Gerald K. Czamanske

Rapid eruption of Siberian flood-volcanic rocks and evidence for coincidence with the Permian–Triassic boundary and mass extinction at 251 Ma

The Siberian flood-volcanic event is the most voluminous and explosive, continental, volcanic event known in the Phanerozoic record. U^Pb perovskite and zircon ages were obtained for lavas of the lowermost unit (251.7 9 0.4 Ma) and near-uppermost unit (251.1 9 0.3 Ma), respectively, of the volcanic sequence in the Maymecha^Kotuy area, Russia. Along with stratigraphic correlations and paleomagnetic evidence, these ages suggest that rapid extrusion of the entire V6500 m thick composite sequence occurred in less than 1million years. The time of extrusion coincides precisely with an age of 251.4 9 0.3 Ma previously obtained for the Permian^Triassic mass-extinction event, the most devastating biotic crisis known. Emplacement of the Noril’sk^Talnakh ore-bearing intrusions, notable for their prodigious Cu^Ni^PGE deposits, was synchronous with these two major geologic events at 251.2 9 0.3 Ma. The Guli volcanic-intrusive complex in the Maymecha^Kotuy area appears to represent the final mafic magmatism of the entire Siberian flood-volcanic event. Baddeleyite from a carbonatite that intrudes the complex gives an age of 250.2 9 0.3 Ma, and shows possible 231 Pa excess. The Bolgokhtokh granodiorite stock has a zircon age of 229.0 9 0.4 Ma, and represents the youngest known magmatism in the region.

Isotopic and trace-element constraints on mantle and crustal contributions to Siberian continental flood basalts, Noril'sk area, Siberia

Abstract We present a tightly controlled and comprehensive set of analytical data for the 250-Ma Siberian flood-basalt province. Consideration of major- and trace-element compositions, along with strontium, lead and neodymium isotopic compositions, strongly supports earlier Russian subdivision of this magmatism into three magmatic cycles, giving rise to three assemblages of eleven basalt suites in the ascending order Ivakinsky-Gudchikhinsky, Khakanchansky-Nadezhdinsky and Morongovsky-Samoedsky. Geochemical and isotopic discontinuities of varying magnitude characterize most of the boundaries between the eleven recognized basalt suites in the Norilsk area. Although we conclude that the dominant volume of erupted magma originated from an asthenospheric mantle plume, none of the lavas is interpreted to directly represent asthenospheric melts, which would have been far more magnesian. On the basis of thermal considerations, we consider it unlikely that vast volumes of basaltic melt were produced directly from the continental lithospheric mantle beneath the Siberian craton. Moreover, there is little evidence from mantle xenoliths that the geochemical signatures of such melts would correspond to those of the Siberian flood basalts. Studies of melt migration lead us to conclude that transport of asthenospheric melt through the lithospheric mantle would be rapid, by fracture propagation. Lavas from the Gudchikhinsky suite have negligible Ta-Nb anomalies and positive ϵ Nd values and their parental magmas presumably interacted little with the continental lithospheric mantle or crust. All other lavas have negative Ta-Nb anomalies and lower ϵ Nd values that we attribute to interaction with continental crust. The model that we have developed requires discrete contributions from the plume and complex processing of all erupted magmas in the continental crust. The earliest magmas represent small percentages of melt formed in equilibrium with garnet. Over time, the percentage of melting in the source region and the volume of magma produced increased, and garnet was no longer stable in the plume source. All of the plume-derived melts initially contained more than 20 wt% MgO and became less Mg rich by fractionation of olivine as they traversed the lithospheric mantle. We conclude, however, that the most significant control on the geochemical and isotopic compositions of all the erupted lavas was processing of mantle-derived magma in crustal reservoirs during periodic replenishment, periodic tapping, continuous crystal fractionation and wallrock assimilation. Rapid eruption of an extremely large volume of processed magma that varied little in chemical and isotopic composition produced the sequence of relatively monotonous tholeiitic basalts that constitute the 2,300-m-thick third assemblage of the Siberian flood-basalt province near Norilsk.

Mantle and crustal contributions to continental flood volcanism

Arndt, N.T., Czamanske, G.K., Wooden, J.L. and Fedorenko, V.A., 1993. Mantle and crustal contributions to continental flood volcanism. In: M.J.R. Wortel, U. Hansen and R. Sabadini (Editors), Relationships between Mantle Processes and Geological Processes at or near the Earths Surface. Tectonophysics, 223: 39–52. Most continental flood basalts are enriched in incompatible elements and have high initial 87Sr/86Sr ratios and low ϵNd values. Many are depleted in Nb and Ta. The commonly-held view that these characteristics are inherited directly from a source in metasomatized lithospheric mantle is inconsistent with the following arguments: (1) thermomechanical modelling demonstrates that flood basalt magmas come mainly from an asthenospheric or plume source, with minimal direct melting of the continental lithospheric mantle. The low water contents of most flood basalts argue against proposals that hydrous lithosphere was the source. (2) Lithospheric mantle normally has low concentrations of incompatible elements, and chondrite-normalized Nb and Ta contents similar to those of other incompatible elements. Such material cannot be the unmodified source of Nb-Ta-depleted basalts such as those from the Karoo, Ferrar, or Columbia River provinces. We suggest there are two main controls on the compositions of continental flood basalts. The first is lithospheric thickness, which strongly influences the depth and degree of mantle melting of a plume or asthenospheric source, and thus has an important influence on the composition of primary magmas. All liquids formed by partial melting of peridotite at sub-lithosphere depths are highly magnesian (20–25 wt.% MgO) but have variable trace-element contents. Where the lithosphere is thick, the source melts at high pressure, garnet is present, the degree of melting is low, and trace-element concentrations are high. This type of magma evolves to produce the high-Ti type of continental flood basalt. Where the lithosphere is thinner, the source ascends to shallower levels, the degree of melting is greater, garnet may be exhausted, and the magmas have lower trace-element contents; these magmas yield low-Ti basalts. The second control is processing of magmas in chambers that were periodically replenished and tapped, while continuously fractionating and assimilating their wall rocks. The uniform compositions of basalts that evolve in such chambers are far removed from those of their picritic parental magmas. Major elements in continental flood basalts reflect control by olivine, pyroxene, and plagioclase crystallization, and this assemblage places the magma chambers at crustal depth. We believe that trace-element and isotopic compositions are also buffered, and that the erupted basalts represent steady-state liquids tapped from these magma chambers. These processes impose a crustal signature on the magmas, as expressed most strongly in the concentrations of incompatible elements (e.g., Nb-Ta anomalies) and their isotopic characteristics.

RE-OS ISOTOPIC EVIDENCE FOR AN ENRICHED-MANTLE SOURCE FOR THE NORIL'SK-TYPE, ORE-BEARING INTRUSIONS, SIBERIA

Magmatic Cu-Ni sulfide ores and spatially associated ultramafic and mafic rocks from the Norilsk I, Talnakh, and Kharaelakh intrusions are examined for Re-Os isotopic systematics. Neodymium and lead isotopic data also are reported for the ultramafic and mafic rocks. The Re-Os data for most samples indicate closed-system behavior since the ca. 250 Ma igneous crystallization age of the intrusions. There are small but significant differences in the initial osmium isotopic compositions of samples from the three intrusions. Ores from the Norilsk I intrusion have γOs values that vary from +0.4 to +8.8, but average +5.8. Ores from the Talnakh intrusion have γOs values that range from +6.7 to +8.2, averaging +7.7. Ores from the Kharaelakh intrusion have γOs values that range from +7.8 to +12.9, with an average value of +10.4. The osmium isotopic compositions of the ore samples from the Main Kharaelakh orebody exhibit minimal overlap with those for the Norilsk I and Talnakh intrusions, indicating that these Kharaelakh ores were derived from a more radiogenic source of osmium than the other ores. Combined osmium and lead data for major orebodies in the three intrusions plot in three distinct fields, indicating derivation of osmium and lead from at least three isotopically distinct sources. Some of the variation in lead isotopic compositions may be the result of minor lower-crustal contamination. However, in contrast to most other isotopic and trace element data, Os-Pb variations are generally inconsistent with significant crustal contamination or interaction with the subcontinental lithosphere. Thus, the osmium and lead isotopic compositions of these intrusions probably reflect quite closely the compositions of their mantle source, and suggest that these two isotope systems were insensitive to lithospheric interaction. Ultramafic and mafic rocks have osmium and lead isotopic compositions that range only slightly beyond the compositions of the ores. These rocks also have relatively uniform ϵNd values that range only from −0.8 to + 1.1. This limited variation in neodymium isotopic composition may reflect the characteristics of the mantle sources of the rocks, or it may indicate that somehow similar proportions of crust contaminated the parental melts. The osmium, lead, and neodymium isotopic data for these rocks most closely resemble the mantle sources of certain ocean island basalts (OIB), such as some Hawaiian basalts. Hence, these data are consistent with derivation of primary melts from a mantle source similar to that of some types of hotspot activity. The long-term Re/Os enrichment of this and similar mantle sources, relative to chondritic upper mantle, may reflect 1. (1) incorporation of recycled oceanic crust into the source more than 1 Ga ago, 2. (2) derivation from a mantle plume that originated at the outer core-lower mantle interface, or 3. (3) persistence of primordial stratification of rhenium and osmium in the mantle.

Composition and phase chemistry of sulfide globules in basalt from the Mid-Atlantic Ridge rift valley near 37°N lat

The electron microprobe was used to determine the bulk composition of immiscible sulfide globules trapped in the glass phase of 25 fresh submarine basalt samples from the Mid-Atlantic Ridge. Twenty-three samples represent a spectrum of primitive through differentiated tholeiites from the FAMOUS dive area; two are differentiated basalts from the Reykjanes Ridge. The analyzed globules range in diameter from 11 to 233 µm. On the average, they constitute only 0.0022 volume percent of the rocks and contain less than 1.5 percent of the sulfur. Compositions of the globules change with differentiation as measured by Fe/(Fe+Mg) or TiO 2 content of the host glass. Globules in glass containing 0.66 to 1.0 wt percent TiO 2 typically contain 20 to 26 wt percent Ni + Cu and have an average atomic Ni/Cu of 1.6. With differentiation toward 1.6 wt percent TiO 2 , Ni + Cu content of the globules falls to less than 10 wt percent and atomic Ni/Cu falls to 0.4. Sulfur content of the host glasses shows a strong correlation with FeO content, increasing from 840 ppm to 1,370 ppm as FeO content increases from 8.0 to 12.6 wt percent. Reference to experimental studies shows that this relationship is consistent with sulfur saturation of the host glass at liquidus temperatures. Crystal fractionation is considered to be the dominant factor in keeping the differentiating melt at sulfur saturation. The sulfide globules may have persisted in the basaltic melt from its place of formation by partial melting in the mantle, or they may have exsolved from the melt as it became sulfur-saturated in a high-level magma chamber. Globule abundance and composition indicate adjustment to the composition of the melt in which they were trapped. Material balance calculations suggest that one-third of the Cu and commensurate amounts of S, Ni, and Fe have settled from the magma as immiscible globules. The sulfide globules contain less than 4 wt percent magnetite, compatible with low f o 2 in the magma. Three sulfide phases coexisted in the globules at about 600 °C: monosulfide solid solution, intermediate solid solution, and pentlandite. At lower temperatures, the intermediate solid solution has broken down, and the monosulfide solid solution has exsolved a second generation of pentlandite.

Applications of the 190Pt186Os isotope system to geochemistry and cosmochemistry

Abstract Platinum is fractionated from osmium primarily as a consequence of processes involving sulfide and metal crystallization. Consequently, the 190Pt186Os isotope system (190Pt → 186Os + α) shows promise for dating some types of magmatic sulfide ores and evolved iron meteorites. The first 190Pt 186Os isochrons are presented here for ores from the ca. 251 Ma Norilsk, Siberia plume, and for group IIAB magmatic iron meteorites. Given the known age of the Norilsk system, a decay constant for 190Pt is determined to be 1.542 × 10−12a−1, with ±1% uncertainty. The isochron generated for the IIAB irons is consistent with this decay constant and the known age of the group. The 186Os/188Os ratios of presumably young, mantle-derived osmiridiums and also the carbonaceous chondrite Allende were measured to high-precision to constrain the composition of the modern upper mantle. These compositions overlap, indicating that the upper mantle is chondritic within the level of resolution now available. Our best estimate for this 186Os/188Os ratio is 0.119834 ± 2 (2σM). The 190Pt/186Os ratios determined for six enstatite chondrites average 0.001659 ± 75, which is very similar to published values for carbonaceous chondrites. Using this ratio and the presumed composition of the modern upper mantle and chondrites, a solar system initial 186Os/188Os ratio of 0.119820 is calculated. In comparison to the modern upper mantle composition, the 186Os/188Os ratio of the Norilsk plume was approximately 0.012% enriched in 186Os. Possible reasons for this heterogeneity include the recycling of Pt-rich crust into the mantle source of the plume and derivation of the osmium from the outer core. Derivation of the osmium from the outer core is our favored model.

A minimum UPb age for Siberian flood-basalt volcanism

Abstract Establishing an accurate and precise age for Siberian flood-basalt volcanism is of great importance in evaluating causes for the unequaled mass extinction of flora and fauna at the Permian-Triassic boundary. We report a new, minimum UPb age obtained from zircon and baddeleyite from the mineralized Norilsk I intrusion that cuts the lower third of this rapidly deposited, 3500-m-thick volcanic sequence near Norilsk. This 251.2 ± 0.3 (2σ) Ma age is within analytical error of the SHRIMP UPb age for zircon from the Permian-Triassic boundary at Meishan, South China [251.1 ± 3.6 Ma (2σ)], and confirms Siberian basaltic volcanism as a possible contributor to the mass extinction.

Demise of the Siberian Plume: Paleogeographic and Paleotectonic Reconstruction from the Prevolcanic and Volcanic Record, North-Central Siberia

The Siberian flood basalts are underlain almost everywhere by terrigenous, coal-bearing sedimentary rocks of the Tungusskaya Series, which is Middle Carboniferous to Late Permian in age and commonly ranges in thickness from 100-150 m to 1400 m. Systematic studies of paleogeographic and paleotectonic conditions during Tungusskaya Series accumulation indicate that its deposition was accompanied by well-balanced subsidence throughout the area occupied by well-developed flood-basalt sequences. The surrounding territories, which experienced denudation and fed this accumulation, subsequently experienced little or no flood-basalt activity. Pronounced inheritance is observed in the evolution of the areas of accumulation and denudation, with no reorganization in the Late Permian that can be ascribed to the influence of a mantle plume. Moderate erosion (from tens to several hundreds of meters), with some local uplifts and local, complex folding, have been observed at the sedimentary/volcanic interface, but these up...

Petrogenesis of the flood-basalt sequence at Noril'sk, North Central Siberia

The 3500-m-thick sequence of volcanic rocks at Norilsk, formed during a brief interval (∼1 m.y.) at the Permian/Triassic time boundary (∼251 Ma), represents the earliest part of the ∼6500-m-thick sequence presently ascribed to the Siberian flood-basalt province. It is composed of picritic and basaltic lavas of both low-Ti and high-Ti parentage. Extensive geological, geochemical, and isotopic study of the lava sequence and related intrusions allows detailed reconstruction of its petrogenesis. Various crustal-related processes-fractionation, crustal contamination, sulfide separation, and magma mixing-participated in the formation of the lavas. The geochemical and isotopic characteristics indicative of these processes, as well as mantle-related signatures of lava compositions, are discussed. Based on these characteristics, detailed interpretations of lava genesis and evolution throughout the Norilsk sequence are presented. Eight varieties of lavas are recognized to be primitive, similar in composition to p...

Osmium and neodymium isotopic constraints on the temporal and spatial evolution of Siberian flood basalt sources

High-Mg volcanic rocks from the ca. 250 Ma old Siberian Flood Basalt Province (SFBP) were analyzed for their osmium and neodymium isotopic compositions in order to help to constrain source characteristics as the system evolved. Picrites from the Gudchikhinsky suite, the oldest rocks examined, have γOs of +5.3 to +6.1 and ϵNd of +3.7 to +4.0. The osmium and neodymium isotopic compositions of these rocks are similar to some modern ocean-island basalts (OIB), consistent with their derivation from a mantle plume, and show little evidence for interaction with either subcontinental lithospheric mantle (SCLM), or the Precambrian Siberian craton through which the parental melts passed. Picrites from the stratigraphically higher Tuklonsky suite have similar γOs of +3.4 to +6.5, but ϵNd Of −0.9 to −2.6. The similar γOs but lower ϵNd for the Tuklonsky picrites as compared with the Gudchikhinsky picrites suggest that some magmas from the same OIB-type, mantle source were contaminated by lithospheric components. The osmium isotopic composition of the Tuklonsky picrites was not significantly affected by this interaction, possibly because Os concentrations in the magmas were substantially greater than those in the contaminant. A differentiated ankaramite flow, associated with the top of the stratigraphically higher Morongovsky suite, has γOs of +9.8 to +10.2 and ϵNd of +1.3 to +1.4. The higher γOs may indicate that the plume source was heterogeneous with respect to osmium isotopic composition, consistent with osmium isotopic measurements in rocks from other plume sources. In contrast to these rocks, Mg-rich, alkaline rocks (meymechites) from the Guli area that erupted much nearer the end of the flood-basalt event have γOs of −1.2 to −2.6 and ϵNd of +3.7 to +4.9. These rocks were probably produced by low degrees of partial melting of mantle after the main stages of flood-basalt production. The relatively low γOs and high ϵNd for the meymechites, together with a variety of trace-element characteristics, are most consistent with derivation from a mixed source—one that included both the 0113-type source that fed the majority of the flood-basalt system and a major component from the SCLM underlying the Siberian craton. These results, taken together with earlier investigations of the Norilsk-type ore-bearing intrusions, suggest that much of the SFBP consists dominantly of plume-derived material, until relatively late in the magmatic event, when the SCLM became a significant source of material.