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New 187Re-187Os, 87Rb-87Sr, triple O-isotope isotope, bulk rock highly siderophile- (HSE: Os, Ir, Ru, Pt, Pd, Re), major- and trace-element abundance data are reported for a variety of carbonaceous, ordinary and enstatite chondrite meteorites. In addition, new mineral chemical data are reported for the Chelyabinsk LL5 ordinary chondrite fall for comparison with existing chondrite data and to investigate element sequestration into metal and mineral phases within some chondrites. The focus of the study is to link the variations observed in the HSE abundances and Re-Os isotopes with other isotopic and elemental data to explore the relative roles of sample sizes, terrestrial alteration and parent body processes more fully on chondrite meteorite compositions. Trace element variations in Chelyabinsk silicate, oxide and metal grains highlight the importance of geochemical heterogeneity imparted by mineralogical variations and mode effects, as well as sample size. Using a range of sample powder aliquot sizes, it is possible to show that this becomes significant for the HSE at

https://hal.archives-ouvertes.fr/hal-03252301; International audience; Summary In the past, highly productive (hyper)saline and alkaline environments have proved propitious for large accumulation of sedimentary OM that led to the formation of Type I kerogens, such as those of the Green River Formation. A sediment core that covers most of the history of Lake Dziani Dzaha, a hypereutrophic saline, alkaline and mostly anoxic crater lakewhich waspostulatedto be a modern analogue of such environments, has been investigated using several analytical approaches comprising bulk (Rock-Eval and C, N, S elemental analyses), isotopic (δ13,14C, δ15N, δ18O, δ33,34,36S), and sedimentological descriptions. This study revealed that, despite strong environmental changes undergone by the lake since its formation, the conditions prevailing in the lake supported by a continuous high primary production have led to an excellent preservation of OM and to early diagenetic reductive and sulfurization processes which have yielded an exceptionally reduced character of the OM with respect to its short diagenetic history (< 6 ky). The similarities observed between the sediments of Lake Dziani Dzaha and some source rocks associated to Type I kerogens suggest that early diagenetic processes can have a strong and rapid impact on the accumulation of OM in some (semi)closed and highly productive aquatic ecosystems.

The origin of most major sulfide Ni-Cu deposits is attributed to the segregation of immiscible sulfide liquids from intraplate silicate melts, in response to assimilation of siliceous or sulfur-bearing rocks. The Algliden gabbroic dike in the Skellefte district (Sweden) contains Ni-Cu sulfide mineralization that is atypical because it formed in an arc setting and has a high Au content and low Ni/Cu ratio. This association led previously to a model in which the mineralization was linked to the assimilation of Cu-Au porphyry wall rocks. Based on new petrological, geochemical, and S isotope analyses of the dike and its wall rocks, we propose that assimilation of mineralized wall rocks was not instrumental in the formation of the deposit. Instead, we propose that the sulfides segregated during olivine crystallization from an evolved arc basalt and were injected into the dike in an olivine-rich crystal mush. This scenario explains the uniform dissemination of sulfide ores within norites, together with the high Au content and low Ni/Cu ratio of the ores. We propose that sulfide segregation from oxidized arc basalt was triggered by a decrease in the oxidation state of the melt, likely due to the cooling of the silicate melt or to the interaction with meta-sediments rich in organic matter, as proposed for other arc-related Cu-Ni sulfide deposits in Fennoscandia and elsewhere. At Algliden, melt reduction would have occurred relatively late during magma evolution, leading to the formation of an uneconomic deposit consisting of low Ni ores and lacking massive sulfides.

https://hal.archives-ouvertes.fr/hal-03252301; International audience

International audience; The deterioration of monument or building stone materials is mostly due to the growth of black crusts that cause blackening and disaggregation of the exposed surface. This study reports on new oxygen (δ17O, δ18O and Δ17O) and sulphur (δ33S, δ34S, δ36S, Δ33S and Δ36S) isotopic analyses of black crust sulphates formed on building stones in Sicily (Southern Italy). The measurements are used to identify the possible influence of volcanic emissions on black crust formation. Black crusts were mostly sampled on carbonate stone substrate in different locations subject to various sulphur emission sources (marine, anthropogenic and volcanic). Unlike atmospheric sulphate aerosols that mostly exhibit Δ33S > 0‰, here most of the analysed black crust sulphates show negative Δ33S. This confirms that black crust sulphates do not result from deposition of sulphate aerosols or of rainwater but mostly from the oxidation of dry deposited SO2 onto the stone substrate. The δ34S and δ18O values indicate that most of black crust sulphate originates from anthropogenic activities. Δ17O values are found to be related to the sampling location. The largest 17O-anomalies (up to ~4‰) are measured in black crust from areas highly influenced by volcanic emissions, which demonstrates the strong involvement of ozone in the formation of black crusts in volcanically influenced environments.

Impact-induced oxidation of Mars’ crust by water >4.4 Ga ago likely supplied enough atmospheric H2 to attain a warm climate., Despite the abundant geomorphological evidence for surface liquid water on Mars during the Noachian epoch (>3.7 billion years ago), attaining a warm climate to sustain liquid water on Mars at the period of the faint young Sun is a long-standing question. Here, we show that melts of ancient mafic clasts from a martian regolith meteorite, NWA 7533, experienced substantial Fe-Ti oxide fractionation. This implies early, impact-induced, oxidation events that increased by five to six orders of magnitude the oxygen fugacity of impact melts from remelting of the crust. Oxygen isotopic compositions of sequentially crystallized phases from the clasts show that progressive oxidation was due to interaction with an 17O-rich water reservoir. Such an early oxidation of the crust by impacts in the presence of water may have supplied greenhouse gas H2 that caused an increase in surface temperature in a CO2-thick atmosphere.

International audience; 16 Evaporites play a major role on the structuration of collisional orogens especially when they 17 act as décollement units. However, their exact pressure-temperature-deformation (P-T-d) paths 18 are poorly documented. In this study, the first direct P-T-d constraints of the "Nappe des 19 Gypses" formation (western French Alps) have been established. An innovative association of 20 structural geology, petrography, crystallochemistry, and detailed study of both fluid inclusions 21 and stable isotopes (C, O) analysis has been applied to this evaporitic facies. Geochemical 22 analysis shows that the "Nappe des Gypses" formation has recorded the three typical 23 metamorphic and deformational events of the Alps (namely D1, D2 and D3). These different 24 constraints allow the determination of the first determination of the P-T path for this unit. 25 Metamorphic peak conditions of the "Nappe des Gypses" are at 16.6 ± 2.3 kbars and 431°C ± 26 28°C. This formation was buried at similar conditions than the oceanic units. During the 27 exhumation path, the D1-D2 transition is reached at 350°C ± 20°C and 6.5 ± 1.8 kbars and the 28 D2-D3 transition is assumed to be at 259°C ± 24°C and 2.0 ± 1.0 kbars (Strzerzynski et al., 29 2012). Peak P-T conditions overlap those of the median Liguro-Piemontese units but are 30 different from those of the Briançonnais units. It implies 1) an active and crucial role of the 31 "Nappe des Gypses" during the exhumation of the Alpine oceanic complex. And 2) confirms the 32 allochthonous and more distal origin of the European Thetysian passive margin of the "Nappe 33 des Gypses" formation. Consideration of sulfates dehydration probably between 15.0 and 16.6 34 kbars and 200 and 300°C, allows to discuss pore pressure excess and its mechanical 35 consequences on the exhumation process. This process is very likely to amplify the 36 "décollement" effect of the evaporites and allow the nappe stack formation. 37 This illustrates the role of this formation as a décollement surface. This difference of 38 evolution highlights the major role of the evaporitic formations on the exhumation and 39 structuration of a collisional chain. Such methodology could contribute to decipher the role of 40 evaporites in the structural context of other collisional chains such as Himalaya, Pyrenees or 41 Zagros. 42

This study reports on measurements of Δ17O (derived from the triple oxygen isotopes) in sulphate from black crust sampled in Sicily. Atmospheric oxidants, such as O3, H2O2, OH and O2 carry specific 17O-anomalies, which are partly transferred to the sulphate during sulphur gas (e.g. SO2) oxidation. Hence, the Δ17O in sulphate can be used as a tracer of sulphur oxidation pathways. So far, this method has been mostly applied on sulphate from aerosols, rainwaters, volcanic deposits and ice cores. Here we propose a new approach, that aims to investigate the dominant oxidants of gaseous sulphur precursors into sulphate extracted from black crust material. Black crusts are mostly found on building/monument/sculpture and are the result of the reaction between sulphur compounds (SO2, H2SO4) and carbonate (CaCO3) from the substrate, which leads to the formation of gypsum (CaSO4, 2H2O). Sicilian black crust from sites under different emission influences (anthropogenic, marine and volcanic) were collected. Multi oxygen and sulphur isotope analyses were performed to better assess the origins of black crust sulphate in these different environments. This is crucial for both a better understanding of the sulphur cycle and the preservation of historical monument.Multi sulphur isotopes show mostly negative values ranging from -0.4 ‰ to 0.02 ‰ ± 0.01 and from -0.59 ‰ to 0.41‰ ± 0.3 for Δ33S and Δ36S respectively. This is unique for natural samples and different from sulphate aerosols measured around the world (Δ33S > 0‰). This tends to indicate that sulphate from black crust is not generated by the same processes as sulphate aerosols in the atmosphere. Instead of SO2 oxidation in the atmosphere, dry deposition of SO2 and its oxidation on the substratum is preferred. The multi oxygen isotopes show a clear dependence with the geographical repartition of the samples. Indeed, black crusts from Palermo (the biggest Sicilian city) show small 17O-anomalies ranging between -0.16 ‰ to 1.02 ‰ with an average value of 0.45 ‰ ± 0.26 (n=12; 2σ). This is consistent with Δ17O values measured in black crust from the Parisian Basin (Genot et al., 2020), which are also formed in an environment influenced by anthropogenic and marine emissions. On the other hand, samples from the eastern part of the Mount Etna region, which are downwind of the volcanic emissions, show the highest 17O-anomalies ranging from 0.48 ‰ to 3.87 ‰ with an average value of 2.7 ‰ ± 0.6 (n=11; 2σ).These results indicate that volcanic emissions influence the oxygen isotopic signature of black crust sulphate. In standard urban areas, SO2 deposited on the substratum is mostly oxidised by O2-TMI and H2O2 to generate the black crust. Yet, under the influence of volcanic emissions, O3 may play the main role in the SO2 oxidation.

In order to establish the role and expression of silicate-metal fractionation in early planetesimal bodies, we have conducted a highly siderophile element (HSE: Os, Ir, Ru, Pt, Pd, Re) abundance and 187 Re- 187 Os study of acapulcoite-lodranite meteorites. These data are reported with new petrography, mineral chemistry, bulk-rock major and trace element geochemistry, and oxygen isotopes for Acapulco, Allan Hills (ALHA) 81187, Meteorite Hills (MET) 01195, Northwest Africa (NWA) 2871, NWA 4833, NWA 4875, NWA 7474 and two examples of transitional acapulcoite-lodranites, Elephant Moraine (EET) 84302 and Graves Nunataks (GRA) 95209. These data support previous studies that indicate that these meteorites are linked to the same parent body and exhibit limited degrees ( 2 × CI chondrite) HSE abundances in transitional acapulcoite-lodranite meteorites (EET 84302, GRA 95209). All of the meteorites have chondritic Re/Os with measured 187 Os/ 188 Os ratios of 0.1271 ± 0.0040 (2 St. Dev.). These geochemical characteristics imply that the precursor material of the acapulcoites and lodranites was broadly chondritic in composition, and were then heated and subject to melting of metal and sulfide in the Fe-Ni-S system. This resulted in metallic melt removal and accumulation to form lodranites and transitional acapulcoite-lodranites. There is considerable variation in the absolute abundances of the HSE, both among samples and between aliquots of the same sample, consistent with both inhomogeneous distribution of HSE-rich metal, and of heterogeneous melting and incomplete mixing of silicate material within the acapulcoite-lodranite parent body. Oxygen isotope data for acapulcoite-lodranites are also consistent with inhomogeneous melting and mixing of accreted components from different nebular sources, and do not form a well-defined mass-dependent fractionation line. Modeling of HSE inter-element fractionation suggests a continuum of melting in the Fe-Ni-S system and partitioning between solid metal and sulfur-bearing mineral melt, where lower S contents in the melt resulted in lower Pt/Os and Pd/Os ratios, as observed in lodranites. The transitional meteorites, EET 84302 and GRA 95209, exhibit the most elevated HSE abundances and do not follow modelled Pt/Os and Pd/Os solid metal-liquid metal partitioning trends. We interpret this to reflect metal melt pooling into domains that were sampled by these meteorites, suggesting that they may originate from deeper within the acapulcoite-lodranite parent body, perhaps close to a pooled metallic ‘core’ region. Petrographic examination of transitional samples reveals the most extensive melting, pooling and networking of metal among the acapulcoite-lodranite meteorites. Overall, our results show that solid metal-liquid metal partitioning in the Fe-Ni-S system in primitive achondrites follows a predictable sequence of limited partial melting and metal melt pooling that can lead to significant HSE inter-element fractionation effects in proto-planetary materials.

International audience; We report new δ 13 C data and N content and aggregation state values for microdiamonds recovered from peridotites and chromitites of the Luobusa ophiolite (Tibet) and chromitites of the Ray-Iz ophiolite in the Polar Urals (Russia). All analyzed microdiamonds contain significant nitrogen contents (from 108 to 589 atomic ppm ± 20%) with a consistently low aggregation state and show identical infrared spectra dominated by strong absorption between 1130 cm –1 and 1344 cm –1 , and therefore characterize type Ib diamond. Microdiamonds from the Luobusa peridotites have δ 13 C (PDB) values ranging from −28.7‰ to −16.9‰, and N contents from 151 to 589 atomic ppm. The δ 13 C and N content values for diamonds from the Luobusa chromitites are −29‰ to −15.5‰ and 152–428 atomic ppm, respectively. Microdiamonds from the Ray-Iz chromitites show δ 13 C values varying from −27.6‰ to −21.6‰ and N contents from 108 to 499 atomic ppm. The carbon isotopes values have features similar to previously analyzed metamorphic diamonds from other worldwide localities, but the samples are characterized by lower N contents. In every respect, they are different from diamonds occurring in kimberlites and impact craters. Our samples also differ from the few synthetic diamonds we analyzed, in that they show enhanced δ 13 C variability and less advanced aggregation state than synthetic diamonds. Our newly obtained N aggregation state and N content data are consistent with diamond formation over a narrow and rather cold temperature range (i.e.

A collection of 61 xenocrystic and 12 eclogite xenolith-derived diamonds from the 600 Ma Lahtojoki kimberlite in central Finland has been investigated. Calculated pressure and temperature conditions for the diamondiferous eclogites are in excess of 5.5 GPa and 1300 °C, suggesting residence depths greater than 180 km, near the base of the Karelian cratonic mantle lithosphere. Geochemically, the eclogite xenoliths have gabbroic compositions showing positive Eu and Sr anomalies, relatively low ΣREE and elevated Al2O3 contents, yet garnets have ambiguous δ18O values of 5.7‰ and 5.9‰. Gabbroic eclogite formation could therefore be linked to either subduction processes during the 1.9 Ga Svecofennian orogeny or to cumulate processes during 2.1 Ga rift-induced magmatism. Determination of the oxygen fugacity of Lahtojoki eclogite xenoliths from both this work and previous studies suggests that diamond-bearing eclogites may be more reduced (ΔFMQ-3.5) compared to barren eclogites (ΔFMQ-1.7). While recycled oceanic crust protoliths for the eclogites remain a possibility, the carbon isotopic compositions and nitrogen abundances of the Lahtojoki diamonds indicate mantle-derived volatile sources. All diamonds (i.e., loose and eclogite xenolith-derived) display a restricted range of δ13C values from −7.8‰ to −3.7‰ that overlaps with the carbon isotopic composition of Earth’s mantle. The Lahtojoki diamond δ13C values form a negatively skewed distribution, indicating diamond growth from reduced mantle-derived carbon sources such as methane- (CH4) bearing fluids. Nitrogen contents of the Lahtojoki diamonds range from 40 to 1830 atomic ppm with a mean of ∼670 atomic ppm; these elevated nitrogen contents combined with the close association to eclogites suggest an eclogitic or crustal volatile source. However, the Karelian craton was periodically intruded by ultramafic alkaline magmas since at least 1.8 Ga, noting in particular the occurrence of phlogopite-rich kimberlites and olivine lamproites between 1200 and 700 Ma. We argue that this punctuated volatile-rich magmatism simultaneously metasomatised the cratonic mantle lithosphere, forming nitrogen enriched phlogopite-bearing metasomes. We propose that reduced, carbon-bearing and nitrogen-rich fluids were remobilized to form the Lahtojoki diamonds. The diamond-forming event(s) most probably occurred during or shortly prior to the entraining kimberlite magmatism as indicated by the diamond nitrogen aggregation systematics. Involvement of reduced diamond-forming fluids is supported by both the negative skewness of Lahtojoki diamond δ13C values and the more reduced nature of the diamondiferous Lahtojoki eclogites compared with their more oxidized barren counterparts. Our results from the diamondiferous eclogites derived from the deepest parts of the Karelian cratonic mantle root are in support of methane being the stable carbon volatile species at the base of thick continental lithosphere.

Ureilite meteorites are partially melted asteroidal-peridotite residues, or more rarely, cumulates that can contain greater than three weight percent carbon. Here we describe an exceptional C-rich lithology, composed of 34 modal % large (up to 0.8 mm long) crystalline graphite grains, in the Antarctic ureilite meteorite Miller Range (MIL) 091004. This C-rich lithology is embedded within a silicate region composed dominantly of granular olivine with lesser quantities of low-Ca pyroxene, and minor FeNi metal, high-Ca pyroxene, spinel, schreibersite and troilite. Petrological evidence indicates that the graphite was added after formation of the silicate region and melt depletion. Associated with graphite is localized reduction of host olivine (Fo88–89) to nearly pure forsterite (Fo99), which is associated with FeNi metal grains containing up to 11 wt.% Si. The main silicate region is typical of ureilite composition, with highly siderophile element (HSE) abundances ∼0.3 × chondrite, 187Os/188Os of 0.1260–0.1262 and Δ17O of −0.81 ± 0.16‰. Mineral trace-element analyses reveal that the rare earth elements (REE) and the HSE are controlled by pyroxene and FeNi metal phases in the meteorite, respectively. Modeling of bulk-rock REE and HSE abundances indicates that the main silicate region experienced ∼6% silicate and >50% sulfide melt extraction, which is at the lower end of partial melt removal estimated for ureilites. Miller Range 091004 demonstrates heterogeneous distribution of carbon at centimeter scales and a limited range in Mg/(Mg + Fe) compositions of silicate grain cores, despite significant quantities of carbon. These observations demonstrate that silicate rim reduction was a rapid disequilibrium process, and came after silicate and sulfide melt removal in MIL 091004. The petrography and mineral chemistry of MIL 091004 is permissive of the graphite representing late-stage C-rich melt that pervaded silicates, or carbon that acted as a lubricant during anatexis and impact disruption in the parent body. Positive correlation of Pt/Os ratios with olivine core compositions, but a wide range of oxygen isotope compositions, indicates that ureilites formed from a compositionally heterogeneous parent body that experienced variable sulfide and metal melt-loss that is most pronounced in relatively oxidized ureilites with Δ17O between −1.5 and ∼0‰.

The geochemical behavior of N during seawater-oceanic crust alteration remains poorly constrained. Yet, it is a central parameter to assess the flux of N to subduction zones. Most studies proposed that hydrothermally altered basaltic rocks are enriched in N relative to fresh basalts. However, published data from DSDP/ODP Hole 504B, a reference site for the composition of the oceanic crust, suggest that seawater alteration leads to the N depletion of the upper ocean crust. To better address this issue, we analyzed N concentration and isotope composition of 21 altered basalts from the lavas and sheeted dikes sampled by Hole 504B. These new analyses show significant N enrichment (up to 14.1 ppm) relative to fresh degassed MORB (∼1 ppm). The differences observed between earlier and modern data are interpreted as resulting from analytical artifact due to the earlier use of a molybdenum crucible for N extraction. Furthermore, our new data show a progressive decrease of N concentration with depth, from 14.1 to 1.4 ppm. Nitrogen isotope compositions display a large range, with δ15N values from −0.9 to +7.3‰, and most likely reflect multiple stages of alteration with fluids of various compositions. In contrast to N concentration, δ15N values do not show a global depth trend but oscillate around a mean value of 3.0 ± 2.2‰ (1SD). The N concentration shows a positive correlation with bulk rock δ18O values, suggesting that N behavior during alteration process is mainly controlled by temperature. We propose that N speciation in the hydrothermal fluid is dominated by NH3/NH4 at low temperature (2, associated with H2, at higher temperature (>200 °C). These new data are used to re-evaluate the global flux of N input into Central American subduction zone, showing that the upper basaltic crust represent about 20% of the total N buried in subduction zone. A comparison with previous results obtained on N degassed in volcanic arc illustrates that, in “warm” subduction zone like Central America, up to 50% of the subducted N may be transferred to the deep mantle. This contrasts with “cold” subduction environments, where >80% of the N inputs escape sub-arc slab devolatilization and supports that the geothermal gradient plays a major role in determining the N fate in subduction zones.

After permanent atmospheric oxygenation, anomalous sulfur isotope compositions were lost from sedimentary rocks, demonstrating that atmospheric chemistry ceded its control of Earth’s surficial sulfur cycle to weathering. However, mixed signals of anoxia and oxygenation in the sulfur isotope record between 2.5 to 2.3 billion years (Ga) ago require independent clarification, for example via oxygen isotopes in sulfate. Here we show, The loss of anomalous sulfur isotope compositions from sedimentary rocks has been considered a symptom of permanent atmospheric oxygenation. Here the authors show sulfur and oxygen isotope evidence from

18 pages; International audience; The Early Triassic is generally portrayed as a time of various, high ecological stresses leading to a delayed biotic recovery after the devastating end-Permian mass extinction. This interval is notably characterized by repeated biotic crises (e.g., during the late Smithian), large-scale fluctuations of the global carbon, nitrogen and sulfur cycles as well as harsh marine conditions including a combination of ocean acidification, anoxia, extreme seawater temperatures and shifting productivity. Observations from different paleolatitudes suggest that sulfidic (H2S-rich) conditions may have developed widely during the Early Triassic, possibly reaching up to ultra-shallow environments in some places. However, the existence and the spatio-temporal extent of such redox swings remain poorly constrained. In order to explore Early Triassic paleoceanographic redox changes and their potential influences on the biotic recovery, we analyzed multiple sulfur isotopes (32S, 33S, 34S, and 36S) of sedimentary pyrite and carbonate associated sulfate (δ34SCAS) from the Mineral Mountains section, Utah. Sediments from this section were mainly deposited in shallow waters and span the Smithian and lower Spathian. We report a 68‰ range of variations in δ34Spy associated with Δ33Spy varying from −0.01‰ to +0.12‰, whereas the δ34SCAS varies between +19.5‰ and + 34.8‰. We interpret the observed signal of multiple sulfur isotopes as reflecting the operation of pore-water synsedimentary microbial sulfate reduction in open system with respect to sulfates before the late Smithian, evolving to a closed system, sulfate limited, Rayleigh-type distillation across the Smithian/Spathian boundary (SSB) and immediately after the SSB. We argue that this marked change is driven by the effectiveness of the connection between the sedimentary pore waters and the overlying water column, which is, in this case, controlled by the local sedimentological conditions such as the bioturbation intensity and the sedimentation rate. Therefore, our results suggest that changes in the sulfur cycle before and across the SSB at Mineral Mountains is probably a local consequence of the loss of the mixed sedimentary layer during the late Smithian extinction event, as opposed to reflecting the development of a lethal anoxic ocean at the global scale.

International audience

International audience; To better address how subducted protoliths drive the Earth's mantle sulfur isotope heterogeneity, we report new data for sulfur (S) and copper (Cu) abundances, S speciation and multiple S isotopic compositions (32S, 33S, 34S, 36S) in 15 fresh submarine basaltic glasses from the Samoan archipelago, which defines the enriched-mantle-2 (EM2) endmember.Bulk S abundances vary between 835 and 2279 ppm. About 17±11%17±11% of sulfur is oxidized (S6+) but displays no consistent trend with bulk S abundance or any other geochemical tracer. The S isotope composition of both dissolved sulfide and sulfate yield homogeneous Δ33S and Δ36S values, within error of Canyon Diablo Troilite (CDT). In contrast, δ34S values are variable, ranging between +0.11 and +2.79‰ (±0.12‰ 1σ) for reduced sulfur, whereas oxidized sulfur values vary between +4.19 and +9.71‰ (±0.80‰, 1σ). Importantly, δ34S of the reduced S pool correlates with the 87Sr/86Sr ratios of the glasses, in a manner similar to that previously reported for South-Atlantic MORB, extending the trend to δ34S values up to +2.79±0.04‰+2.79±0.04‰, the highest value reported for undegassed oceanic basalts.As for EM-1 basalts from the South Atlantic ridge, the linear δ34S–87Sr/86Sr trend requires the EM-2 endmember to be relatively S-rich, and only sediments can account for these isotopic characteristics. While many authors argue that both the EM-1 and EM-2 mantle components record subduction of various protoliths (e.g. upper or lower continental crust, lithospheric mantle versus intra-metasomatized mantle, or others), it is proposed here that they primarily reflect sediment recycling. Their distinct Pb isotope variation can be accounted for by varying the proportion of S-poor recycled oceanic crust in the source of mantle plumes.

Unravelling the origin of organic compounds that were accreted into asteroids requires better constraining the impact of asteroidal hydrothermal alteration on their isotopic signatures, molecular structures, and spatial distribution. Here, we conducted a multi-scale/multi-technique comparative study of the organic matter (OM) from two CM chondrites (that originate from the same parent body or from identical parent bodies that accreted the same mixture of precursors) and underwent a different degree of hydrothermal alteration: Paris (a weakly altered CM chondrite – CM 2.8) and Murchison (a more altered one – CM 2.5). The Paris insoluble organic matter (IOM) shows a higher aliphatic/aromatic carbon ratio, a higher radical abundance and a lower oxygen content than the Murchison IOM. Analysis of the OM in situ shows that two texturally distinct populations of organic compounds are present within the Paris matrix: sub-micrometric individual OM particles and diffuse OM finely distributed within phyllosilicates and amorphous silicates. These results indicate that hydrothermal alteration on the CM parent body induced aromatization and oxidation of the IOM, as well as a decrease in radical and nitrogen contents. Some of these observations were also reported by studies of variably altered fragment of Tagish Lake (C2), although the hydrothermal alteration of the OM in Tagish Lake was apparently much more severe. Finally, comparison with data available in the literature suggests that the parent bodies of other chondrite petrologic groups could have accreted a mixture of organic precursors different from that accreted by the parent body of CMs.

We report in situ and bulk S-isotope analyses of the 3.2 Ga old barite deposit of the Sargur Group, Dharwar Craton, India. Compared with the two other coeval barite deposits in Western Australia (Pilbara Craton, Warrawoona and Sulfur Springs Group) and South Africa (Kaapvaal Craton, Onverwacht and Fig Tree Group), the rocks of the Sargur Group have experienced pervasive ductile deformation and equilibrated under high-grade metamorphic conditions, which could have caused partial S-isotope re-equilibration. However, in situ techniques revealed high spatial heterogeneities in δ 34 S of up to ∼18‰ in barite-bearing pyrites, as well as similar Δ 33 S anomalies compared to the associated sulfate indicating bacterial sulfate reduction isotope effects and a minor impact of the pervasive ductile deformation and metamorphism on the primary/diagenetic signatures. Our data in associated silicate rocks define consistent trends in Δ 33 S/δ 34 S and Δ 36 S/Δ 33 S reflecting metamorphic reworking of primary photolytic signatures with sulfide deriving from thermal reduction of the barite sulfate at >400 °C. Our results match the previous data obtained on other Early Archean barite deposits. This strongly suggests that similar sulfur sources were at play over the three continents during the production and preservation of atmospheric sulfate and elemental sulfur aerosols. This also confirms that the different Early Archean barite deposits known on Earth formed during periods of intense subaerial volcanism and record volcanic photolytic signatures different from those common in the Archean.

Modern nitrogen (N) fixation is primarily mediated by biological processes. However, in the early Earth where biological activity was absent or limited, abiotic N reduction in hydrothermal systems is thought to be a key process to transform atmospheric N2 and NOx to ammonium, an essential nutrient to support the emergence of life and also an N form that can be incorporated into rocks. Surprisingly, evidence for abiotic N reduction in the rock record has not been clearly identified. In this study, we reported anomalously low N isotope compositions ( δ N 15 values as low as − 15.8 ‰ ) of mica samples in ultrahigh-pressure metamorphic rocks from the Donghai area in the Sulu orogenic belt, eastern China. Compared with mica samples with typical crustal δ N 15 values ( 3 – 9 ‰ ) in similar metamorphic rocks from the western Dabie orogen, the 15N-depleted mica samples from the Sulu orogen are characterized by significant N enrichment (10 times higher) and extreme 18O depletion ( δ O 18 values as low as − 9 ‰ ). These features can be best explained by assimilation of N from a source characterized by extremely low δ N 15 values (less than ∼ − 16 ‰ ). The extremely low δ N 15 value would be produced by abiotic N reduction during reaction of a meteoric-hydrothermal fluid with crustal rocks before subduction. This observation provides a clue to the occurrence of abiotic N reduction in continental supracrustal rocks and infer that abiotic N reduction process could be a fundamental process driving the geological N cycling in early Earth.

International audience; The occurrence of Early Archean barite deposits is intriguing since this type of sediment requires high availability of dissolved sulfate (SO42-), the oxidized form of sulfur, although most authors argued that the Archean eon was dominated by reducing conditions, with low oceanic sulfate concentration (56Fe values varying between -2.04‰ in Fe sulfide-dominated barite beds, to +2.14‰ in Fe oxide-bearing cherts. δ34S values of sulfides vary between -10.84 and +3.56‰, with Δ33S in a range comprised between -0.35 and +2.55‰, thus supporting an O2-depleted atmosphere (-5 PAL). Iron isotope variations together with major element correlations show that, although the sediments experienced a pervasive stage of hydrothermal alteration, the rocks preserved a primary/authigenic signature predating subsequent hydrothermal stage. Highly positive δ56Fe values recorded in primary Fe-oxides from ferruginous cherts support partial Fe oxidation in a reducing oceanic environment (O2 < 10-4 μM), but are incompatible with a model of complete oxidation at the redox boundary of a stratified water column. Iron oxide precipitation under low O2 levels was likely mediated by anoxygenic photosynthesis, and/or abiotic photo-oxidation processes. Our results are consistent with global anoxic conditions in the 3.2 Ga-old sediments, implying that the barite deposits were most likely sourced by atmospheric photolysis of S gases produced by large subaerial volcanic events, and possibly SO42- produced by magmatic SO2 disproportionation in hydrothermal systems.

Introduction Earth’s carbon, derived from planetesimals in the 1 AU region during accretion of the Solar System, still retains similarities to carbon found in meteorites (Marty et al. 2013) even after 4.57 billion years of geological processing. The range in isotopic composition of carbon on Earth versus meteorites is nearly identical and, for both, diamond is a common, if volumetrically minor, carbon mineral (Haggerty 1999). Diamond is one of the three native carbon minerals on Earth (the other two being graphite and lonsdaleite). It can crystallize throughout the mantle below about 150 km and can occur metastably in the crust. Diamond is a rare mineral, occurring at the part-per-billion level even within the most diamondiferous volcanic host rock although some rare eclogites have been known to contain 10–15% diamond. As a trace mineral it is unevenly distributed and, except for occurrences in metamorphosed crustal rocks, it is a xenocrystic phase within the series of volcanic rocks (kimberlites, lamproites, ultramafic lamprohyres), which bring it to the surface and host it. The occurrence of diamond on Earth’s surface results from its unique resistance to alteration/dissolution and the sometimes accidental circumstances of its sampling by the volcanic host rock. Diamonds are usually the chief minerals left from their depth of formation, because intact diamondiferous mantle xenoliths are rare. Diamond has been intensively studied over the last 40 years to provide extraordinary information on our planet’s interior. For example, from the study of its inclusions, diamond is recognized as the only material sampling the “very deep” mantle to depths exceeding 800 km (Harte et al. 1999; McCammon 2001; Stachel and Harris 2009; Harte 2010) although most crystals (~95%) derive from shallower depths (150 to 250 km). Diamonds are less useful in determining carbon fluxes on Earth because they provide only a small, …

We report the quadruple sulfur isotope compositions, sulfur contents and speciation major and trace elements (including copper and chlorine abundances) of eleven basalts collected in the Garrett transform fault. We combine these data to discuss the absence of S isotopic fractionation along both partial melting and low-pressure fractional crystallization. The variations of K2O/TiO2 and La/SmN-ratios (respectively between 0.017 and 0.067, and between 0.31 and 0.59) suggest a range of depletion in Garrett lavas that includes ultra depleted samples ( K 2 O/TiO 2 0.03 ). The remarkable level of incompatible element depletion is consistent with re-melting of a depleted source. Contrasting with incompatible element depletion, all samples display similar S and Cu abundance (at a given major-element composition) to mid-ocean ridge basalts (MORB). This indicates that Garrett Intra Transform Lavas (ITL) are sulfide saturated as MORB are. Copper content for Garrett parental melts (MgO >8%) are ∼80 ppm, indistinguishable from MORBs. This requires their mantle sources, variably depleted in incompatible element, to host residual sulfide buffering the Cu content of all erupted melts. We calculate a minimum S content for the source of ultra-depleted Garrett lavas of 100 ± 40 ppm S , i.e. roughly a factor of 2 below the MORB mantle source. After exclusion of a single sample with Cl/K ratio >0.1 that likely experienced hydrothermal sulfide assimilation, Garrett ITLs display homogeneous δ 34 S , Δ 33 S and Δ 36 S values with averages of − 0.68 ± 0.08 ‰ , + 0.010 ± 0.005 ‰ and − 0.04 ± 0.04 ‰ , respectively (all 1σ, n = 10 ). The δ 34 S values display no relationship with either K2O/TiO2 variations or extent sulfide fractionation. From these observations, we derive a 34S/32S fractionation factor between exsolved sulfides and sulfide dissolved in silicate melts of 1.0000 ± 0.0003 . The S isotopic fractionation during partial melting can thus be considered as negligible, and both MORBs and ITLs record the 34S/32S ratio of their mantle source. The concept of sulfide melts segregating from the mantle, sinking and being added to the core during planetary differentiation was termed the ‘Hadean Matte’. The segregation of sulfides from the mantle to the core during planetary differentiation could account for various geochemical features of the Earth's mantle. Based on S isotopic mass balance, we derive a lower and upper limit for the hadean matte. While the lower bound corresponds to a virtually negligible hadean matte, the upper limit is 3.36 × 10 24 g S (i.e. ∼10% of the bulk terrestrial S), which remains 5 to 10 times lower than previous estimates. This upper bound nonetheless requires high mantle S content >1000 ppm S before the extraction of the hadean matte. This suggestion would have chronological requirements, requiring any sulfide melt to have formed after the core extraction but before late accretion of the highly siderophile elements.

Nitrogen contents and isotope compositions together with major and trace element concentrations were determined in a sequence of metagabbros from the western Alps (Europe) in order to constrain the evolution and behavior of N during hydrothermal alteration on the seafloor and progressive dehydration during subduction in a cold slab environment (8 C/km). The rocks investigated include: (i) low-strain metagabbros that equilibrated under greenschist to amphibolite facies (Chenaillet Massif), blueschist facies (Queyras region) and eclogite facies (Monviso massif) conditions and (ii) highly-strained mylonites and associated eclogitic veins from the Monviso Massif. In all samples, nitrogen (2.6–55 ppm) occurs as bound ammonium (NH4 þ ) substituting for K or Na–Ca in minerals. Cu concentrations show a large variation, from 73.2 to 6.4 ppm, and are used as an index of hydrothermal alteration on the seafloor because of Cu fluid-mobility at relatively high temperature (>300 C). In low-strain metagabbros, d 15 N values of +0.8& to +8.1& are negatively correlated with Cu concentrations. Eclogitic mylonites and veins display Cu concentrations lower than 11 ppm and show a d 15 N–Cu relationship that does not match the d 15 N–Cu correlation found in low-strain rocks. This d 15 N–Cu correlation preserved in low-strain rocks is best interpreted by leaching of Cu–N compounds, possibly of the form Cu(NH3)2 2+ , during hydrothermal alteration. Recognition that the different types of low-strain metagabbros show the same d 15 N–Cu correlation indicates that fluid release during subduction zone metamorphism did not modify the original N and Cu contents of the parent hydrothermally-altered metagabbros. In contrast, the low Cu content present in eclogitic veins and mylonites implies that ductile deformation and veining were accompanied either by a loss of copper or that externally-derived nitrogen was added to the system. We estimate the global annual flux of N subducted by metagabbros as 4.2 (±2.0) � 10 11 g/yr. This value is about half that of sedimentary rocks, which suggests that gabbros carry a significant portion of the subducted nitrogen. The net budget between subducted N and that outgassed at volcanic arcs indicates that � 80% of the subducted N is not recycled to the surface. On a global scale, the total amount of N buried to the mantle via subduction zones is estimated to be three times higher than that released from the mantle via mid-ocean ridges, arc and intraplate volcanoes and back-arc basins. This implies that N contained in Earth surface reservoirs, mainly in the atmosphere, is progressively transferred and sequestered into the mantle, with a net flux of � 9.6 � 10 11 g/yr. Assuming a constant flux of subducted N over the Earth’s history indicates that an amount equivalent to the present atmospheric N may have been sequestered into the silicate Earth over a period of 4 billion years.

Sub-micrometer inclusions in diamonds carry high-density fluids (HDF) from which the host diamonds have precipitated. The chemistry of these fluids is our best opportunity of characterizing the diamond-forming environment. The trace element patterns of diamond fluids vary within a limited range and are similar to those of carbonatitic/kimberlitic melts that originate from beneath the lithospheric mantle. A convecting mantle origin for the fluid is also implied by C isotopic compositions and by a preliminary Sr isotopic study (Akagi, T., Masuda, A., 1988. Isotopic and elemental evidence for a relationship between kimberlite and Zaire cubic diamonds. Nature 336, 665–667.). Nevertheless, the major element chemistry of HDFs is very different from that of kimberlites and carbonatites, varying widely and being characterized by extreme K enrichment (up to ∼ 39 wt.% on a water and carbonate free basis) and high volatile contents. The broad spectrum of major element compositions in diamond-forming fluids has been related to fluid–rock interaction and to immiscibility processes. Elemental signatures can be easily modified by a variety of mantle processes whereas radiogenic isotopes give a clear fingerprint of the time-integrated evolution of the fluid source region. Here we present the results of the first multi radiogenic-isotope (Sr, Nd, Pb) and trace element study on fluid-rich diamonds, implemented using a newly developed off-line laser sampling technique. The data are combined with N and C isotope analysis of the diamond matrix to better understand the possible sources of fluid involved in the formation of these diamonds. Sr isotope ratios vary significantly within single diamonds. The highly varied but unsupported Sr isotope ratios cannot be explained by immiscibility processes or fluid-mineral elemental fractionations occurring at the time of diamond growth. Our results demonstrate the clear involvement of a mixed fluid, with one component originating from ancient incompatible element-enriched parts of the lithospheric mantle while the trigger for releasing this fluid source was probably carbonatitic/kimberlitic melts derived from greater depths. We suggest that phlogopite mica was an integral part of the enriched lithospheric fluid source and that breakdown of this mica releases K and radiogenic Sr into a fluid phase. The resulting fluids operate as a major metasomatic agent in the sub-continental lithospheric mantle as reflected by the isotopic composition and trace element patterns of G10 garnets.

In order to address diamond formation and origin in the lithospheric mantle underlying the Central Slave Craton, we report N- and C-stable isotopic compositions and N-contents and aggregation states for 85 diamonds of known paragenesis (73 peridotitic, 8 eclogitic and 4 from lower mantle) from the Panda kimberlite (Ekati Mine, Lac de Gras Area, Canada). For 12 peridotitic and two eclogitic sulfide inclusion-bearing diamonds from this sample set, we also report multiple-sulfur isotope ratios. The 73 peridotitic diamonds have a mean δ13C-value of − 5.2‰ and range from − 6.9 to − 3.0‰, with one extreme value at − 14.1‰. The associated δ15N-values range from − 17.0 to + 8.5‰ with a mean value of − 4.0‰. N-contents range from 0 to 1280 ppm. The 8 eclogitic diamonds have δ13C-values ranging from − 11.2 to − 4.4‰ with one extreme value at − 19.4‰. Their δ15N ranges from − 2.1 to + 7.9‰ and N-contents fall between 0 and 3452 ppm. Four diamonds with an inferred lower mantle origin are all Type II (i.e. nitrogen-free) and have a narrow range of δ13C values, between − 4.5 and − 3.5‰. The δ34S of the 14 analyzed peridotitic and eclogitic sulfide inclusions ranges from − 3.5 to +5.7‰. None of them provide evidence for anomalous δ33S-values; observed variations in δ33S are from +0.19 to − 0.33‰, i.e. within the 2 sigma uncertainties of mantle sulfur (δ33S = 0‰). At Panda, the N contents and the δ13C of sulfide-bearing peridotitic diamonds show narrower ranges than silicate-bearing peridotitic diamonds. This evidence supports the earlier suggestion established from eclogitic diamonds from the Kaapvaal that sulfide-(±silicate) bearing diamonds sample a more restricted portion of sublithospheric mantle than silicate-(no sulfide) bearing diamonds. Our findings at Panda suggest that sulfide-bearing diamonds should be considered as a specific diamond population on a global-scale. Based on our study of δ34S, Δ33S, δ15N and δ13C, we find no evidence for subduction-related isotopic signatures in the mantle sampled by Panda diamonds.

article In order to study the geochemistry of diamonds formed during similar growth conditions in a very localised environment in the mantle, we carried out a detailed study of the variation in δ 15 N, δ 13 C, N content, and N aggregation state of 35 diamonds in a single eclogite xenolith from the Kaalvallei kimberlite in South Africa. Diamond nitrogen contents determined by infrared spectroscopy range from 239 ppm to 1272 ppm, and are positivelycorrelated with nitrogen aggregation states which vary from 11.5% to 43.7% of IaB defects. Modelling of these parameters using second order reaction kinetics suggests that the diamonds likely represent a single population which has been resident in the mantle at temperatures of 1090 °C to 1190 °C. δ 13 C values of the diamonds analysed range from −6.0‰ to −4.2‰, while δ 15 N values vary from −8.9‰ to −4.1‰, with no correlation between δ

The formation of diamond through metasomatic events, from volatiles-enriched fluids/melts brought into pre-existing mantle rocks, has been suggested from a series of independent studies. However, the link between these hypothetical volatile-rich fluids and deep-seated mineral inclusions (mostly silicate and sulphides) entrapped in diamonds remains unclear, yet the relationship between these two species may provide the key to our understanding of diamond crystallization. In order to address the relationship between the origin and formation of diamonds and their mineral inclusions, we carried out the first coupled stable isotopic study ( δ 13 C, δ 15 N and multiple S-isotopes as δ 34 S, δ 33 S, Δ 33 S) of diamonds containing sulphide inclusions, using samples from the Jwaneng kimberlite as our model system. Sulphides extracted from the present collection of 55 diamonds belong to either eclogitic (E-type, 52 diamonds) or peridotitic (P-type, 3 diamonds) paragenetic suites, as attested by their Cr and Ni content (Cr 0.18 wt.% and Ni > 14 wt.% for P-types). Sulphur isotopic compositions have been measured in-situ by multicollector secondary ion mass spectrometry and reveal that peridotitic sulphide inclusions in diamonds ( n = 4) lie on the terrestrial fractionation line whereas Mass Independent sulphur isotope Fractionations (MIF) are preserved inside eclogitic sulphides inclusions (− 0.5‰ 33 S n = 33). Such large MIF values, extended here to the negative values recorded in Archean sediments, are thought to be produced through photochemical reactions in the Archean atmosphere, which implies that sulphides inclusions contain an Archean sedimentary sulphur component that was transferred to the Earth's mantle. In contrast to this isotopic dichotomy between E-type and P-type sulphides, the geochemical characteristics of their host diamonds ( δ 13 C, δ 15 N, N-content, N-speciation) are largely homogeneous throughout the population, and differ from silicate-bearing diamonds previously studied in the same locality. Sulphide-bearing diamonds appear to define a distinct population, which might reflect a different crystallization process characterized by carbon and nitrogen isotopic compositions falling within the range of unfractionated mantle values. The lack of a distinct isotopic signature of recycled sedimentary nitrogen and carbon in sulphide-bearing diamonds leads us to infer that sulphide-bearing Jwaneng diamonds were not derived from the same chemical reservoir as their inclusions. One possible formation mechanism that is compatible with our observations is diamond crystallization from a mantle-derived carbon-bearing fluid associated with pre-existing sedimentary sulphide minerals. This model has important implications for the interpretation of diamond ages obtained from sulphide inclusions using Re–Os systematics. Here we suggest that the incorporation of a significant amount of mantle-related sulphur during a diamond growth event into the original sulphide could lead to a re-equilibration of the Re–Os isotope composition in any inclusion, and accordingly, account for some scatter on Re–Os isochrons.

In order to better investigate the compositions and the origins of fluids associated with diamond growth, we have carried-out combined noble gas (He and Ar), C and N isotope, K, Ca and halogen (Cl, Br, I) determinations on fragments of individual microinclusion-bearing diamonds from the Panda kimberlite, North West Territories, Canada. The fluid concentrations of halogens and noble gases in Panda diamonds are enriched by several orders of magnitude over typical upper mantle abundances. However, noble gas, C and N isotopic ratios (3He/4He = 4–6 Ra, 40Ar/36Ar = 20,000–30,000, δ13C = −4.5‰ to −6.9‰ and δ15N = −1.2‰ to −8.8‰) are within the worldwide range determined for fibrous diamonds and similar to the mid ocean ridge basalt (MORB) source value. The high 36Ar content of the diamonds (>1 × 10−9 cm3/g) is at least an order of magnitude higher than any previously reported mantle sample and enables the 36Ar content of the subcontinental lithospheric mantle to be estimated at ∼0.6 × 10−12 cm3/g, again similar to estimates for the MORB source. Three fluid types distinguished on the basis of Ca–K–Cl compositions are consistent with carbonatitic, silicic and saline end-members identified in previous studies of diamonds from worldwide sources. These fluid end-members also have distinct halogen ratios (Br/Cl and I/Cl). The role of subducted seawater-derived halogens, originally invoked to explain some of the halogen ratio variations in diamonds, is not considered an essential component in the formation of the fluids. In contrast, it is considered that large halogen fractionation of a primitive mantle ratio occurs during fluid–melt partitioning in forming silicic fluids, and during separation of an immiscible saline fluid.

In order to better characterise mantle CO2/Nb-variability, we obtained and compiled major and trace elements, content and isotope composition of both CO2 and water on two series of mid-ocean ridge basalt (MORB) samples dredged at ∼ 14° N (n = 6) and 34° N (n = 11) on the mid-Atlantic ridge. All samples are carbon-saturated. One, the so-called popping rock 2ΠD43 kept its vesicles, the initial (pre-degassing) C-contents of the 16 other samples being reconstructed from their assumed degassing history. For water, the samples show large variations, from 1300 to 6900 ppm and from 1900 to 7900 ppm with associated δD-values ranging from − 55 to − 79‰ and from − 55 to − 88‰ for samples at 14° N and 34° N respectively. For carbon, the inferred initial predegassing contents vary greatly, from 660 to 14,700 ppmCO2 and from 1400 to 57,600 ppmCO2 for samples at 14° N and 34° N respectively. Measured Nb-contents range from 4.5 to 29.6 ppm show both good agreement with previously published data and positive correlations with reconstructed initial CO2-contents. The mean CO2/Nb range from ∼ 570 to ∼ 730 at 14° N and 34° N respectively. CO2 and Nb data for the two undegassed samples available so far (i.e. the popping rock of the present study and the basaltic glasses from the Siqueiros transform fault from the study of Saal et al., 2002) show significant variations in CO2/Nb over a factor of 2 and thus questions the constant CO2/Nb previously emphasised for these two samples, this view being supported by CO2/Nb-ratios of samples whose initial C-contents were reconstructed. For incompatible elements such as Ce, K and including water, a comparison of the geochemical characteristics of transform fault basaltic magmatism with other MORB systems shows magma transform fault magmatism to be unrepresentative of mantle compositions. Assuming a more appropriate average MORB CO2/Nb-ratio of ∼ 530 and a mean MORB Nb-content of 3.31−1.8+3.99, we computed a mantle carbon flux of 2.3−1.3+2.7 × 1012 mol/yr, a value actually consistent with that derived from C/3He systematics.

The terminal Neoproterozoic Era (850–542 Ma) is characterized by the most pronounced positive sulfur isotope (34S/32S) excursions in Earth's history, with strong variability and maximum values averaging δ34S∼+38‰. These excursions have been mostly interpreted in the framework of steady-state models, in which ocean sulfate concentrations do not fluctuate (that is, sulfate input equals sulfate output). Such models imply a large pyrite burial increase together with a dramatic fluctuation in the isotope composition of marine sulfate inputs, and/or a change in microbial sulfur metabolisms. Here, using multiple sulfur isotopes (33S/32S, 34S/32S and 36S/32S ratios) of carbonate-associated sulfate, we demonstrate that the steady-state assumption does not hold in the aftermath of the Marinoan Snowball Earth glaciation. The data attest instead to the most impressive event of oceanic sulfate drawdown in Earth's history, driven by an increased pyrite burial, which may have contributed to the Neoproterozoic oxygenation of the oceans and atmosphere., Large positive sulphur isotope excursions, recorded in the wake of the Marinoan glaciation have previously been interpreted assuming stable ocean sulphate concentrations. Here, using multiple sulphur isotopes, the authors instead suggest significant ocean sulphate drawdown, driven by increased pyrite burial.

–Olivine-dominated (70–80 modal %) achondrite meteorite Lewis Cliff (LEW)88763 originated from metamorphism and limited partial melting of a FeO-rich parentbody. The meteorite experienced some alteration on Earth, evident from subchondritic Re/Os, and redistribution of rhenium within the sample. LEW 88763 is texturally similar towinonaites, has a D 17 O value of 1.19 0.10‰, and low bulk-rock Mg/(Mg+Fe) (0.39),similar to the FeO-rich cumulate achondrite Northwest Africa (NWA) 6693. The similarbulk-rock major-, minor-, and trace-element abundances of LEW 88763, relative to somecarbonaceous chondrites, including ratios of Pd/Os, Pt/Os, Ir/Os, and 187 Os/ 188 Os (0.1262),implies a FeO- and volatile-rich precursor composition. Lack of fractionation of the rareearth elements, but a factor of approximately two lower highly siderophile elementabundances in LEW 88763, compared with chondrites, implies limited loss of Fe-Ni-S meltsduring metamorphism and anatexis. These results support the generation of high Fe/Mg,sulfide, and/or metal-rich partial melts from FeO-rich parent bodies during partial melting.In detail, however, LEW 88763 cannot be a parent composition to any other meteoritesample, due to highly limited silicate melt loss (0 to <

Mass-independent fractionation of sulfur isotopes (S-MIF) results from photochemical reactions involving short-wavelength UV light. The presence of these anomalies in Archean sediments [(4-2.5 billion years ago, (Ga)] implies that the early atmosphere was free of the appropriate UV absorbers, of which ozone is the most important in the modern atmosphere. Consequently, S-MIF is considered some of the strongest evidence for the lack of free atmospheric oxygen before 2.4 Ga. Although temporal variations in the S-MIF record are thought to depend on changes in the abundances of gas and aerosol species, our limited understanding of photochemical mechanisms complicates interpretation of the S-MIF record in terms of atmospheric composition. Multiple sulfur isotope compositions (δ(33)S, δ(34)S, and δ(36)S) of the3.8 billion-year-old Nuvvuagittuq Greenstone Belt (Ungava peninsula) have been investigated to track the early origins of S-MIF. Anomalous S-isotope compositions (Δ(33)S up to +2.2‰) confirm a sedimentary origin of sulfide-bearing banded iron and silica-rich formations. Sharp isotopic transitions across sedimentary/igneous lithological boundaries indicate that primary surficial S-isotope compositions have been preserved despite a complicated metamorphic history. Furthermore, Nuvvuagittuq metasediments recorded coupled variations in (33)S/(32)S, (34)S/(32)S, and (36)S/(32)S that are statistically indistinguishable from those identified several times later in the Archean. The recurrence of the same S-isotope pattern at both ends of the Archean Eon is unexpected, given the complex atmospheric, geological, and biological pathways involved in producing and preserving this fractionation. It implies that, within 0.8 billion years of Earth's formation, a common mechanism for S-MIF production was established in the atmosphere.

This paper reports on the petrology and geochemistry of a diamondiferous peridotite xenolith from the Premier diamond mine in South Africa. The xenolith is altered with pervasive serpentinisation of olivine and orthopyroxene. Garnets are in an advanced state of kelyphitisation but partly fresh. Electron microprobe analyses of the garnets are consistent with a lherzolitic paragenesis (8.5 wt.% Cr2O3 and 6.6 wt.% CaO). The garnets show limited variation in trace element composition, with generally low concentrations of most trace elements, e.g. Y (

The present paper provides C- and N-stable isotope characteristics, N-contents and N-aggregation states for alluvial diamonds of known paragenesis from placers along the Namibian coast. The sample set includes diamonds with typical peridotitic and eclogitic inclusions and the recently reported “undetermined” suite of Leost et al. [Contrib. Mineral. Petrol. 145 (2003) 15] which resulted from infiltration of high temperature, carbonate-rich melts. δ13C-values range from −20.3‰ to −0.5‰ (n=48) for peridotitic diamonds and from −38.5‰ to −1.6‰ (n=45) for eclogitic diamonds. Diamonds belonging to the “undetermined” suite span a narrower range in δ13C from −8.5‰ to −2.7‰ (n=13). When compared with previous studies, diamonds from Namibia are characterised by unusually low proportions of N-free (i.e. Type II) peridotitic and eclogitic diamonds (3% and 2%, respectively) and an unprecedented high proportion of N-rich diamonds (15% and 73%, respectively, have N-contents >600 ppm). δ15N-values for diamonds of the peridotitic, eclogitic and “undetermined” suites range from −10‰ to +13‰ without correlations with either N-content or δ13C. The similarity in N-isotopic composition and the N-rich character of diamonds belonging to the eclogitic, peridotitic and “undetermined” suites is striking and suggests a close genetic relationship. We propose that a large part of the diamonds mined in Namibia formed during metasomatic events of similar style that introduced carbon and nitrogen into a range of different host lithologies.

The Archean lithospheric mantle beneath the Kaapvaal–Zimbabwe craton of Southern Africa shows ±1% variations in seismic P-wave velocity at depths within the diamond stability field (150–250 km) that correlate regionally with differences in the composition of diamonds and their syngenetic inclusions. Seismically slower mantle trends from the mantle below Swaziland to that below southeastern Botswana, roughly following the surface outcrop pattern of the Bushveld-Molopo Farms Complex. Seismically slower mantle also is evident under the southwestern side of the Zimbabwe craton below crust metamorphosed around 2 Ga. Individual eclogitic sulfide inclusions in diamonds from the Kimberley area kimberlites, Koffiefontein, Orapa, and Jwaneng have Re–Os isotopic ages that range from circa 2.9 Ga to the Proterozoic and show little correspondence with these lithospheric variations. However, silicate inclusions in diamonds and their host diamond compositions for the above kimberlites, Finsch, Jagersfontein, Roberts Victor, Premier, Venetia, and Letlhakane do show some regional relationship to the seismic velocity of the lithosphere. Mantle lithosphere with slower P-wave velocity correlates with a greater proportion of eclogitic versus peridotitic silicate inclusions in diamond, a greater incidence of younger Sm–Nd ages of silicate inclusions, a greater proportion of diamonds with lighter C isotopic composition, and a lower percentage of low-N diamonds whereas the converse is true for diamonds from higher velocity mantle. The oldest formation ages of diamonds indicate that the mantle keels which became continental nuclei were created by middle Archean (3.2–3.3 Ga) mantle depletion events with high degrees of melting and early harzburgite formation. The predominance of sulfide inclusions that are eclogitic in the 2.9 Ga age population links late Archean (2.9 Ga) subduction-accretion events involving an oceanic lithosphere component to craton stabilization. These events resulted in a widely distributed younger Archean generation of eclogitic diamonds in the lithospheric mantle. Subsequent Proterozoic tectonic and magmatic events altered the composition of the continental lithosphere and added new lherzolitic and eclogitic diamonds to the already extensive Archean diamond suite.

The Earth’s upper mantle is depleted in 15N isotope, with a N15N value around 35x [where N15N= (N/Nsample/N/NAir31)1000], relative to atmosphere and crust. The 15N-depleted character ¢rst identi¢ed from recent (i.e. 6 380 Ma) diamonds ([1^3], see [4] for review) was con¢rmed by studies focusing on volatiles extracted from Mid-Ocean Ridge Basalts (MORB) vesicles [5^ 8]. Archean diamonds as old as 3.2 Ga ([9], for review see [10] and references therein) con¢rmed the negative N15N values, showing that the 15Ndepleted character of the Earth’s upper mantle is a long term feature [11,12]. If the nitrogen present within the external reservoirs of the Earth (i.e. atmosphere, sediments and crust) had been simply outgassed from the mantle, then one would expect them to display also negative N15N values. However, the atmosphere, sediments and crust have all N15Nv 0x. In addition, the fractionation of Nisotopes occurring during basaltic melt degassing at ridges is opposite to what would be required to achieve positive N15N in vesicles ([8,13], see below and [1,14] for discussion). Accordingly, there is a so-called isotopic imbalance of nitrogen between the mantle and the external reservoirs of the Earth (Fig. 1) which cannot be accounted for by outgassing of the Earth’s mantle only. Several models have been proposed to account for the nitrogen isotopic imbalance (see Fig. 1): (1) a heterogeneous accretion of the Earth’s volatiles (e.g. [15]), (2) core-mantle isotopic fractionation [7,13], (3) hydrodynamic escape and fractionation of the atmospheric nitrogen [6], and (4) the new model presented by Marty and Dauphas [16] who consider that the nitrogen now present in the diierent mantle reservoirs is dominantly recycled organic nitrogen. According to these authors, the nitrogen in the upper mantle, as sampled by diamonds and MORB, has a N15N value around 35x because it contains nitrogen of organic origin recycled in the Archean which would have had a N15N around 35x. The nitro

Nitrogen and hydrogen isotopic compositions together with N, K, Rb, Cs and H 2 O contents were measured on several high-pressure (HP) to ultrahigh-pressure (UHP) metasediments from the Schistes Lustres nappe (western Alps) and on unmetamorphosed sedimentary protoliths from the Apennines (Italy). These samples represent a sequence of pelagic sediments subducted to different depths down to 90 km along a ‘cold’ geothermal gradient (∼8°C/km). Nitrogen isotopic composition (δ 15 N between +2.6 and +4.8‰) does not show any specific evolution with increasing metamorphic conditions and can be considered constant during subduction. Large variations of the N content (between 169 and 1721 ppm N) together with K, Rb and Cs content are observed but the constancy of K/N (14), K/Rb (385) and K/Cs (10 190) molar ratios in protoliths and metamorphic rocks indicates that none of these fluid-mobile elements was lost through devolatilization processes. This suggests that fluid circulation was limited to sample scale and that the rocks behaved as closed systems during subduction. This interpretation is also supported by the small range of δD values (from −54.1 to −78.0‰). The present results indicate that N, K, Rb and Cs were retained in the subducted sedimentary veneer at least down to the depth locus of island arc magmatism. Based on the correlation between N and K contents, the flux of sedimentary N recycled in subduction zones is estimated at 7.6×10 11 g/yr. Mass balance calculations strongly support the fact that nitrogen is efficiently recycled to the deeper mantle.

The lithospheric mantle beneath the Kaapvaal-Zimbabwe craton of southern Africa shows variations in seismic P-wave velocity at depths within the diamond stability field that correlate with differences in the composition of diamonds and their syngenetic inclusions. Middle Archean mantle depletion events initiated craton keel formation and early harzburgitic diamond formation. Late Archean accretionary events involving an oceanic lithosphere component stabilized the craton and contributed a younger Archean generation of eclogitic diamonds. Subsequent Proterozoic tectonic and magmatic events altered the composition of the continental lithosphere and added new lherzolitic and eclogitic diamonds to the Archean diamond suite.

Primarily on the basis of C, N, S, and O stable isotope systematics, this article reviews recent achievements in understanding diamond formation and growth in Earth's mantle. Diamond is a metasomatic mineral that results from either the reduction or oxidation of mobile C-bearing liquids (fluids or melts) that intrude preexisting lithologies (eclogites, peridotites, and metamorphic rocks). This process seems ubiquitous, as it occurs over a large range of depths and extends through time. Diamond-forming carbon derives mainly from the convective asthenosphere. Most of its isotopic anomalies reflect fractionation processes in the lithospheric mantle, which are attributed to diamond precipitation itself and/or a mineralogical control occurring prior to diamond precipitation. Evidence for a mineralogical control would be the decoupling of the 15N/14N ratios in eclogitic diamond from other tracers of subduction in inclusions in the same diamond. C isotope anomalies related to subduction are rare and are probably best seen in diamonds from the transition zone.

In order to better constrain the origin of volatile variability of mid ocean ridge basalt (MORB), we have performed crushing analyses on 33 fresh basaltic glasses. Most samples originate from the Southwest Indian Ridge (i.e. 78–49°E, EDUL cruise, August 97), the Southeast Indian Ridge, the Central Indian Ridge and the Rodriguez Triple Junction. δ13C and δ18O of CO2, δ15N of N2 together with C, N and Ar contents were determined. δ15N values vary between −5.9‰ and +2.1‰ while δ13C values range between −11.4 and −4.3‰. C/N2 ratios vary by one order of magnitude (316–3900). Most N2/Ar ratios fall within a narrow range of values (48–90) but four samples yield higher values up to 220. Overall, the data for δ13C, δ18O, δ15N, C/N2 and N2/Ar are within the range of those previously reported for Pacific and Atlantic oceans. No volatile DUPAL anomaly has been detected in the present study. The correlations between N2/Ar, C/Ar and C/N2 together with δ13C and δ15N show that major volatile signatures, including δ15N, are more influenced by degassing-induced fractionation than by mantle heterogeneity and/or late atmospheric contamination. A two stage degassing model (a closed-system degassing followed by a Rayleigh distillation) can be used to explain the data set. This model gives initial Indian MORB values similar to those for the Atlantic and Pacific oceanic basalts δ13C0∼−4.5‰, δ15N0∼−6.0‰, (C/N2)0∼130, (C/Ar)0∼14 000 and (N2/Ar)0∼110 and C0 between 1100 and 5000 ppm C. This large range of possible initial carbon concentrations (due to the lack of constraints on the extent of degassing under closed-system conditions) results in a large range of mantle flux estimates (i.e. from 0.4 to 1.8×1013 mol/yr). The correlations induced by these degassing processes permit an estimation of the relative solubilities of C, N and Ar: SC/SN2∼5 and SAr/SN2∼1.2 as well as an evaluation of the fractionation of nitrogen isotopes between the vesicles and the melt: ΔN=−1.6‰. The present study proposes a coherent data set for volatile mantle fluxes with φC∼1.3±0.1×102φN2∼1.4±0.1×104φAr (molar).

This study reports δ13C, δ15N and N-content values for microdiamonds from ultrahigh-pressure metamorphic rocks of the Kokchetav massif in Kazakhstan. Both alluvial diamonds and in-situ diamonds from a garnet–clinopyroxene rock and a marble (i.e. a garnet–pyroxene dolomitic rock) were investigated. In-situ diamonds were analysed in batches, because of their small size (average 40 μm), whereas the larger alluvial diamonds were analysed individually. The latter group has δ13C-values ranging from −15.92‰ to −10.57‰, δ15N from −1.8‰ to +1.1‰ and N-contents from 2300 to 3650 ppm. Diamonds from the garnet–clinopyroxene rock yield mean values of −10.50‰ for δ13C, +5.9‰ for δ15N and a high average nitrogen content of 11,150 ppm. Values for diamonds in marble are −10.19‰, +8.5‰ and 2650 ppm, respectively. For diamonds from garnet–clinopyroxene rock and marble, there is more nitrogen released by bulk combustion than estimated by infrared (IR) spectroscopy, the differences being of about 7000 and 1500 ppm, respectively. These differences suggest that a significant quantity of nitrogen is IR-inactive and may be present as fluid inclusions. Their carbon and nitrogen isotopic compositions are compatible with an in-situ crystallisation of diamond from dominantly metasedimentary sources, suggesting that sedimentary nitrogen can be subducted to very high pressures. Carbon isotopic fractionation between coexisting carbonate and diamond suggests crystallisation temperatures before the peak of metamorphism at temperatures probably below 700°C and deduced pressures of 3 GPa. Relative to the isotopic data reported for sediments, metasediments and in-situ diamonds, the slightly 15N-depleted compositions of alluvial diamonds is striking. These values suggest that the contribution of any metasedimentary source is unlikely and may point toward a mafic/ultramafic protolith.

Microdiamonds from garnet clinopyroxenites of the Kokchetav massif (northern Kazakhstan) and associated alluvial diamonds have been investigated using infrared spectroscopy. All diamonds have high nitrogen contents (from 747 up to 4488 ± 20% at. ppm) and are of type Ib-IaA. Nitrogen data are consistent with a diamond formation over a narrow temperature range. Microdiamonds from garnet clinopyroxenites are characterized by water and carbonate inclusions, similar to those ocurring in fibrous coated diamonds from kimberlite, which suggest that diamonds grew from a C-H-O fluid. In contrast, alluvial microdiamonds do not contain any carbonate inclusions and H2O is absent or present in only minor amounts. Alluvial diamonds are considerably more N-rich but have a similar range of nitrogen aggregation states. It is suggested that alluvial diamonds belong to a separate diamond population compared with the microdiamonds from garnet clinopyroxenites. The Kokchetav microdiamonds are distinct from kimberlitic fibrous diamonds in that they contain abundant Ib centres, have high nitrogen contents and show different populations depending on host rock type. These characteristics support a metamorphic origin for the microdiamonds from the UHPM rocks of the Kokchetav massif.