Siberian Flood Basalts Research Articles

Sr, Nd, and Pb isotope provenance of surface sediments on the East Siberian Arctic Shelf and implications for transport pathways

The East Siberian Arctic Shelf (ESAS) is a crucial area for sea ice formation and exportation in the Arctic Ocean. Precise sedimentary identification in the ESAS is vital to trace sea ice transport. However, tracking sea ice transport is challenging due to the complexity of the sediment sources and transport processes in the ESAS. In this study, we investigated Sr, Nd, and Pb isotopes in residual fractions of recent surface sediments in the ESAS to explore the distribution, sources, and transport pathways of terrigenous materials and track regional sea ice transport. Our results show that approximately 20–30% of the Western Laptev Sea (WLS) sediments were derived from Siberian flood basalt, characterized by a high εNd (ca. -9.0) and a high 87Sr/86Sr ratio (ca. 0.715). Furthermore, most sediments of the Eastern Laptev Sea–Western East Siberian Sea (ELS–WESS) were derived from ancient clastic and metamorphic rocks, with the lowest εNd values (ca. -12.3), highest 87Sr/86Sr ratios (ca. 0.717), and highest 207Pb/206Pb ratios (ca. 0.836). Additionally, sediments in the Eastern East Siberian Sea–Western Chukchi Sea (EESS–WCS) had the highest εNd values (ca. -7.6), lowest 87Sr/86Sr ratios (ca. 0.712), and relatively high 207Pb/206Pb ratios (ca. 0.830) due to the influence of Okhotsk–Chukotka volcanic belt (OCVB) weathering products, which contributed approximately 10–20% of the local sediments. Eastern Chukchi Sea (ECS) sediments showed relatively low εNd values (ca. -9.2), low 87Sr/86Sr ratios (ca. 0.712), and the lowest 207Pb/206Pb ratios (ca. 0.810), which probably influenced by coast erosion inputs from Alaska and Yukon River material transported via Pacific Sea Water. Based on the sediment provenance with Nd isotope, sea ice formed in the WLS and ELS–WESS is predominantly transported by the Trans-Polar Drift (TPD) to the Eurasian Basin and Fram Strait. This study provides insights for further understanding and tracking sea ice transport processes from the ESAS to the Arctic Ocean.

U–Pb geochronology of calcite carbonatites and jacupirangite from the Guli alkaline complex, Polar Siberia, Russia

AbstractThis work is the first in situ U–Pb geochronological study of perovskite and calcic garnet (andradite) from the Guli complex in the Maimecha–Kotuy alkaline province (Polar Siberia, Russia). The U–Pb isotopic compositions of perovskite from contact zones of the two carbonatite stocks (Southern and Northern) and from jacupirangite separating the stocks were determined by laser-ablation inductively-coupled-plasma mass-spectrometry (LA-ICPMS) and isotope-dilution thermal-ionisation mass-spectrometry (ID-TIMS). The LA-ICPMS and ID-TIMS data for perovskite from the Northern carbonatite stock are in good agreement, yielding 206Pb/238U ages of 250.4 ± 1.1 Ma and 249 ± 2 Ma, respectively. These ages are also within the analytical uncertainty from the ID-TIMS results for perovskite from jacupirangites (250 ± 1 Ma). The LA-ICPMS results for perovskite from the Southern carbonatite stock indicate its somewhat older age (255.3 ± 2.4 Ma), implying the possibility of small-volume mantle magmatism predating the eruption of the Siberian flood basalts at ca. 252–251 Ma. This interpretation is supported by reports of pre-flood magmatism elsewhere in eastern Siberia. Andradite crystals from the contact between the Southern stock and metasomatised melilitolite were analysed by ID-TIMS. These measurements are inconclusive (247 ± 6 Ma) and could not be used to constrain further the timing of carbonatitic magmatism in the southern part of the complex. The present contribution also presents a refined methodology for LA-ICPMS geochronological studies of perovskite with elevated levels of common lead, and addresses some of the problems with previously proposed calibration standards.

Linking the Siberian Flood Basalts and Giant Ni‐Cu‐PGE Sulfide Deposits at Norilsk

AbstractThe world‐class magmatic sulfide deposits in the Norilsk region exhibit a remarkable spatial and temporal association with the Siberian large igneous province (LIP). However, the details of the causal connection between the Siberian LIP and the ore deposits have remained contentious. Here we address the problem by modeling of assimilation, crystallization and flow behavior of magmas based on aggregated data from the Norilsk camp. Crustal assimilation at depth by mantle‐derived picritic magma can account for the compositional and isotopic variations of the early tholeiitic basalts and the sequestering of sulfide liquid in a mid‐crustal conduit system. When the rate and volume of asthenospheric magma production dramatically increased, the metal tenors of the mid‐crustal sulfide reservoir were first upgraded by consequent reaction with the fresh, undepleted magmas, then entrained and flushed to higher crustal levels. The resulting sulfide‐rich emulsions were too dense to reach the surface to form lavas but were instead emplaced in multiple pulses within blind sill‐like bodies at shallow depth. Sulfide ores transported from the mid‐crust were deposited at the bases of the upper‐crustal sills, where sedimentary host rocks were assimilated wholesale by the already sulfide‐charged replenishing magmas; this shallow assimilation was itself not instrumental in forming the ores. Although the evolution and dynamics of flood basalts are thus inextricably linked with coeval subvolcanic economic intrusions, the mineralization itself is argued not to occupy feeders with direct links to the overlying lavas, suggesting major implications for exploration in other LIPs.

Nitrogen and noble gases reveal a complex history of metasomatism in the Siberian lithospheric mantle

The Siberian flood basalts (SFB) erupted at the end of the Permian period (∼250 Ma) in response to a deep-rooted mantle plume beneath the Siberian Sub-Continental Lithospheric Mantle (SCLM). Plume-lithosphere interaction can lead to significant changes in the structure and chemistry of the SCLM and trigger the release of metasomatic material that was previously stored within the stable craton. Here, we investigate the nature of the Siberian-SCLM (S-SCLM) by measuring nitrogen abundances and isotopes (δ15N) in 11 samples of two petrologically-distinct suites of peridotitic xenoliths recovered from kimberlites which bracket the eruption of the SFB: the 360 Myr old Udachnaya and 160 Myr old Obnazhennaya pipes. Nitrogen isotope (δ15N) values range from -5.85 ± 1.29‰ to +3.94 ± 0.63‰, which encompasses the entire range between depleted Mid-Ocean Ridge Basalt (MORB) mantle (DMM; -5 ± 2‰) and plume-derived (+3 ± 2‰) endmembers. In addition, we present neon (n=7) and argon (n=8) abundance and isotope results for the same two suites of samples. The 20Ne/22Ne and 21Ne/22Ne range from atmospheric-like values of 9.88 up to 11.35 and from 0.0303 to 0.0385, respectively, suggesting an admixture of DMM and plume-derived components. Argon isotopes (40Ar/36Ar) range from 336.7 to 1122 and correlate positively with 40Ar contents. We show that volatile systematics of Siberian xenoliths: (1) exhibit evidence of ancient metasomatic and/or recycled signatures, and (2) show evidence of subsequent plume-like re-fertilization, which we attribute to the emplacement of the SFB. Metasomatic fluids are highly enriched in radiogenic gases and have elevated Br/Cl and I/Cl values, consistent with an ancient subducted crustal component. The metasomatic component is marked by light N isotope signatures, suggesting it may be derived from an anoxic Archean subducted source. Taken together, these N2-Ne-Ar isotope results suggest that mantle plume impingement has profoundly modified the S-SCLM, and that N, Ne and Ar isotopes are sensitive tracers of metasomatism in the S-SCLM. Metasomatic fluids that permeate the S-SCLM act to archive a “subduction-fingerprint” that can be used to probe relative volatile-element recycling efficiencies and thus provide insight into volatile transport between the surface and mantle reservoirs over Earth history.

Tellurium in Late Permian‐Early Triassic Sediments as a Proxy for Siberian Flood Basalt Volcanism

AbstractWe measured the concentrations of trace elements in Late Permian to Early Triassic sediments from Spitsbergen. High mercury concentrations in sediments from the level of the Permo‐Triassic Mass Extinction (PTME) at this location were previously attributed to the emplacement of the Siberian Traps Large Igneous Province and used to link the timing of volcanism with the record of environmental change and extinction in these sediments. We investigated the use of the moderately to highly volatile, siderophile elements Ni, Zn, Cd, Sb, Te, Re, and Tl as proxies for the intensity of Siberian volcanism. These trace elements, like Hg, have high concentrations in volcanic gas compared to crustal rocks. Tellurium is highly enriched at the PTME, and Te/Th ratios increase by a factor of ∼20 across the PTME, similar to the variation in Hg/total organic carbon (TOC) in the same samples. Te/Th and Hg/TOC values imply that Siberian volcanism initiated at the onset of the PTME, coincident with the start of the δ13Corganic excursion and abrupt warming. Based on Te and Hg, most Siberian volcanism occurred between the two phases of the PTME boundary (a period of less than 100 ky), but also continued into the Early Triassic. The duration of Siberian volcanism inferred from Te/Th and Hg/TOC is shorter than that indicated by recent high‐precision U‐Pb ages of Siberian intrusive and extrusive rocks. Te concentrations and Te/Th ratios in sediments represent a useful new proxy for volcanism, which can be used to link the marine sedimentary record with large volcanic events on land.

Field evidence for coal combustion links the 252 Ma Siberian Traps with global carbon disruption

Abstract The Permian-Triassic extinction was the most severe in Earth history. The Siberian Traps eruptions are strongly implicated in the global atmospheric changes that likely drove the extinction. A sharp negative carbon isotope excursion coincides within geochronological uncertainty with the oldest dated rocks from the Norilsk section of the Siberian flood basalts. We focused on the voluminous volcaniclastic rocks of the Siberian Traps, relatively unstudied as potential carriers of carbon-bearing gases. Over six field seasons we collected rocks from across the Siberian platform, and we show here the first direct evidence that the earliest eruptions in the southern part of the province burned large volumes of a combination of vegetation and coal. We demonstrate that the volume and composition of organic matter interacting with magmas may explain the global carbon isotope signal and may have significantly driven the extinction.

Correlation high-calc undersaturated earth silicon complex of Maymecha-Kotuy province with Siberian flood basalts. New age data of massif Kugda (Polar Siberia)

Perovskite crystals from ore-bearing olivinite and phoscorite of Kugda Massif were analyzed by LA-ICP-MS. Age obtained for alkaline rocks of Kugda Massif is 2576 Ma. It means that Kugda massif was formed simultaneously with the Siberian flood basalts and Guli Massif (2509 Ma) which is the largest pluton in the Maymecha-Kotuy Province and is highly differentiated. The melilite-bearing Kugda Complex was formed synchronously with Siberian Trapps and Guli Massif, which suggests that these rocks might be genetically related.

Correlation of High-Calcium Silica-Undersaturated Complex of the Maymecha–Kotuy Province with Siberian Flood Basalts: New Age Data on the Kugda Massif (Polar Siberia)

Abstract—Perovskite crystals from the ore-bearing olivinite and phoscorite of the Kugda massif were analyzed by LA-ICP-MS. Age obtained for the alkaline rocks of the Kugda massif is 257 ± 6 Ma. This implies that the Kugda massif was formed simultaneously with the Siberian flood basalts and Guli Massif (250 ± 9 Ma), which is the largest and highly differentiated pluton in the Maymecha–Kotuy Province. The simultaneous formation of the melilite-bearing Kugda Complex, Siberian flood basalts, and Guli Massif suggests the possible genetic relation of these rocks.

End-Permian extinction amplified by plume-induced release of recycled lithospheric volatiles

Magmatic volatile release to the atmosphere can lead to climatic changes and substantial environmental degradation including the production of acid rain, ocean acidification and ozone depletion, potentially resulting in the collapse of the biosphere. The largest recorded mass extinction in Earth’s history occurred at the end of the Permian, coinciding with the emplacement of the Siberian large igneous province, suggesting that large-scale magmatism is a key driver of global environmental change. However, the source and nature of volatiles in the Siberian large igneous province remain contentious. Here we present halogen compositions of sub-continental lithospheric mantle xenoliths emplaced before and after the eruption of the Siberian flood basalts. We show that the Siberian lithosphere is massively enriched in halogens from the infiltration of subducted seawater-derived volatiles and that a considerable amount (up to 70%) of lithospheric halogens are assimilated into the plume and released to the atmosphere during emplacement. Plume–lithosphere interaction is therefore a key process controlling the volatile content of large igneous provinces and thus the extent of environmental crises, leading to mass extinctions during their emplacement. Halogens in Siberian xenoliths show that plume–lithosphere interaction controls the volatile content of large igneous provinces. The seawater-derived volatiles, implicated in the end-Permian mass extinction, infiltrated the lithosphere during subduction.

In situ LA-ICPMS Isotopic and Geochronological Studies on Carbonatites and Phoscorites from the Guli Massif, Maymecha-Kotuy, Polar Siberia

In this study we present a fresh isotopic data, as well as U–Pb ages from different REE-minerals in carbonatites and phoscorites of Guli massif using in situ LA-ICPMS technique. The analyses were conducted on apatites and perovskites from calcio-carbonatite and phoscorite units, as well as on pyrochlores and baddeleyites from the carbonatites. The 87Sr/86Sr ratios obtained from apatites and perovskites from the phoscorites are 0.70308–0.70314 and 0.70306–0.70313, respectively; and 0.70310–0.70325 and 0.70314–0.70327, for the pyrochlores and apatites from the carbonatites, respectively. Furthermore, the in situ laser ablation analyses of apatites and perovskites from the phoscorite yield eNd from 3.6 (±1) to 5.1 (±0.5) and from 3.8 (±0.5) to 4.9 (±0.5), respectively; eNd of apatites, perovskites and pyrochlores from carbonatite ranges from 3.2 (±0.7) to 4.9 (±0.9), 3.9 (±0.6) to 4.5 (±0.8) and 3.2 (±0.4) to 4.4 (±0.8), respectively. Laser ablation analyses of baddeleyites yielded an eHf(t)d of +8.5 (± 0.18); prior to this study Hf isotopic characteristic of Guli massif was not known. Our new in situ eNd, 87Sr/86Sr and eHf data on minerals in the Guli carbonatites imply a depleted source with a long time integrated high Lu/Hf, Sm/Nd, Sr/Rb ratios. In situ U–Pb age determination was performed on perovskites from the carbonatites and phoscorites and also on pyrochlores and baddeleyites from carbonatites. The co-existing pyrochlores, perovskites and baddeleyites in carbonatites yielded ages of 252.3 ± 1.9, 252.5 ± 1.5 and 250.8 ± 1.4 Ma, respectively. The perovskites from the phoscorites yielded an age of 253.8 ± 1.9 Ma. The obtained age for Guli carbonatites and phoscorites lies within the range of ages previously reported for the Siberian Flood Basalts and suggest essentially synchronous emplacement with the Permian-Triassic boundary.

Late Triassic (Carnian) lamproites from Noril'sk, polar Siberia: Evidence for melting of the recycled Archean crust and the question of lamproite source for some placer diamond deposits of the Siberian Craton

Two typical lamproitic dykes were found in Noril'sk region of the north-western Siberian Craton, which according to mineralogical, geochemical and isotopic criteria belong to anorogenic, non-diamondiferous type of lamproites. According to the geologic relationships, they cut through the Noril'sk-1 intrusion of the Siberian flood basalt province and thus are younger than ~251Ma. 40Ar/39Ar dating of the two dykes yielded ages of 235.24±0.19Ma and 233.96±0.19Ma, showing that they were emplaced in Carnian of the Late Triassic, about 16Ma after the flood basalt event. There are some indications that there were multiple lamproitic dyke emplacements, including probably emplacement of diamondiferous lamproites, which produced Carnian-age diamond-rich placer deposits in other parts of the Siberian Craton and in adjacent regions. Lead isotope modelling shows that the source of the studied lamproites was formed with participation of recycled crust, which underwent modification of its U/Pb ratio as early as 2.5Ga. However, the exact mechanism of the recycling cannot be deciphered now. It could be either through delamination of the cratonic crust or subduction of a mix of ancient terrigenous sediments into the mantle transition zone.

Precious metal enrichment at low-redox in terrestrial native Fe-bearing basalts investigated using laser-ablation ICP-MS

Primary native Fe is a rare crystallizing phase from terrestrial basaltic magmas, requiring highly reducing conditions (fO2 <iron-wüstite buffer) in order to form. Reducing conditions in basaltic magmas can be achieved through assimilation of carbonaceous crustal material, which leads to formation of an immiscible, molten, C-rich, native Fe alloy liquid. If this liquid also contains sufficient sulfur, it can undergo further division into conjugate Fe–C-rich and a Fe–S-rich immiscible melts that can effectively scavenge the highly siderophile elements (HSE: Re, Au, and the platinum group elements [PGE], Pd, Pt, Rh, Ru, Ir, Os), as well as Ni and Cu, to economic abundances. Three localities are known globally where native Fe bearing mafic rocks occur: (1) Paleocene basalts of Disko Island, West Greenland; (2) a Miocene lava of the Bühl basalts, Germany; and (3) mafic intrusions associated with the Late Permian Siberian flood basalts. In this contribution, we report major- and minor-element compositions and HSE concentrations for the main alloy phases (FeNi metal and cohenite) and sulfide, for all three known global occurrences of native Fe bearing basalt. Total HSE abundances in metal grains, obtained by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), are lowest in the Bühl basalt, (∼0.05ppm), intermediate in the Disko Island basalts (4–8ppm), and highest the Siberian Khungtukun and Dzhaltul intrusions (10–30ppm). These differences demonstrate that, while native Fe formation is the result of carbonaceous crustal assimilation, HSE enrichment is not ubiquitous during this process. The Siberian occurrences are characterized by Pt PGE (PPGE: Pt, Pd) enrichment relative to the Ir PGE (IPGE: Rh, Ru, Ir, Os), consistent with models of early stage fractionation of olivine, chromite and metallic IPGE in staging magma reservoirs, prior to the addition of C-rich crustal materials in the shallow crust. Relative to Noril’sk Ni–Cu-PGE sulfide ores, the Siberian native Fe basalts are enriched in the PPGE relative to the IPGE, but exhibit Ru enrichments. In contrast, Disko Island native Fe rocks do not show significant fractionation of the PPGE from the IPGE, but have positive Re and Ru anomalies and high Os/Ir ratios. To reconcile these observations, we present a general model where some parental melts experienced early-stage crustal assimilation in staging magma reservoirs, prior to reduction by carbon-rich materials (e.g., Khungtukun, Dzhaltul), whereas basaltic parental melts to Disko Island and Bühl solely experienced interaction with carbonaceous material, resulting in extensive devolatilization, native Fe segregation, and native Fe formation.

Atmospheric outgassing and native-iron formation during carbonaceous sediment–basalt melt interactions

Organic carbon-rich sediment assimilation by basaltic magmas leads to enhanced emission of greenhouse gases during continental flood basalt eruptions. A collateral effect of these interactions is the generation of low oxygen fugacities (fO2) (below the iron-wüstite [IW] buffer curve) during magmatic crystallization, resulting in the precipitation of native-iron. The occurrence of native-iron bearing terrestrial basaltic rocks are rare, having been identified at three locations: Siberia, West Greenland, and Central Germany. We report the first combined study of Re–Os isotopes, highly siderophile element (HSE: Os, Ir, Ru, Pt, Pd, Re), and trace-element abundances for these three occurrences, in addition to host sediments at West Greenland. To quantify the amount of crustal assimilation experienced by the magmas, we present combined crystallization and assimilation models, together with fractional crystallization models, to assess how relative abundances of the HSE have been modified during crystallization. The radiogenic osmium isotopic compositions (γOsinitial +15 to +193) of mafic igneous samples are consistent with assimilation of old high Re/Os crustal contaminants with radiogenic 187Os/188Os, whereas the HSE inter-element fractionations (Pd/Os 2 to >10,000) suggest that some Siberian samples underwent an early stage of sulfide removal.Metalliferous samples from the Siberian intrusions of Khungtukun and Dzhaltul (associated with the Siberian flood basalts) yield internal 187Re–187Os ages of 266±83Ma and 249±50Ma, respectively, reflecting late-Permian emplacement ages. These results imply that crustal assimilation took place prior to crystallization of native-Fe. In contrast, metalliferous samples from Disko Island and Bühl (associated with the West Greenland flood basalts, and the Central European Volcanic Province, respectively) have trends in 187Re/188Os–187Os/188Os space corresponding to apparent ages older than their reported crystallization ages. These anomalous ages probably reflect concurrent assimilation of high Re/Os, radiogenic 187Os crust during crystallization of native-Fe, consistent with the character of local West Greenland sediments. In all three locations, calculations of combined assimilation of crustal sediments and fractional crystallization indicate between 1–7% assimilation can account for the Os-isotope systematics. In the case of Siberian samples, incompatible trace-element abundances indicate that lower crustal assimilation may have also occurred, consistent with the suggestion that crustal assimilation took place prior to native-Fe precipitation. The extent of local crustal contamination at Siberia, West Greenland, and Bühl necessitates that significant quantities of CH4, CO, CO2, SO2 and H2O were released during assimilation of carbonaceous sediments. Consequently, carbonaceous sediment–basalt melt interactions have collateral effects on total gas output from flood basalt volcanism into the atmosphere. However, the amount of carbonaceous sediment contamination experienced by melts forming the Khungtukun and Dzhaltul intrusions alone, cannot explain the major C-isotope excursions at the Permo–Triassic mass-extinction event. Instead, further unsampled intrusions that also experienced significant carbonaceous sediment–melt interactions would be required. Enhanced greenhouse gas-emission during the Permo–Triassic mass extinction may have been facilitated by a combination of mantle melting and carbonaceous sediment–melt interactions, together with other proposed mechanisms, including wildfires, or by microbial metabolic exhalation.

Test of the Ballhaus–Berry–Green Ol–Opx–Sp oxybarometer and calibration of a new equation for estimating the redox state of melts saturated with olivine and spinel

Testing the Ballhaus–Berry–Green Ol–Opx–Sp oxybarometer (BBG) on independent experimental data indicates that it overestimates the oxygen fugacity by 0.6–1.3 log units under mildly reduced conditions (near the C–CO buffer) and by as much as 2–3 log units under reduced conditions (at the IW buffer and below it). A newly developed oxibarometer is suggested to minimize this effect and enhance the capabilities of redoxometry of low-pressure mineral associations, including magmatic melts undersaturated with respect to orthopyroxene (Opx). The new empirical equation of the oxybarometer is applicable to a wide range of mafic–ultramafic magmas of normal alkalinity, including terrestrial, lunar, and meteoritic systems under pressures of 0.001–25 kbar and oxygen fugacity ranging from IW–3 to NNO + 1. The derived regression fits the ΔQFM values of the calibration dataset (154 experiments) accurate to ~0.5 log units. The new oxybarometer eliminates systematic errors when redox parameters are evaluated for the reduced region (from IW–3 to C–CO) and for crystallization of magmas without Opx on the liquidus. The efficiency of the suggested model is demonstrated by its application to natural rocks: (1) low-Ti lunar basalts, (2) tholeiites from the Shatsky Rise, (3) Siberian flood basalts, (4) rocks of the layered series of the Yoko-Dovyren intrusion, and (5) mantle xenoliths collected in southern Siberia, Mongolia, China, and the southern Russian Far East. The values yielded by such oxybarometers for intrusive rocks, which underwent long-lasting cooling and postcumulus reequilibration, should be regarded with reserve.

Highly productive polar forests from the Permian of Antarctica

Two stratigraphically closely spaced bedding planes exposed at Lamping Peak in the Upper Buckley Formation, Beardmore Glacier area, Antarctica contain abundant in situ stumps (n=53, n=21) and other plant fossils that allow reconstruction of forest structure and biomass of Glossopteris forests that thrived at ~75° S paleolatitude in the Permian. Mean trunk diameter is 14 and 25cm, corresponding to estimated mean maximum heights of 12 and 19m. Basal areas are 65 and 80m2ha-1. The above ground biomass was calculated using allometric equations for Ginkgo biloba, yielding biomasses of 147 and 178Mgha−1. Biomass estimates based on comparison with biomass of modern forests with equivalent basal areas are higher (225–400Mgha−1). The amount of above ground biomass added each year (annual net primary productivity), based on biomass estimates and growth rings in silicified plant material from the Buckley Formation nearby, is poorly constrained, ranging from ~100–2000gm−2yr−1.Compared to modern forests at all latitudes, the Permian forests have high basal areas and high biomass, exceeded in both only by forests of the U.S. Pacific northwest and Sequoia forests. The estimated range of productivity (ANPP) is within that of many very productive modern forests. The Lamping Peak forests' basal areas and calculated biomass are also larger than younger high paleolatitude fossil forests except for Arctic Cenozoic forests.The presence of these highly productive fossil forests at high paleolatitude is consistent with hothouse conditions during the Late Permian, prior to the eruption of the Siberian flood basalts.

Sources of the magmatic rocks of the Mendeleev Rise, Arctic Ocean: Evidence from isotope-geochemical data

Mineralogical-petrographic and isotope-geochemical studies were carried out for basic and intermediate magmatic rocks derived by deep-water drilling and dredging at the test sites from 79.02° N to 83.09° N, Arctic Ocean (AO), during expedition “Arktika-2012” with the icebreaker Kapitan Desnitsyn. Volcanic bedrock samples derived from the drill holes at the foot of the Mendeleev Rise are grouped into the low- and moderate alkali rocks, showing narrow ranges in the initial Nd (ɛNd(T) from +4.3 to +7.0) and Sr (87Sr/86Sr from 0.70365 to 0.70495) isotope ratios, and correspond to the volcanic rocks of the oceanic islands and rises in the discriminant isotope and geochemical diagrams. Gabbro-dolerite samples dredged from the slopes of the Mendeleev Rise are grouped into the low-alkali field and show wide variations in Nd (ɛNd from −33.1 to +2.9) and Sr (87Sr/86Sr from 0.7050 to 0.7233) isotope compositions, being similar to the Siberian flood basalts and, possibly, other rocks of continental genesis.

High influx of carbon in walls of agglutinated foraminifers during the Permian–Triassic transition in global oceans

The Permian–Triassic mass extinction is postulated to be related to the rapid volcanism that produced the Siberian flood basalt (Traps). Unrelated volcanic eruptions producing several episodes of ash falls synchronous with the Siberian Traps are found in South China and Australia. Such regional eruptions could have caused wildfires, burning of coal deposits, and the dispersion of coal fly ash. These eruptions introduced a major influx of carbon into the atmosphere and oceans that can be recognized in the wall structure of foraminiferal tests present in survival populations in the boundary interval strata. Analysis of free specimens of foraminifers recovered from residues of conodont samples taken at a Permian–Triassic boundary section at Lung Cam in northern Vietnam has revealed the presence of a significant amount of elemental carbon, along with oxygen and silica, in their test wall structure, but an absence of calcium carbonate. These foraminifers, identified as Rectocornuspira kalhori, Cornuspira mahajeri, and Earlandia spp. and whose tests previously were considered to be calcareous, are confirmed to be agglutinated, and are now referred to as Ammodiscus kalhori and Hyperammina deformis. Measurement of the 207Pb/204Pb ratios in pyrite clusters attached to the foraminiferal tests confirmed that these tests inherited the Pb in their outer layer from carbon-contaminated seawater. We conclude that the source of the carbon could have been either global coal fly ash or forest fire-dispersed carbon, or a combination of both, that was dispersed into the Palaeo-Tethys Ocean immediately after the end-Permian extinction event.

Plume impingement on the Siberian SCLM: Evidence from Re–Os isotope systematics

We report Re–Os and platinum group element (PGE) systematics for a suite of 16 mantle peridotites from the Udachnaya (360Ma) and Obnazhennaya (160Ma) kimberlite pipes, Siberia. Xenoliths from these pipes bracket the thermal climax of the Siberian plume, which is represented by the emplacement of the ~250Ma Siberian Flood Basalts (SFBs). Thus, these xenoliths represent snapshots of the sub-continental lithospheric mantle (SCLM) before and after plume modification. Pre-plume Udachnaya peridotite xenoliths generally display unradiogenic Os-isotopes with respect to CI-chondrite (expressed by γOs, the percentage difference between the Os-isotope composition of a sample and the average chondrite composition; 187Os/188Os — 0.127), coupled with low [Pd/Ir]N, for both whole-rock and olivine mineral-fraction analyses. Such signatures are typical of an ancient depleted cratonic mantle that underwent melt extraction. The preservation of unradiogenic Os-isotope compositions (γOs −5 to −14), coupled with low (<0.4) 187Re/188Os ratios, provides robust melt extraction age estimates, ranging from ~3Ga to ~1.2Ga. This indicates that craton stabilization/growth events not only occurred during the Archean, but also extended into the Proterozoic. A number (4) of post-plume Obnazhennaya peridotites display 187Os/188Os ratios (>0.1292), which overlap the convecting mantle range. At first glance, these observations are in agreement with garnet chemistry data, which indicate that high-degrees of silicate-melt percolated through the lithosphere during the emplacement of the SFB. However, Obnazhennaya olivine mineral-separates display ‘depleted’ systematics (>Fo 92 and low [Pd/Ir]N), consistent with ‘pristine’ melt residues. We suggest that these Obnazhennaya xenoliths represent ‘newly formed’ residues associated with partial melts extracted from the impinging Siberian plume on the SCLM. During plume impingement, thermo-chemical erosion of the lithosphere is thought to be an important process, acting to remove the original depleted material. In the space created, new refractory residues may be able to infill this void; i.e., plume-subcretion. Importantly, two Obnazhennaya peridotites display very unradiogenic Os-isotope compositions (γOs −9 and −10) indicating that ancient depleted lithosphere is preserved after the emplacement of the SFB. Overall, we suggest that plume impingement may not be ubiquitous within the SCLM, leaving a lithosphere that is characterized by both its original depleted residues and newly formed, plume related residues.In addition, two samples from Obnazhennaya and one from Udachnaya, contain highly radiogenic 187Os/188Os ratios (γOs>+85). Such radiogenic values cannot be accounted for by kimberlitic or plume-related metasomatism. Instead, these samples reflect metasomatism from fluids that derived from a source characterized by long-term high 187Re/188Os, such as recycled subducted material (i.e., oceanic crust and sediments). These lithologies were introduced into the SCLM during craton growth events from ~1.8 to 3.0Ga. The identification of such samples reflects an important metasomatic source within the SCLM. Overall, these two suites of mantle peridotites reveal the complex evolution of the Siberian cratonic lithosphere, from birth in the Archean, followed by major depletion events during the Proterozoic, and ending with major tectonothermal perturbation during the impingement of the Siberian plume.

Helium isotopic evidence for modification of the cratonic lithosphere during the Permo-Triassic Siberian flood basalt event

Major flood basalt emplacement events can dramatically alter the composition of the sub-continental lithospheric mantle (SCLM). The Siberian craton experienced one of the largest flood basalt events preserved in the geologic record — eruption of the Permo-Triassic Siberian flood basalts (SFB) at ~ 250 Myr in response to upwelling of a deep-rooted mantle plume beneath the Siberian SCLM. Here, we present helium isotope (3He/4He) and concentration data for petrologically-distinct suites of peridotitic xenoliths recovered from two temporally-separated kimberlites: the 360 Ma Udachnaya and 160 Ma Obnazhennaya pipes, which erupted through the Siberian SCLM and bracket the eruption of the SFB. Measured 3He/4He ratios span a range from 0.1 to 9.8 RA (where RA = air 3He/4He) and fall into two distinct groups: 1) predominantly radiogenic pre-plume Udachnaya samples (mean clinopyroxene 3He/4He = 0.41 ± 0.30 RA (1σ); n = 7 excluding 1 outlier), and 2) ‘mantle-like’ post plume Obnazhennaya samples (mean clinopyroxene 3He/4He = 4.20 ± 0.90 RA (1σ); n = 5 excluding 1 outlier). Olivine separates from both kimberlite pipes tend to have higher 3He/4He than clinopyroxenes (or garnet). Helium contents in Udachnaya samples ([He] = 0.13–1.35 μcm3STP/g; n = 6) overlap with those of Obnazhennaya ([He] = 0.05–1.58 μcm3STP/g; n = 10), but extend to significantly higher values in some instances ([He] = 49–349 μcm3STP/g; n = 4). Uranium and thorium contents are also reported for the crushed material from which He was extracted in order to evaluate the potential for He migration from the mineral matrix to fluid inclusions. The wide range in He content, together with consistently radiogenic He-isotope values in Udachnaya peridotites suggests that crustal-derived fluids have incongruently metasomatized segments of the Siberian SCLM, whereas high 3He/4He values in Obnazhennaya peridotites show that this section of the SCLM has been overprinted by Permo-Triassic (plume-derived) basaltic fluids. Indeed, the stark contrast between pre- and post-plume 3He/4He ratios in peridotite xenoliths highlights the potentially powerful utility of He-isotopes for differentiating between various types of metasomatism (i.e., crustal versus basaltic fluids).

The end-Permian mass extinction: a still unexplained catastrophe

The end-Permian mass extinction is widely regarded as the largest mass extinction in the past 542 million years with loss of about 95% of marine species and 75% of terrestrial species. There has been much focus and speculation on what could have caused such a catastrophe. Despite decades of study the cause or causes remain mysterious. Numerous scenarios have been proposed, including asteroid impact, Siberian flood basalt volcanism, marine anoxia and euxinia, sea-level change, thermogenic methane release and biogenic methane release due to explosive growth of a methanogenic microbe. It is now clear that a number of major environmental perturbations are approximately coincident with the end-Permian mass extinction. These include global negative excursions of both  13 Ccarb and  13 Corg near the extinction interval (see a review by [1] and a recent study by [2]); distinctive calcium isotope excursions [3]; a sudden expansion of microbialites [4]; a rapid temperature rise of ~8 o C in the extinction interval [5] followed by a long “hothouse” period in the Early Triassic [6], large regression followed by rapid transgression [7], evidence for wildfires and cyanobacteria blooms [8] etc. There remains disagreement over the nature, timing, and duration of the environmental perturbations and how they relate to detailed patterns of extinction, resolution of which are critical for understanding the causative mechanism(s).