasedimentary rocks (e.g. crustal thickening or underplating of mafic magmas) occurred during generation of both the Archean and Proterozoic SPGs. Third, bulk-rock CaO/Na2O, Rb/Sr, and Rb/Ba ratios indicate that Archean and Proterozoic SPGs were derived from partial melting of both clay-rich (i.e. pelites) and clay-poor (i.e. greywackes) source regions that are locality specific, but not defined by age. This observation, although based on a relatively limited dataset, indicates that the source regions of Archean and Proterozoic SPGs were similar in terms of sediment maturity (i.e. clay component). Last, existing oxygen isotope data for quartz, zircon, and whole-rocks from Proterozoic SPGs show higher values than those of Archean SPGs, suggesting that bulk sedimentary 18O/16O ratios increased across the Archean–Proterozoic boundary. The existing geochemical datasets for Archean and Proterozoic SPGs, however, are limited in size and further work on these rocks is required. Future work must include detailed field studies, petrology, geochronology, and constraints on sedimentary source ages to fully interpret the chemistry of this uniquely useful suite of granites.

Phenomenon of the Evolution of Hydrothermal Fluids of Mineral Formation at the Archean-Proterozoic Boundary

Based on isotope-geochemical study of mineral-forming fluid of the gold and uranium deposits of two megablocks of the Ukrainian shield, a significant difference between the hydrothermal systems of the Archean and Proterozoic (3042—1750 million years) has been established. The study included the experimental determination of the hydrogen, oxygen and carbon isotopic composition, and the ratio of water and carbon dioxide of fluids of gas—liquid inclusions in quartz, pyrite and feldspars.

Slow Cooling at Higher Temperatures Recorded within High-PMafic Granulites from the Southern Granulite Terrain, India: Implications for the Presence and Style of Plate Tectonics near the Archean–Proterozoic Boundary

Slow Cooling at Higher Temperatures Recorded within High-<i>P</i>Mafic Granulites from the Southern Granulite Terrain, India: Implications for the Presence and Style of Plate Tectonics near the Archean–Proterozoic Boundary

Pervasively anoxic surface conditions at the onset of the Great Oxidation Event: New multi-proxy constraints from the Cooper Lake paleosol

Oceanic element inventories derived from marine sedimentary rocks place important constraints on oxidative continental weathering in deep time, but there remains a scarcity in complementary observations directly from continental sedimentary reservoirs. This study focuses on better defining continental weathering conditions near the Archean-Proterozoic boundary through the multi-proxy (major and ultra-trace element, Fe and Cr stable isotopes, µ-XRF elemental mapping, and detrital zircon U-Pb geochronology) investigation of the ca. 2.45 billion year old (giga annum, Ga) Cooper Lake paleosol (saprolith) developed on a sediment-hosted mafic dike within the Huronian Supergroup (Ontario, Canada).Throughout the variably altered Cooper Lake saprolith, ratios of immobile elements (Nb, Ta, Zr, Hf, Th, Al, Ti) are constant, indicating a uniform pre-alteration dike composition, lack of extreme pH weathering conditions, and no major influence from ligand-rich fluids during weathering or burial metasomatism/metamorphism. The loss of Mg, Fe, Na, Sr, and Li, a signature of albite and ferromagnesian silicate weathering, increases towards the top of the preserved profile (unconformity) and dike margins. Coupled bulk rock behaviour of Fe-Mg-Mn and co-localization of Fe-Mn in clay minerals (predominantly chlorite) indicates these elements were solubilized primarily in their divalent state without Fe/Mn-oxide formation. A lack of a Ce anomaly and immobility of Mo, V, and Cr further support pervasively anoxic weathering conditions. Subtle U enrichment, if primary, is the only geochemical evidence that could be consistent with oxidative element mobilization. The leaching of ferromagnesian silicates was accompanied by variable mobility and depletion of transition metals with a relative depletion order of Fe ≈ Mg ≈ Zn > Ni > Co > Cu (Cu being significantly influenced by secondary sulfide formation). Mild enrichment of heavy Fe isotopes (δ56/54Fe from 0.169 to 0.492‰) correlating with Fe depletion in the saprolith indicates open-system loss of isotopically light aqueous Fe(II). Minor REE + Y fractionation with increasing alteration intensity, including a decreasing Eu anomaly and Y/Ho ratio, is attributed to albite breakdown and preferential scavenging of HREE > Y by clay minerals, respectively. Younger metasomatism resulted in the addition of several elements (K, Rb, Cs, Be, Tl, Ba, Sn, In, W), partly or wholly obscuring their earlier paleo-weathering trends.The behavior of Cr at Cooper Lake can help test previous hypotheses of an enhanced, low pH-driven continental weathering flux of Cr(III) to marine reservoirs between ca. 2.48–2.32 Ga and the utility of the stable Cr isotope proxy of Mn-oxide induced Cr(III) oxidation. Synchrotron µ-XRF maps and invariant Cr/Nb ratios reveal complete immobility of Cr despite its distribution amongst both clay-rich groundmass and Fe-Ti oxides. Assuming a pH-dependent, continental source of Cr(III) to marine basins, the Cr immobility at Cooper Lake indicates either that signatures of acidic surface waters were localized to uppermost and typically unpreserved regolith horizons or were geographically restricted to acid-generating point sources. However, given detrital pyrite preservation in overlying fluvial sequences, it is probable that the oxidative sulfide corrosion required to drive surface pH < 4 lagged behind in this region relative to other early Proterozoic sequences. The entire saprolith exhibits a consistently light stable Cr isotope composition (δ53/52Cr: −0.321 ± 0.038‰, 2sd, n = 34) that cannot be linked to Cr(III) oxidation and is instead interpreted to have a magmatic origin.The combined paleosol chemical signatures and preservation of detrital pyrite/uraninite indicate low atmospheric O2 during weathering at ca. 2.45 Ga in the rift-related environment of the Lower Huronian. The aqueous flux from the reduced weathering of mafic rocks was presumably characterized by a greater transition metal (Fe, Mn, Zn, Co, Ni) load with isotopically light Fe(II) compared to modern environments, as well as higher Eu/Eu* and Y/Ho than the source rock. In most models of Precambrian ocean element inventories, hydrothermal fluids are viewed as the main supplier of several metals (e.g., Fe, Zn), although the results herein suggest that a riverine metal supply may have been substantial and that using Eu-excess as a strict proxy for hydrothermal flux may be misleading in near-shore marine sedimentary deposits.