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Highly positive δ 34 S values in sulphide minerals are a common feature of shale hosted massive sulphide deposits (SHMS). Often this is attributed to near quantitative consumption of seawater sulphate, and for Paleozoic strata of the Selwyn Basin (Canada), this is thought to occur during bacterial sulphate reduction (BSR) in a restricted, euxinic water column. In this study, we focus on drill-core samples of sulphide and barite mineralisation from two Late Devonian SHMS deposits (Tom and Jason, Macmillan Pass, Selwyn Basin), to evaluate this euxinic basin model. The paragenetic relationship between barite, pyrite and hydrothermal base metal sulphides has been determined using transmitted and reflected light microscopy, and backscatter electron imaging. This petrographic framework provides the context for in-situ isotopic microanalysis (secondary ion mass spectrometry; SIMS) of barite and pyrite. These data are supplemented by analyses of δ 34 S values for bulk rock pyrite ( n = 37) from drill-core samples of un-mineralised (barren), siliceous mudstone, to provide a means by which to evaluate the mass balance of sulphur in the host rock. Three generations of barite have been identified, all of which pre-date hydrothermal input. Isotopically, the three generations of barite have overlapping distributions of δ 34 S and δ 18 O values (+22.5‰ to +33.0‰ and +16.4‰ to +18.3‰, respectively) and are consistent with an origin from modified Late Devonian seawater. Radiolarian tests, enriched in barium, are abundant within the siliceous mudstones, providing evidence that primary barium enrichment was associated with biologic activity. We therefore propose that barite formed following remobilisation of productivity-derived barium within the sediment, and precipitated within diagenetic pore fluids close to the sediment water interface. Two generations of pyrite are texturally associated with barite: framboidal pyrite (py-I), which has negative δ 34 S values (−23‰ to −28‰; n = 9), and euhedral pyrite (py-II), which has markedly more positive δ 34 S values (+8‰ to +26‰; n = 86) . We argue that stratiform pyrite and barite developed along diagenetic redox fronts, where the isotopic relationships (δ 34 S pyrite ≈ δ 34 S barite ) are explained by anaerobic oxidation of methane coupled to sulphate reduction (AOM-SR). Furthermore, the relatively narrow distribution of δ 34 S barite values is consistent with an open system model of sulphate reduction, in which reduced sulphur generation occurred with a reduced isotopic fractionation (ε 34 S = <15‰) linked to higher rates of sulphate reduction and AOM-SR. Importantly, hydrothermal sulphides (pyrite, sphalerite and galena) all post-date this diagenetic barite-pyrite assemblage, and textural and mineralogical evidence indicates barite replacement to be an important process during hydrothermal mineralisation. Neither the textures nor the documented isotopic relationships can be produced by processes operating in a euxinic water column, which represents a major departure from the conventional model for SHMS formation at Macmillan Pass. We suggest that positive δ 34 S values in sulphides, a common feature of SHMS systems both in the Selwyn Basin and throughout the geologic record, could be linked to AOM-SR. At Macmillan Pass, positive δ 34 S pyrite values developed during open system diagenesis, which was critical for rapid sulphur cycling and the development of an effective metal trap. [ABSTRACT FROM AUTHOR]

Abstract: One hundred and twenty-one micro-diamonds (<1mm) and 90 macro-diamonds (2.5mm to 3.4mm) from the Panda kimberlite (Ekati mine, Central Slave Craton, Canada) were analyzed for nitrogen content, nitrogen aggregation state (%B) and platelet and hydrogen peak areas (cm−2). Micro-diamond nitrogen concentrations range from <10at.ppm to 1696at.ppm (median=805at.ppm) and the median aggregation state is 23%B. Macro-diamonds range from <10at.ppm to 1260at.ppm (median=187at.ppm) nitrogen and have a median nitrogen aggregation of 26%B. Platelet and hydrogen peaks were observed in 37% and 79% of the micro-diamonds and 79% and 56% of the macro-diamonds, respectively. Nitrogen based time averaged residence temperatures indicate that micro- and macro-diamonds experienced similar thermal mantle residence histories, both populations displaying bimodal residence temperature distributions with a gap between 1130°C and 1160°C (at 3.5Ga residence). In addition, SIMS carbon isotopic analyses for the micro-diamonds were obtained: δ13C compositions range from −6.9‰ to +1.8‰ (median=−4.3‰). CL imaging reveals distinct growth layers that in some samples differ by >2‰, but mostly vary by <0.5‰. Comparison of only the “gem-quality” samples (n=49 micro- and 90 macro-diamonds) between the two diamond sets, indicates a statistically significant shift of +1.3‰ in average δ13C from macro- to micro-diamonds and this shift documents distinct diamond forming fluids, fractionation process or growth histories. A broad transition to heavier isotopic values is also observed in connection to decreasing mantle residence temperatures. The bimodal mantle residence temperature distribution may coincide with the transition from highly depleted shallow to more fertile deep lithospheric mantle observed beneath the Central Slave Craton. The increase in δ13C with decreasing residence temperature (proxy for decreasing depth) is interpreted to reflect diamond formation from a carbonate-bearing metasomatic fluid/melt that isotopically evolves as it percolates upward through the lithosphere. [Copyright &y& Elsevier]

Numerous geologic proxies for Earth system processes track dramatic changes at the atmosphere-lithosphere or atmosphere-ocean interface during the early Paleoproterozoic Era. The presence of a geodynamic driver for these changes and how this might have affected the deeper lithosphere is more cryptic. Here we present temporally constrained δ18O and εHf in detrital zircon from Paleoproterozoic sedimentary successions in Western Australia and Canada that chart a rapid change in the oxygen isotopic composition from <7.5‰ prior to Great Oxidation Event (GOE) to 9–11‰ by ∼2.3 Ga. Intriguingly, we show that the timing of this zircon δ18O isotopic shift directly coincides with the GOE and the rapid development of continental freeboard evidenced by the shift from predominantly subaqueous to subaerial large igneous provinces and a rapid decrease in Δ17O in shale. Importantly, no correlation exists between zircon δ18O and εHf or to known periods of enhanced tectonic reworking of sedimentary material (e.g. ∼2.3–2.2 Ga tectono-magmatic lull). We propose that the development of continental freeboard led to the appearance of an isotopically distinct sedimentary reservoir with high δ18O that was incorporated into subduction zone magmas. The sedimentary contamination of subduction zone magmas led to a globally rapid change in average continental composition as recorded by δ18O in zircon grains. [ABSTRACT FROM AUTHOR]

• The Nulliak supracrustal assemblage confirmed to be Eoarchean. • Zircon Hf isotope results indicate the presence of depleted mantle domains underneath the Saglek Block during the Eoarchean. • O-isotope signatures of Eoarchean metamorphic zircon indicate the reworking of material that experienced alteration at low temperature. The Archean Saglek Block, North Atlantic craton, is one of the oldest crustal segments preserved on Earth. This area records two granulite facies metamorphic events: one in the late-Eoarchean at ca. 3.7 Ga and one in the Neoarchean, at ca. 2.75 Ga. Here, we report the results of combined U–Pb/Hf/O in-situ isotope analyses of detrital and metamorphic zircon domains from two meta -sedimentary samples, one psammitic, and one pelitic. Both samples are components of the oldest supracrustal assemblage of the Saglek Block: the Eoarchean Nulliak supracrustal assemblage. The isotopic data presented in this study: (i) confirm deposition age of the Nulliak supracrustal assemblage before ca. 3.65 Ga, (ii) indicate local provenance of the detrital zircon, (iii) suggest deposition of the sedimentary protoliths in a convergent setting, such as an arc, (iv) crystallization of the oldest zircon during the early Eoarchean in a protolith derived from a depleted mantle component, (v) involvement of material that experienced prolonged low temperature alteration during the late Eoarchean granulite facies metamorphism and (vi) selective metamorphic overprinting during the Neoarchean granulite facies metamorphism. The new zircon U–Pb/Hf/O isotope data presented here in conjunction with previously published analyses suggest that a depleted mantle domain existed in the early Archean, as reflected in the positive εHf signature of zircon from the Saglek Block. Yet, such positive Hf isotope deviations are not the norm, but rather the exception in the preserved early Earth zircon record. In addition, mildly elevated zircon δ18O values, i.e. those ≥ 5.9 ‰, are as common in the early Archean as they are in the late-Archean zircon record, implying that crustal reworking was an active and efficient process all through the Archean. Hence, no significant variation in the nature of the reworking and broader geodynamic processes occurred during this period of time. [ABSTRACT FROM AUTHOR]

When and how Earth's ancient crust – the cratons – became underpinned by cool, thick lithospheric mantle roots capable of hosting diamonds are among the most controversial aspects of Archean geology. Alluvial diamonds in cratonic sedimentary cover rocks, whose minimum age is determined by detrital-zircon geochronology, provide a unique perspective on this topic. A new discovery of a diamond-bearing quartz-pebble conglomerate from the northern Slave craton, Canada contains detrital zircon with a restricted U-Pb age distribution that has a dominant peak at ∼2.94 Ga and depositional age of ∼2.83 Ga. Pressure-temperature constraints derived from an olivine-diamond host pair lie on a conductive Mesoarchean geotherm of ∼36–38 mW/m2, comparable to the coolest modern lithospheric geotherms. This result is at odds with a hotter geothermal gradient related to nearby Mesoarchean komatiites. We propose a model whereby early building blocks for cratons were small but with deep cool roots that formed by slab-stacking, and were subsequently juxtaposed with regions of thinner, hotter lithosphere. This heterogeneous initial architecture later amalgamated and thickened through lateral accretion forming the more uniformly thick cratonic lithosphere observed today. Thermal modelling indicates that stacking/thickening of cool initial lithosphere into a lithospheric keel thick enough to stabilise diamonds is the most likely way of generating the observed geotherm by Mesoarchean times. • Detrital zircon in host sedimentary rock has a Mesoarchean U-Pb age of >2.83 Ga. • Oxidizing diamond-forming fluids had crustal component (non-mantle δ 13 C- δ 15 N). • PT constraints show cool conductive Mesoarchean mantle geotherm of ∼36–38 mW/m2. • Early cratonic building blocks were small with deep cool roots. • These blocks likely formed through slab-stacking. [ABSTRACT FROM AUTHOR]

Abstract: A suite of 30 eclogite and pyroxenite xenoliths recovered from the Jurassic Victor kimberlite in the western Superior Province are investigated to determine their formation and emplacement in the sub-continental lithospheric mantle (SCLM). The samples have a wide compositional range, including low-Mg and high-Mg varieties. The low-Mg eclogites have a shallow origin as plagioclase-bearing protoliths that were subsequently subducted and emplaced into the SCLM. This is supported by their generally flat MREE to HREE compositions, the presence of kyanite and a positive Eu anomaly in the kyanite-bearing sample, as well as δ 18O in three low-Mg eclogites that are higher than the pristine mantle value. LREE depletion in the low-Mg eclogites, along with unradiogenic 87Sr/86Sr indicate that they were not affected by widespread metasomatism after emplacement in the SCLM. The high-Mg eclogites and pyroxenites have compositional characteristics that require a distinct origin to the low-Mg eclogites. Their bulk compositions, LREEN-enriched trace element patterns and in particular, occurrence of unradiogenic 187Os/188Os in pyroxenite, is consistent with formation by reaction of broadly siliceous melts (generated from the melting of low-Mg eclogites) with depleted peridotite. A subduction origin of the eclogites studied here is consistent with seismic and field-based studies that have reported terrane accretion by successive subduction of the west–east orientated terranes in the western Superior Province. Although the timing of eclogite and pyroxenite formation could not be constrained, radiogenic 187Os/188Os require long-term isolation from the convecting mantle and supports a Neorchaean age for their formation. [Copyright &y& Elsevier]

Abstract: Size frequency distributions are the principal tool for predicting the macro-diamond grade of new kimberlite discoveries, based on micro-diamonds (i.e., diamond ≤0.5mm) recovered from small exploration samples. Lognormal size frequency distributions – as observed for the Artemisia kimberlite (Slave Craton, Canada) – suggest a common source for micro- and macro-diamonds recovered from single samples, an implication that has never been conclusively tested. We analyzed 209 diamonds between 0.2 and 2mm in size from the Artemisia kimberlite for their carbon isotopic compositions and nitrogen characteristics to determine the nature of the micro-/macro-diamond relationship. Despite overall similarity in the δ13C distributions of micro- and macro-diamonds – both are bimodal with peaks in classes −5.0 to −4.5‰ and −3.5 to −3.0‰ – rare diamonds with δ13C between −14.2 and −24.5‰ of presumed eclogitic origin are restricted to macro-diamonds, whereas positive values are only observed for micro-diamonds. In addition, a shift in main mode and median value in δ13C of about +1‰ is observed for micro- relative to macro-diamonds. Fundamental differences between micro- and macro-diamonds at Artemisia were revealed through the analysis of nitrogen concentrations: 68% of micro-diamonds are Type II (“nitrogen free”) versus 21% of macro-diamonds, and only 19% of micro-diamonds have nitrogen contents >100atomic ppm versus 43% of macro-diamonds. Similarly, the presence of a detectable hydrogen related peak (at 3107cm−1) increases from 40% for micro-diamonds to 94% for macro-diamonds. Previous studies on diamond populations from individual deposits have documented that single batches of ascending kimberlite or lamproite magma sample multiple diamond subpopulations formed during distinct growth events in compositionally variable sources and at various depth levels. The Artemisia data clearly show that even over a fairly narrow size interval, spanning the micro- to macro-diamond transition, the specific diamond subpopulations present and their relative proportions may vary significantly with diamond size. At Artemisia, we conclude that the observed lognormal size distribution is not a reflection of an entirely common origin of micro- and macro-diamonds. [Copyright &y& Elsevier]

• Large new time–space geochronological, geochemical and isotopic dataset used to analyse crustal evolution in the Archean south-east Superior Craton. • We propose a new geodynamic model for the Neoarchean south-east Superior Craton. • δ18O, ΔFMQ, and other zircon and whole-rock geochemical proxies, indicate two geodynamic processes in operation at different times. • The transition from plume-rift to subduction tectonics occurred at ca. 2704–2695 Ma. • The advent of subduction would have had major implications for the Neoarchean environment via increased oxidation of associated magmatism and crustal recycling. The formation of the continental crust in the early Earth, and the geodynamics that drove it, are fundamental to understanding the evolution of our planet, but remain intensely debated. Here, we analysed 148 archived zircon separates of magmatic rocks for in-situ zircon U-Pb-Hf-O-trace element data, and compiled published geochronological, whole-rock geochemical and Sm-Nd isotopic data from across the south-east Superior Craton, Canada. We combine these data spatially and temporally to investigate the crustal evolution of this part of the craton in the Neoarchean, with a view to understanding its tectonic setting. In terms of zircon data, at >2704–2695 Ma, the central and north-west Abitibi demonstrate more juvenile εHf, light to mantle-like δ18O, lower (Eu/Eu*)/Y (drier/shallower crust), reduced ΔFMQ, less continental initial-U (U i)/Yb, and more mantle-like U i /Nb, relative to surrounding crust, which contains older, ca. 2800–2750 Ma inherited and magmatic zircon ages. Furthermore, whole-rock Sr/Y and La/Sm demonstrate the presence of a high Sr/Y TTG component (mainly intrusive) surrounding zones of low Sr/Y (mainly volcanic) component, the latter of which shows contamination trends with Mesoarchean crust. We interpret this to represent a continental-rift setting, driven by plume magmatism as represented by multiple komatiite suites. At ca. 2704–2695 Ma, there is a marked transition in multiple datasets, including; increases in δ18O, (Eu/Eu*)/Y, ΔFMQ, U i /Yb and U i /Nb data, together with more distinct arc-like trace element trends, suggesting a transition to north-dipping subduction. This process closed the rift system and initiated orogenesis. Subsequently, this study constrains the geodynamic setting which formed the majority of Neoarchean continental crust in the south-east Superior Craton, and the timing at which it transitioned to subduction. If these findings are replicated in other cratons, it suggests that plate tectonics was active by, or started at, ca. 2.7 Ga. [ABSTRACT FROM AUTHOR]