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Comments on an article in the 1998 issue of the periodical 'Physics of Plasma,' which identified neutral particle recycling as one important aspect of severe heating and cooling cycles observed in large-amplitude drift waves. Evidence that ions heat stochastically in the wave electric field before escaping confinement and reaching the vacuum chamber wall.

Abstract: The origin of variability in δ 13C values of cratonic diamonds is controversial, particularly for diamonds associated with eclogitic source rocks. The variability may be due to distinct primordial reservoirs, high-temperature isotope fractionation, recycling of crustal carbon, or any combination of the three. In contrast, the interpretation of variability in δ 18O values of silicate minerals in eclogite xenoliths and eclogitic inclusions in diamonds is less contentious—the hypothesis that they represent rocks that were exposed to weathering and hydrothermal alteration at the surface of Earth is broadly, but not universally, accepted. Here, we report high-precision SIMS oxygen isotope measurements of 15 eclogitic garnet inclusions in diamond from the Damtshaa mine, which comprises four kimberlites within the Orapa cluster of kimberlites. The results demonstrate a link between δ 18O values of inclusions and δ 13C values of their host diamonds. The δ 18OVSMOW values range from +4.7‰ to +8.8‰ and have a median value of +5.7‰, similar to the distribution exhibited by cratonic eclogite xenoliths worldwide, at the nearby Orapa Mine, and of oceanic crust. Contrary to previous suggestions, there is no evidence for a unique oxygen isotope distribution for inclusions in diamond. The oxygen isotope ratios do not correlate with garnet compositions; they have, however, a strong negative correlation with the δ 13C values of their host diamonds. We interpret this correlation to have geochemical significance. It cannot be due to primordial heterogeneities or high-temperature isotope fractionation, but is most likely due to an association between recycled near-surface crustal rocks and recycled carbon. We suggest that protoliths to the eclogites at Damtshaa that were more strongly affected by low-temperature seafloor weathering (recorded by δ 18O≥∼6‰) also have concentrations of primary organic carbon, and, therefore, low-δ 13C Damtshaa diamonds are associated with recycling of crustal carbon. [Copyright &y& Elsevier]

Abstract The North Atlantic craton hosts extensive exposures of Eoarchean crust, spread through areas of Western Greenland and Northern Labrador (Canada). Of these two areas, the crust of the Saglek Block of Northern Labrador has received far less attention from the scrutiny of modern analytical methods than its better documented Western Greenland equivalent, the Itsaq Gneiss Complex. Here, we present the first coupled trace element and U–Pb/Hf/O isotope dataset for zircon from an early TTG component of the Saglek Block. The combination of textural, elemental and isotopic in-situ analyses enables selection of the least disturbed zircon domains. From these it is demonstrated that the oldest felsic remnants exposed in the Saglek Block were emplaced 3.86 ± 0.01 billion yr (Ga) ago through partial melting of basaltic protoliths. The Hf isotope signature of the oldest zircon domains from the Saglek Block TTG indicates derivation from sources that did not undergo substantial Lu/Hf fractionation, resulting in initial Hf isotope compositions that are chondritic within uncertainty. The oxygen isotope ratios of the least disturbed zircon portions vary from 5.38 ± 0.16‰ to 6.64 ± 0.19‰ and document the interaction of the TTG protoliths with Earth's early hydrosphere at low temperature (≤150–200 °C) prior partial melting in the Eoarchean. The results support TTG production in the Eoarchean from variably hydrated basaltic protoliths. Highlights • Investigation of the older TTG component of the Saglek Block (Northern Labrador). • New in-situ zircon U–Pb/Hf/O isotope data coupled with trace element analyses. • Chondritic Hf composition of the oldest (ca. 3.86 Gyr) zircon domains. • Zircon δ 18 O VMOW values, up to 6.6 ± 0.2‰, i.e. higher than the typical mantle range. • Low-T hydrothermal alteration of the basaltic protoliths of Eoarchean TTG. [ABSTRACT FROM AUTHOR]

Major- and trace-element compositions of garnet and clinopyroxene, as well as 87 Sr/ 86 Sr in clinopyroxene and δ 18 O in garnet in eclogite and pyroxenite xenoliths from Orapa, at the western margin of the Zimbabwe craton (central Botswana), were investigated in order to trace their origin and evolution in the mantle lithosphere. Two groups of eclogites are distinguished with respect to 87 Sr/ 86 Sr: One with moderate ratios (0.7026–0.7046) and another with 87 Sr/ 86 Sr >0.7048 to 0.7091. In the former group, heavy δ 18 O attests to low-temperature alteration on the ocean floor, while 87 Sr/ 86 Sr correlates with indices of low-pressure igneous processes (Eu/Eu ∗ , Mg#, Sr/Y). This suggests relatively undisturbed long-term ingrowth of 87 Sr at near-igneous Rb/Sr after metamorphism, despite the exposed craton margin setting. The high- 87 Sr/ 86 Sr group has mainly mantle-like δ 18 O and is suggested to have interacted with a small-volume melt derived from an aged phlogopite-rich metasome. The overlap of diamondiferous and graphite-bearing eclogites and pyroxenites over a pressure interval of ∼3.2 to 4.9 GPa is interpreted as reflecting a mantle parcel beneath Orapa that has moved out of the diamond stability field, due to a change in geotherm and/or decompression. Diamondiferous eclogites record lower median 87 Sr/ 86 Sr (0.7039) than graphite-bearing samples (0.7064) and carbon-free samples (0.7051), suggesting that interaction with the – possibly oxidising – metasome-derived melt caused carbon removal in some eclogites, while catalysing the conversion of diamond to graphite in others. This highlights the role of small-volume melts in modulating the lithospheric carbon cycle. Compared to diamondiferous eclogites, eclogitic inclusions in diamonds are restricted to high FeO and low SiO 2 , CaO and Na 2 O contents, they record higher equilibrium temperatures and garnets have mostly mantle-like O isotopic composition. We suggest that this signature was imparted by a sublithospheric melt with contributions from a clinopyroxene-rich source, possibly related to the ca. 2.0 Ga Bushveld event. [ABSTRACT FROM AUTHOR]

Sublithospheric (ultra-deep) diamonds provide a unique window into the deepest parts of Earth's mantle, which otherwise remain inaccessible. Here, we report the first combined C- and N-isotopic data for diamonds from the Monastery and Jagersfontein kimberlites that sample the deep asthenosphere and transition zone beneath the Kaapvaal Craton, in the mid Cretaceous, to investigate the nature of mantle fluids at these depths and the constraints they provide on the deep volatile cycle. Both diamond suites exhibit very light δ 13 C values (down to − 26‰) and heavy δ 15 N (up to + 10.3‰), with nitrogen abundances generally below 70 at. ppm but varying up to very high concentrations (2520 at. ppm) in rare cases. Combined, these signatures are consistent with derivation from subducted crustal materials. Both suites exhibit variable nitrogen aggregation states from 25 to 100% B defects. Internal growth structures, revealed in cathodoluminescence (CL) images, vary from faintly layered, through distinct cores to concentric growth patterns with intermittent evidence for dissolution and regular octahedral growth layers in places. Modelling the internal co-variations in δ 13 C-δ 15 N-N revealed that diamonds grew from diverse C-H-O-N fluids involving both oxidised and reduced carbon species. The diversity of the modelled diamond-forming fluids highlights the complexity of the volatile sources and the likely heterogeneity of the deep asthenosphere and transition zone. We propose that the Monastery and Jagersfontein diamonds form in subducted slabs, where carbon is converted into either oxidised or reduced species during fluid-aided dissolution of subducted carbon before being re-precipitated as diamond. The common occurrence of recycled C and N isotopic signatures in super-deep diamonds world-wide indicates that a significant amount of carbon and nitrogen is recycled back to the deep asthenosphere and transition zone via subducting slabs, and that the transition zone may be dominated by recycled C and N. [ABSTRACT FROM AUTHOR]

Most apple cultivars are self-sterile and completely dependent on cross-pollination from a different cultivar in order to set fruit. Various insects may be pollinators, but the main one is the honey bee [HB] ( Apis mellifera ). However, despite the advantages of the honey bee as pollinator of many plants, it is a relatively inefficient pollinator of apple flowers. The main reason for this is the tendency of HBs to visit the apple flower from the side (sideworker), thus “stealing” nectar without touching the flower’s reproductive organs – stamens and stigma. In contrast, a bee that visits the flower from the top (topworker) contacts the flower’s reproductive organs, which results in better pollination. Due to the low pollination efficiency, few seeds are formed, and often the resulting fruit is too small to be of commercial value. Experiments conducted in Israel over the last few years have shown for the first time that adding bumblebees [BB] ( Bombus terrestris ) into pear orchards improved cross-pollination, thus increasing the number of seeds and subsequently fruit size. The goal of the present work was to test the hypothesis that adding BBs to apple orchards may improve cross-pollination. We found that adding BBs to the HBs in the apple orchard improved pollination in all tested cultivars, especially in ‘Gala’, which naturally suffers from relatively few seeds in the fruit. It appears that the addition of BBs did not only increase the number of pollinating insects in the orchard that could perform cross-pollination, including in the cool mornings and in adverse weather conditions, but that it also changed HB foraging behavior, which resulted in improved cross-pollination and increased efficiency, and subsequently more seeds and larger fruit. The improved pollination was due to the greater mobility of HBs between rows of pollinated cultivar and pollenizer, and to the greater proportion of topworkers, which are more efficient pollinators. [ABSTRACT FROM AUTHOR]

This study provides an experimental calibration of the empirical Al-in-olivine thermometer for mantle peridotites proposed by De Hoog et al. (2010). We report Al concentrations measured by secondary ion mass spectrometry (SIMS) in olivines produced in the original high-pressure, high-temperature, four-phase lherzolite experiments by Brey et al. (1990). These reversed experiments were used for the calibration of the two-pyroxene thermometer and Al-in-orthopyroxene barometer by Brey and Köhler (1990). The experimental conditions of the runs investigated here range from 28 to 60 kbar and 1000 to 1300 °C. Olivine compositions from this range of experiments have Al concentrations that are consistent, within analytical uncertainties, with those predicted by the empirical calibration of the Al-in-olivine thermometer for mantle peridotites. Fitting the experimental data to a thermometer equation, using the least squares method, results in the expression: T ° C = 11245 + 46.0 ∗ P kbar / ( 13.68 – ln Al ppm – 273 . This version of the Al-in-olivine thermometer appears to be applicable to garnet peridotites (lherzolites and harzburgites) well outside the range of experimental conditions investigated here. However, the thermometer is not applicable to spinel-bearing peridotites. We provide new trace element criteria to distinguish between olivine from garnet-, garnet-spinel-, and spinel-facies peridotites. The estimated accuracy of the thermometer is ± 20 °C. Thus, the thermometer could serve as a useful tool in settings where two-pyroxene thermometry cannot be applied, such as garnet harzburgites and single inclusions in diamond. [ABSTRACT FROM AUTHOR]

The age and origin of the past-producing Nanisivik carbonate-hosted Zn Pb deposit in Nunavut, Canada, have been controversial for decades. Various direct and indirect dating methods have produced results ranging from Mesoproterozoic to Ordovician in age, and previous studies of the mineralising fluids have suggested that the fluids were anomalously hot (> 150 °C). This study combines Re Os (pyrite) geochronology, in-situ sulphur isotope analysis, and fluid inclusion analysis to refine both the timing of mineralisation and the nature of mineralising fluids. Re Os pyrite analysis shows that the Nanisivik deposit formed ca. 1.1 Ga, broadly similar to the depositional age of the host rock and with the Grenville orogeny, making it one of few known Precambrian carbonate-hosted Zn Pb deposits. In-situ sulphur isotope measurements from Nanisivik show a narrow δ 34 S range of 27.54 ± 0.72, very similar to what has been reported before in bulk sample analyses. New fluid inclusion data show that the mineralising fluids were ~ 100 °C, which is not anomalous in the context of carbonate-hosted base-metal deposits. The fluids exhibit no significant spatial variation in homogenisation temperature in the 2-km-long ‘upper lens’ of the ore deposit, but recrystallisation and modification of fluid inclusions took place in the immediate vicinity of the cross-cutting ~ 720 Ma “mine dyke”. The deposit is broadly inferred to have formed during late Mesoproterozoic assembly of supercontinent Rodinia, when regional hydrostatic head developed under the influence of far-field stresses originating in the developing Grenville orogen. The Nanisivik deposit remains anomalous only in its age; most other aspects of this ore deposit are now shown to be quite typical for carbonate-hosted ore deposits. [ABSTRACT FROM AUTHOR]

A subducted oceanic crustal origin for most eclogite xenoliths in kimberlites has long been a cornerstone of tectonic models for craton development. However, eclogite xenoliths often have protracted and complex histories involving multiple metasomatic events that could overprint some of the key geochemical indicators typically taken as evidence of a subducted origin (e.g., garnet δ 18 O-values and mineral 87 Sr/ 86 Sr compositions). To assess the potential for disturbance of oxygen isotopic compositions in mantle eclogites via diamond-forming and other possible metasomatic fluids, we have conducted a multi-technique in situ study of a diamondiferous eclogite xenolith from the Roberts Victor kimberlite, S. Africa. Using SIMS we provide the first texturally-controlled in situ measurements of δ 18 O-values in eclogitic garnet in close proximity to diamond. Garnet and clinopyroxene modal proportions are heterogeneous in the xenolith and garnet compositions vary from Mg# = 75.8–79.2; grossular proportions = 8.05–10.14 mol.%, and omphacitic pyroxene has Jd 13–24 and Mg# = 86.6–90.0. Rare earth element patterns of minerals across the xenolith, including grains close to diamond, are typical LREE-depleted garnets and markedly LREE-enriched pyroxenes. These silicate minerals also record detectable intra- and inter-grain LREE abundance variations. Clinopyroxenes of the studied xenoliths show HFSE and Sr abundance variations that are decoupled from LREE contents and major-element variations. Mineralogical constraints and bulk-rock reconstructions indicate that the studied sample likely experienced selective incompatible element enrichment during small-volume (<<0.03 wt.%) infiltration of metasomatic fluid(s) potentially linked to ancient diamond evolution. Intra-grain major-element, LREE and HFSE variations in clinopyroxene resulted from late-stage metasomatism. Oxygen isotope compositions in garnet are decoupled from all major- and trace-element variations, with garnet δ 18 O-values being uniform across the xenolith in a wide variety of textural settings. Garnet δ 18 O-values of +6.5 ± 0.2 ‰ are higher than the mean (+5.19 ± 0.26 ‰) of the mantle garnet range (+4.8–5.5 ‰). Modelling of the buffering effect of mantle peridotite on CO 2 -rich and H 2 O-rich metasomatic fluids at temperatures within the diamond stability field indicates that the likelihood of a metasomatic fluid with exotic oxygen isotopic composition arriving at a mantle eclogite body with its isotopic composition unmodified, after percolative flow through dominantly peridotitic mantle at great depth, is very low. As we find no evidence of metasomatically induced garnet oxygen isotope variations in the studied diamondiferous eclogite xenolith we conclude that the most likely origin for the elevated garnet δ 18 O-values is via inheritance from a crustal protolith altered at relatively low temperatures. These results have broader relevance and support the hypothesis of a low-pressure protolith for mantle eclogite xenoliths, demonstrating the robust nature of garnet oxygen isotope compositions – even in diamond-bearing eclogites. [ABSTRACT FROM AUTHOR]

For more than 20 years, the reported occurrence of diamonds in the chromites and peridotites of the Luobusa massif in Tibet (a complex described as an ophiolite) has been widely ignored by the diamond research community. This skepticism has persisted because the diamonds are similar in many respects to high-pressure high-temperature (HPHT) synthetic/industrial diamonds (grown from metal solvents), and the finding previously has not been independently replicated. We present a detailed examination of the Luobusa diamonds (recovered from both peridotites and chromitites), including morphology, size, color, impurity characteristics (by infrared spectroscopy), internal growth structures, trace-element patterns, and C and N isotopes. A detailed comparison with synthetic industrial diamonds shows many similarities. Cubo-octahedral morphology, yellow color due to unaggregated nitrogen (C centres only, Type Ib), metal–alloy inclusions and highly negative δ C 13 values are present in both sets of diamonds. The Tibetan diamonds ( n = 3 ) show an exceptionally large range in δ N 15 (−5.6 to + 28.7 ‰ ) within individual crystals, and inconsistent fractionation between {111} and {100} growth sectors. This in contrast to large synthetic HPHT diamonds grown by the temperature gradient method, which have with δ N 15 = 0 ‰ in {111} sectors and + 30 ‰ in {100} sectors, as reported in the literature. This comparison is limited by the small sample set combined with the fact the diamonds probably grew by different processes. However, the Tibetan diamonds do have generally higher concentrations and different ratios of trace elements; most inclusions are a NiMnCo alloy, but there are also some small REE-rich phases never seen in HPHT synthetics. These characteristics indicate that the Tibetan diamonds grew in contact with a C-saturated Ni–Mn–Co-rich melt in a highly reduced environment. The stable isotopes indicate a major subduction-related contribution to the chemical environment. The unaggregated nitrogen, combined with the lack of evidence for resorption or plastic deformation, suggests a short (geologically speaking) residence in the mantle. Previously published models to explain the occurrence of the diamonds, and other phases indicative of highly reduced conditions and very high pressures, have failed to take into account the characteristics of the diamonds and the implications for their formation. For these diamonds to be seriously considered as the result of a natural growth environment requires a new understanding of mantle conditions that could produce them. [ABSTRACT FROM AUTHOR]

[Display omitted] • Large zircon Hf-O isotopic and trace element dataset maps the crustal architecture of the south-east Superior Craton. • Syn-volcanic (2750-2695 Ma) mineral systems (VMS, Ni-Cu-PGE) occur in the most juvenile crust, interpreted as a rift zone. • Post-volcanic (<2695 Ma) mineral deposits (orogenic gold) also formed in this area during subsequent orogenesis. • These results show the first-order localising effect and long-lived control of crustal architecture on mineral systems. Crustal and lithospheric architecture provide a first-order control on Earth evolution, including mineral systems. The ability to understand the internal nature and margins of crustal and lithospheric blocks, in both space and time, is vital in order to use continent architecture as a predictive tool in mineral exploration. In this study, we use U-Pb-Hf-O-trace element (TE) geochemical data from zircon grains in felsic magmatic rocks to isotopically map the south-east Superior Craton, producing a time-constrained architecture of the Archean crust in this area. We then assess the localization of volcanogenic massive sulphide (VMS), komatiite-hosted Ni-Cu-PGE, and syn- to post-tectonic Au systems within that architecture, constraining the first-order crustal-scale controls on these mineral systems. In terms of zircon data, at ca. >2750–2695 Ma, the central and north-west Abitibi subprovince has more juvenile εHf, light to mantle-like δ18O, lower (Eu/Eu*)/Y*10000 (drier/shallower crust), reduced ΔFMQ, less continental initial-U (U i)/Yb, and more mantle-like U i /Nb, relative to surrounding crust. The syn-volcanic mineral systems are localised in this juvenile zone. At ca. 2704–2695 Ma, there is a marked transition in multiple datasets, including increases in δ18O, (Eu/Eu*)/Y*10000, ΔFMQ, U i /Yb and U i /Nb data, and a decrease in εHf. These data represent a more evolved continental signature and the transition to a relatively homogenous regional Hf-O-TE architecture. World-class orogenic gold mineralization occurring at ca. <2680 Ma is predominantly localized into the same region as the syn-volcanic deposits, demonstrating the profound role of early architecture on the localisation of later mineral systems. [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]

To determine the magnitude of N-isotope fractionation between diamond and its parental fluid, detailed C- and N-isotope analyses of a complexly-zoned, eclogitic diamond (JDE-25) were undertaken using secondary ion mass spectrometry. Combined C- and N-isotope and N-abundance measurements were made across four distinct growth zones and show the following range of values: δ 13 C = − 5.7 to − 2.1‰; δ 15 N = − 7.0 to + 5.5‰; [N] = 104 to 5420 at. ppm. The core zone displays a continuous, rimward increase in δ 13 C and δ 15 N values and decreases in N-abundance, and is interpreted to have formed by fractional crystallization of diamond from a single pulse of fluid (i.e., closed system). Modelling of the isotopic and abundance data from the core zone yields a diamond–fluid nitrogen partition coefficient ( K N ) of 4.4 and a N-isotope fractionation factor (∆ 15 N diam–fluid ) of − 4.0 ± 1.2‰ (2σ) at ~ 1100 °C, for precipitation from a pure carbonate fluid. Calculated K N and ∆ 15 N diam–fluid values would have larger magnitudes if JDE-25 formed from a more complex fluid, in which the carbonate species formed only a minor component. Theoretical calculations of N-isotope fractionation between the principal N-species associated with upper mantle fluids (N 2 , NH 3 or NH 4 + ) and the CN − molecule, as an analogue for the carbon–nitrogen bond in diamond, yield the following ∆ 15 N diam (CN)–fluid estimates at 1100 °C: − 3.6‰ for NH 4 + , − 2.1‰ for N 2 and − 1.4‰ for NH 3 . The theoretical calculations provide only minimum estimates of the true diamond–fluid N-isotope fractionation factor, given that the C–N single bond in diamond would have a lower affinity for 15 N than the stronger C–N triple bond in the CN − molecule. Accordingly, the theoretical N-isotope fractionation factors are consistent with the empirical fractionation factor derived from diamond JDE-25. As a consequence of the large magnitude of ∆ 15 N diam–fluid , intracrystalline N-isotope variations in diamond should provide a sensitive test for fluid-related, fractional crystallization processes. Furthermore, the large magnitude of ∆ 15 N diam–fluid could be reflected in the wide range of δ 15 N values for natural diamonds and the absence of clearly defined modes for the N-isotope compositions of peridotitic and eclogitic diamonds. [ABSTRACT FROM AUTHOR]

Abstract: Despite several decades of research on growth of the continental crust, it remains unclear whether the production of juvenile continental crust has been continuous or episodic throughout the Precambrian. Models for episodic crustal growth have gained traction recently through compilations of global U–Pb zircon age frequency distributions interpreted to delineate peaks and lulls in crustal growth through geologic time. One such apparent trough in zircon age frequency distributions between ∼2.45 and 2.22 Ga is thought to represent a pause in crustal addition, resulting from a global shutdown of magmatic and tectonic processes. The ∼2.45–2.22 Ga magmatic shutdown model envisions a causal relationship between the cessation of plate tectonics and accumulation of atmospheric oxygen over the same period. Here, we present new coupled U–Pb, Hf, and O isotope data for detrital and magmatic zircon from the western Churchill Province and Trans-Hudson orogen of Canada, covering an area of approximately 1.3 million km2, that demonstrate significant juvenile crustal production during the ∼2.45–2.22 Ga time interval, and thereby argue against the magmatic shutdown hypothesis. Our data is corroborated by literature data showing an extensive 2.22–2.45 Ga record in both detrital and magmatic rocks on every continent, and suggests that the operation of plate tectonics continued throughout the early Paleoproterozoic, while atmospheric oxygen rose over the same time interval. We argue that uninterrupted plate tectonics between ∼2.45 and 2.22 Ga would have contributed to efficient burial of organic matter and sedimentary pyrite, and the consequent rise in atmospheric oxygen documented for this time interval. [Copyright &y& Elsevier]

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]