Asish R. Basu

Synchrony and causal relations between Permian-Triassic boundary crises and siberian flood volcanism

The Permian-Triassic boundary records the most severe mass extinctions in Earths history. Siberian flood volcanism, the most profuse known such subaerial event, produced 2 million to 3 million cubic kilometers of volcanic ejecta in approximately 1 million years or less. Analysis of 40Ar/39Ar data from two tuffs in southern China yielded a date of 250.0 � 0.2 million years ago for the Permian-Triassic boundary, which is comparable to the inception of main stage Siberian flood volcanism at 250.0 � 0.3 million years ago. Volcanogenic sulfate aerosols and the dynamic effects of the Siberian plume likely contributed to environmental extrema that led to the mass extinctions.

Rapid Eruption of the Siberian Traps Flood Basalts at the Permo-Triassic Boundary

The Siberian Traps represent one of the most voluminous flood basalt provinces on Earth. Laser-heating 40Ar/39Ar data indicate that the bulk of these basalts was erupted over an extremely short time interval (900,000 � 800,000 years) beginning at about 248 million years ago at mean eruption rates of greater than 1.3 cubic kilometers per year. Such rates are consistent with a mantle plume origin. Magmatism was not associated with significant lithospheric rifting; thus, mantle decompression resulting from rifting was probably not the primary cause of widespread melting. Inception of Siberian Traps volcanism coincided (within uncertainty) with a profound faunal mass extinction at the Permo-Triassic boundary 249 � 4 million years ago; these data thus leave open the question of a genetic relation between the two events.

Major element, REE, and Pb, Nd and Sr isotopic geochemistry of Cenozoic volcanic rocks of eastern China: implications for their origin from suboceanic-type mantle reservoirs

Major- and rare-earth-element (REE) concentrations and UThPb, SmNd, and RbSr isotope systematics are reported for Cenozoic volcanic rocks from northeastern and eastern China. These volcanic rocks, characteristically lacking the calc-alkaline suite of orogenic belts, were emplaced in a rift system which formed in response to the subduction of the western Pacific plate beneath the eastern Asiatic continental margin. The rocks sampled range from basanite and alkali olivine basalt, through olivine tholeiite and quartz tholeiite, to potassic basalts, alkali trachytes, pantellerite, and limburgite. These rock suites represent the volcanic centers of Datong, Hanobar, Kuandian, Changbaishan and Wudalianchi in northeastern China, and Mingxi in the Fujian Province of eastern China. The major-element and REE geochemistry is characteristic of each volcanic suite broadly evolving through cogenetic magmatic processes. Some of the outstanding features of the isotopic correlation arrays are as follows: (1) NdSr shows an anticorrelation within the field of ocean island basalts, extending from the MORB end-member to an enriched, time-averaged high Rb/Sr and Nd/Sr end-member (EM1), (2) SrPb also shows an anticorrelation, similar to that of Hawaiian and walvis Ridge basalts, (3) NdPb shows a positive correlation, and (4) the 207Pb/204Pb vs 206Pb/204Pb plot shows linear arrays parallel to the general trend (NHRL) for MORB on both sides of the geochron, although in the 208Pb/204Pb vs 206Pb/204Pb plot the linear array is significantly displaced above the NHRL in a pattern similar to that of the oceanic island basalts that show the Dupal signatures. In all isotope correlation patterns, the data arrays define two different mantle components—a MORB-like component and an enriched mantle component. The isotopic data presented here clearly demonstrate the existence of Dupal compositions in the sources of the continental volcanic rocks of eastern China. We suggest that the subcontinental mantle beneath eastern China served as the reservoir for the EMI component, and that the MORB component was either introduced by subduction of the Kula-Pacific Ridge beneath the Asiatic plate in the Late Cretaceous, as proposed by Uyeda and Miyashiro, or by upwellings in the subcontinental asthenosphere due to subduction.

Sr, Nd, and Pb isotopes of ultramafic xenoliths in volcanic rocks of Eastern China: enriched components EMI and EMII in subcontinental lithosphere

The U-Th-Pb, Sm-Nd, and Rb-Sr isotopic systematics of mafic and ultramafic xenolithic rocks and associated megacrystic inclusions of aluminous augite and garnet, that occur in three alkalic volcanic suites: Kuandian in eastern Liaoning Province, Hanluoba in Hebei Province, and Minxi in western Fujian Province, China are described. In various isotopic data plots, the inclusion data invariably fall outside the isotopic ranges displayed by the host volcanic rocks, testifying to the true xenolithic nature of the inclusions. The major element partitioning data on Ca, Mg, Fe, and Al among the coexisting silicate minerals of the xenoliths establish their growth at ambient mantle temperatures of 1000–1100°C and possible depths of 70–80 km in the subcontinental lithosphere. Although the partitioning of these elements reflects equilibrium between coexisting minerals, equilibria of the Pb, Nd, and Sr isotopic systems among the minerals were not preserved. The disequilibria are most notable with respect to the206Pb/204Pb ratios of the minerals. On a Nd-Sr isotopic diagram, the inclusion data plot in a wider area than that for oceanic basalts from a distinctly more depleted component than MORB with higher143Nd/144Nd and a much broader range of87Sr/86Sr values, paralleling the theoretical trajectory of a sea-water altered lithosphere in Nd-Sr space. The garnets consistently show lowerμ andκ values than the pyroxenes and pyroxenites, whereas a phlogopite shows the highestμ andκ values among all the minerals and rocks studied. In a plot ofΔ207 andΔ208, the host basalts for all three areas show lowerΔ207 and higherΔ208 values than do the xenoliths, indicating derivation of basalts from Th-rich (relative to U) sources and xenoliths from U-rich sources. The xenolith data trends toward the enriched mantle components, EMI and EMII-like, characterized by high87Sr/86Sr andΔ207 values but with slightly higher143Nd/144Nd. The EMI trend is shown more distinctly by the host basalts. The EMII mantle domain may be present in the Chinese continental lithosphere just above the EMI domain of the basalt source at the lower part of the lithosphere. We argue that the ancient depleted continental lithosphere was metasomatized, imparting the EMI signature, in earlier times ( > 1000 m.y.), and U migrated upward, resulting in highTh/U ratios in the lower portion of the lithosphere. Observed highTh/U,Rb/Sr,87Sr/86Sr andΔ208, lowSm/Nd ratios, and a large negativeɛNd in phlogopite pyroxenite with a depleted mantle model age of 2.9 Ga, support our contention that metasomatized continental lower mantle lithosphere is the source for the EMI component. We also suggest that the EMII signature may have been introduced later (less than ∼ 500 Ma) by another metasomatic event during the subduction of an oceanic plate, which was partially responsible for some of the observed inter-mineral isotopic disequilibria.

Early and Late Alkali Igneous Pulses and a High-3He Plume Origin for the Deccan Flood Basalts

Several alkalic igneous complexes of nephelinite-carbonatite affinities occur in extensional zones around a region of high heat flow and positive gravity anomaly within the continental flood basalt (CFB) province of Deccan, India. Biotites from two of the complexes yield 40Ar/39Ar dates of 68.53 � 0.16 and 68.57 � 0.08 million years. Biotite from a third complex, which intrudes the flood basalts, yields an 40Ar/39Ar date of 64.96 � 0.1 1 million years. The complexes thus represent early and late magmatism with respect to the main pulse of CFB volcanism 65 million years ago. Rocks from the older complexes show a 3He/4He ratio of 14.0 times the air ratio, an initial 87Sr/86Sr ratio of 0.70483, and other geochemical characteristics similar to ocean island basalts; the later alkalic pulse shows isotopic evidence of crustal contamination. The data document 3.5 million years of incubation of a primitive, high-3He mantle plume before the rapid eruption of the Deccan CFB.

High-3He plume origin and temporal-spatial evolution of the Siberian flood basalts

An olivine nephelinite from the lower part of a thick alkalic ultrabasic and mafic sequence of volcanic rocks of the northeastern part of the Siberian flood basalt province (SFBP) yielded a 40Ar/39Ar plateau age of 253.3 � 2.6 million years, distinctly older than the main tholeiitic pulse of the SFBP at 250.0 million years. Olivine phenocrysts of this rock showed 3He/4He ratios up to 12.7 times the atmospheric ratio; these values suggest a lower mantle plume origin. The neodymium and strontium isotopes, rare earth element concentration patterns, and cerium/lead ratios of the associated rocks were also consistent with their derivation from a near-chondritic, primitive plume. Geochemical data from the 250-million-year-old volcanic rocks higher up in the sequence indicate interaction of this high-3He SFBP plume with a suboceanic-type upper mantle beneath Siberia.

Temporal Sr-, Nd- and Pb-isotopic variations in the Siberian flood basalts: Implications for the plume-source characteristics

The Sr-, Nd- and Pb-isotopic compositions of lavas from the 248-Ma-old Siberian Flood Basalt Province (SFBP) become increasingly uniform with height as the main phase of volcanic activity is approached. The late-stage lava sequences from the Putorana subprovince, which constitute approximately 90 vol% of the SFBP, require a homogeneous mantle as their source. Volcanostratigraphic and volume arguments imply that extreme isotopic compositions, displayed by the early lavas from Norilsk subprovince, are not the end-member compositions for the bulk of the SFBP. In 147/Sm144Nd-initial eNd space, most of the Putorana basalts define a horizontal array with a majority clustering at (0.17, +1.8). In contrast, the Norilsk lavas show a positively correlated trend indicative of continental lithospheric contributions to the primary plume. Among the three isotopic systems considered, Nd shows the least effect of crustal contamination. We estimate an initial eNd value of +1.8 ± 0.7 (1 σ) for the uncontaminated Siberian plume, which points to a near-chondritic, slightly depleted lower mantle source. The Siberian basalts, which show an eNd of ⩾ 0, display a remarkably uniform Sr-isotopic composition with an average 87/Sr86Sr of 0.7050 ± 3 (1 σ). However, in a (87/Sr86Sr)i-eNd diagram these lavas are significantly displaced to the right-hand side of a mixing line between bulk earth and depleted mantle. We suggest that these basalts suffered selective contamination of Sr during ascent through the continental crust. This element-specific contamination, which may also be concluded from Nd—Pb and Sr—Pb isotopic variations in the basalts, is at a maximum for Pb-isotopes and at a minimum for Nd. The Pb-isotopic ratios of the contaminated basalts indicate significant contributions from the lower crust with little or no input from the upper crust. The early Norilsk lavas define the extreme compositions and help constrain the initial Pb-isotopic composition of the plume source as 206/Pb204Pb= 18.3, 207/Pb204Pb= 15.5 and 208/Pb204Pb= 38.0, which falls slightly to the right of the geochron at 248 Ma B.P.. We propose a two-stage growth history of Pb for the plume source: An early core-mantle differentiation stage, with a μ value of 0.21 for the first 100–150 m.y. after the Earth formed, was followed in the second stage by an increase in the μ value to about 9.2 for the rest of Earths evolution.

The groundwater geochemistry of the Bengal Basin: Weathering, chemsorption, and trace metal flux to the oceans

Sixty-eight groundwater samples from the Ganges-Brahmaputra floodplain in the Bengal Basin were analyzed to assess the groundwater geochemistry, the subsurface hydrology, the buffering effects of sediments on trace metal concentrations and their isotopic compositions, and the magnitude of the subsurface trace element flux to the Bay of Bengal and to the global ocean. Samples obtained from depths of 10 to 350 m were measured for major and trace elements, dissolved gas, and tritium. On the basis of the 3He/3H ages, the groundwater at depth (30–150 m) appears to be continually replenished, indicating that this recharge of groundwater to depth must ultimately be balanced by a significant quantity of submarine discharge into the Bay of Bengal. Using the 3He/3H groundwater age–depth relationship to calculate a recharge rate of 60 ± 20 cm/yr, we estimate a subsurface discharge into the Bay of Bengal of 1.5 ± 0.5 × 1011 m3/yr, or 15% of the surface Ganges-Brahmaputra river (GBR) flux. Several trace elements, especially Sr and Ba, display elevated concentrations averaging 7 to 9 times the surface GBR water values. The submarine groundwater fluxes of Sr and Ba to the oceans are 8.2 ± 2 × 108 and 1.5 ± 0.3 × 108 mol/yr, or 3.3 and 1.2%, respectively, of the world total, or equal to the surface GBR Sr and Ba estimated fluxes. Our groundwater flux for Ba agrees with the estimate of Moore (1997) (3 × 108–3 × 109 mol/yr), on the basis of measured Ba and Ra excesses in the Bay of Bengal. Other trace metals, such as U and Mo, are at low but measurable levels and are not major contributors to the global flux in this river system. A comparison of the Sr and Ba concentrations, plus 87Sr/86Sr ratios in groundwater to the oxalate extractable fractions of a coastal sediment core, suggests that weathering of carbonates and minor silicates, coupled with cation exchange plus adsorption and desorption reactions, controls the trace element concentrations and 87Sr/86Sr isotopic compositions in both the groundwater and river water. Our data also imply that other coastal floodplains (e.g., the Mekong and the Irrawaddy rivers) that have high precipitation rates and rapid accumulation of immature sediments are likely to make significant contributions to the global oceanic trace metal budgets and have an impact on the Sr isotopic evolution in seawater.

Origin of the Sudbury Complex by Meteoritic Impact: Neodymium Isotopic Evidence

Samarium-neodymium isotopic data on whole rocks and minerals of the Sudbury Complex in Canada gave an igneous crystallization age of 1840 � 21 x 106 years. The initial epsilon neodymium values for 15 whole rocks are similar to those for average upper continental crust, falling on the crustal trend of neodymium isotopic evolution as defined by shales. The rare earth element concentration patterns of Sudbury rocks are also similar to upper crustal averages. These data suggest that the Sudbury Complex formed from melts generated in the upper crust and are consistent with a meteoritic impact.

Source of Oligocene to Pliocene sedimentary rocks in the Linxia basin in northeastern Tibet from Nd isotopes: Implications for tectonic forcing of climate

We used Nd isotopes and trace element data to determine the provenance of sedimentary rocks in the Linxia basin, northeastern Tibet, whose Oligocene through Pliocene sedimentation history has been interpreted to refl ect deposition in a fl exural basin associated with contractional deformation along the northeastern margin of the Tibetan Plateau. Paleozoic‐early Mesozoic metasedimentary source rocks from the KunlunQaidam and Songpan-Ganzi terranes have e Nd values between −11.8 and −16.1, whereas Paleozoic and Mesozoic plutonic source rocks that intrude the metasedimentary rocks have more positive e Nd values between −3.6 and −11.2. Cretaceous sedimentary source rocks display e Nd values of −9.7 and ‐9.9 in the Maxian Shan, north of the Linxia basin, and ‐15.3 in the plateau margin south of the basin. With e Nd values that range between −8.4 and −10.4 before ca. 15 Ma, and −6.2 and −11.8 after ca. 14 Ma, sedimentary rocks of the Linxia basin are less negative than metasedimentary rocks, which are dominant source rocks within the margin of the Tibetan Plateau today. The relatively positive e Nd values of Linxia basin sedimentary rocks could refl ect several possible sources, including (1) a mixture of plutonic and metasedimentary rocks within the northeastern margin of Tibet, (2) Cretaceous sedimentary rocks derived from the north, or (3) loess derived from central Asian deserts. A mass balance calculation indicates that plutonic rocks are not volumetrically signifi cant enough to generate the e Nd values observed in Linxia basin sedimentary rocks through mixing of plutonic and metasedimentary sources. Rare earth element patterns suggest that Cretaceous rocks were not a dominant source of sediment. The Nd isotopic composition and rare earth element pattern of Quaternary loess are similar to older deposits in the Linxia basin and refl ect loess deposited elsewhere in the Loess plateau and the North Pacifi c (e Nd = −8.6 to ‐10.5). In addition, the modern Daxia River, which drains the margin of the plateau today, transports clay and silt with e Nd values of ‐10.5 to ‐10.8 despite the river’s source in more negative metasedimentary rocks of the Kunlun-Qaidam and Songpan-Ganzi terranes, which indicates that the modern fi ne-grained sedimentary budget is dominated by recent loess deposits. Considering the slow sedimentation rates in the Linxia basin, it is likely that loess sources have contributed a signifi cant volume of fi grained sediment to this basin throughout its history. An increase in the range of e Nd values at ca. 14 Ma in the Linxia basin may refl ect increased unroofi ng of the northeastern margin of Tibet, which slightly preceded a change in climate between ca. 13 and 12 Ma in the Linxia basin. A 1.5‰ increase in baseline δ 18 O values of lacustrine carbonates has been interpreted as the result of reorganization of atmospheric circulation and an increase in aridity on the northeastern margin of the Tibetan Plateau, perhaps associated with the plateau having achieved an elevation suffi cient to block moisture from the Indian Ocean and/or Pacifi c Ocean. Similar timing of exhumation and climate change suggests that northeastward and eastward propagation of the plateau margin was responsible for the middle Miocene climate change observed in the Linxia basin.