During chemical weathering, magnesium (Mg) is released by the dissolution of both carbonate and silicate sources. The degree to which solute concentrations and isotopic compositions of rivers reflect the relative proportions of these two inputs, or cycling by a series of processes associated with weathering is poorly constrained. In the river waters of the Mackenzie Basin (Canada), the Mg content is high and Mg isotope ratios (Mg-26/Mg-24 expressed as delta Mg-26) show in excess of one per mil variability. Part of this variability is attributed to the 3 parts per thousand range in the carbonate and silicate rocks drained. Despite this inherent lithological control on river water delta Mg-26 values, there is also evidence for a fractionation control. A linear positive covariation between lithium (Li-7/Li-6, expressed as delta Li-7) and Mg isotope ratios in the river waters of the Mackenzie Basin is reported. This covariation is not expected because previously reported fractionation related to physicochemical processes associated with clays or oxides should induce a negative covariation with Mg isotope ratios.This continental-scale covariation can be resolved by either process-related fractionation or mixing. Evidence for fractionation associated with clays is provided firstly by comparing Mg and Li isotopes in both the waters and sediments carried in suspension. Secondly a linear covariation between the sediment concentrations of large ion lithophile elements caesium and rubidium (a proxy for clay content of the sediment) and delta Mg-26 values of the water suggests that processes linked to clay, such as neoformation of clay, cation exchange or adsorption may be important. Simple models illustrate that if the covariation is induced by fractionation, there is either more than one process acting, or a single process is kinetically limited. Alternatively, the data can be reconciled by mixtures between at least three different water bodies, two of which have similar isotopic compositions but differing Li/Mg ratios. This intriguing data set highlights the challenges associated with distinguishing mixing from process with stable isotope data. Despite the complexity, the data question to what extent and by what mechanism clays mediate river water chemistry, at least in terms of the stable isotope compositions of Mg and Li. These questions are fundamental to the quantification of carbon dioxide consumption by silicate weathering and its role in climatic feedback. Crown Copyright (C) 2012 Published by Elsevier B.V. All rights reserved.

Mg isotope ratios (26Mg/24Mg) are reported in soil pore-fluids, rain and seawater, grass and smectite from a 90 kyr old soil, developed on an uplifted marine terrace from Santa Cruz, California. Rain water has an invariant 26Mg/24Mg ratio (expressed as δ26Mg) at −0.79 ± 0.05‰, identical to seawater δ26Mg. Detrital smectite (from the base of the soil profile, and therefore unweathered) has a δ26Mg value of 0.11‰, potentially enriched in 26Mg by up to 0.3‰ compared to the bulk silicate Earth Mg isotope composition (although within the range of all terrestrial silicates). The soil pore-waters show a continuous profile with depth for δ26Mg, ranging from −0.99‰ near the surface to −0.43‰ at the base of the profile. Shallow pore-waters (<1 m) have δ26Mg values that are similar to, or slightly lower than the rain waters. This implies that the degree of biological cycling of Mg in the pore-waters is relatively small and is quantified as <32%, calculated using the average Mg isotope enrichment factor between grass and rain (δ26Mggrass-δ26Mgrain) of 0.21‰. The deep pore-waters (1–15 m deep) have δ26Mg values that are intermediate between the smectite and rain, ranging from −0.76‰ to −0.43‰, and show a similar trend with depth compared to Sr isotope ratios. The similarity between Sr and Mg isotope ratios confirms that the Mg in the pore-waters can be explained by a mixture between rain and smectite derived Mg, despite the fact that Mg and Sr concentrations may be buffered by the exchangeable reservoir. However, whilst Sr isotope ratios in the pore-waters span almost the complete range between mineral and rain inputs, Mg isotopes compositions are much closer to the rain inputs. If Mg and Sr isotope ratios are controlled uniquely by a mixture, the data can be used to estimate the mineral weathering inputs to the pore-waters, by correcting for the rain inputs. This isotopic correction is compared to the commonly used chloride correction for precipitation inputs. A consistent interpretation is only possible if Mg isotope ratios are fractionated either by the precipitation of a secondary Mg bearing phase, not detected by conventional methods, or selective leaching of 24Mg from smectite. There is therefore dual control on the Mg isotopic composition of the pore-waters, mixing of two inputs with distinct isotopic compositions, modified by fractionation. The data provide (1) further evidence for Mg isotope fractionation at the surface of the Earth and (2) the first field evidence of Mg isotope fractionation during uptake by natural plants. The coherent behaviour of Mg isotope ratios in soil environments is encouraging for the development of Mg isotope ratios as a quantitative tracer of both weathering inputs of Mg to waters, and the physicochemical processes that cycle Mg, a major cation linked to the carbon cycle, during continental weathering.

We have re-assessed the Sr isotopic budget of the modern ocean taking into account the high erosion rates of volcanic islands, and especially of island arcs, emphasizing important contribution from subsurface weathering to global budgeting. We propose that intensive weathering on volcanic islands, island arcs and oceanic islands, coupled with large surface and subsurface water fluxes is the missing source of mantle-derived 87Sr/86Sr (0.703) in seawater Sr isotope balance. In our approach, it represents 60% of the actual mantle-like input of Sr to the oceans, the remaining 40% supplied by ridge-crest hydrothermal activity and sea-floor low-temperature alteration of basalts. The seawater Nd isotopic budget is consistent with this interpretation and explains well the regional contributions from ridge crest and island arc activity among the oceans.

The standard addition method is evaluated to verify the accuracy and precision of Mg and Ca isotope data with complex matrices, using the standard-sample bracketing technique and analysis by MC-ICP-MS. The 44Ca/42Ca ratio of seawater (expressed as δ4442Ca relative to SRM915a) was determined as 0.93±0.03‰ (95% confidence), in agreement with estimates obtained by the double spike method. Using standard addition, the seawater 26Mg/24Mg ratio (expressed as δ26Mg relative to the DSM3 standard) was determined as −0.80±0.06‰ (95% confidence) in agreement with previous estimates. Four terrestrial silicate rocks (MORB, flood basalt, glacial flour, and granodiorite) and olivine mineral separates from an island basalt are shown to exhibit no scatter within the error of the method, averaging a δ26Mg of −0.20±0.05‰ (95% confidence). Although a number of silicate rock data for Mg isotope ratios have already been reported, this is the first detailed effort to validate the accuracy of such data and test for residual analytical artifact after chemical purification of samples. Data regressions were evaluated statistically using the mean square weighted deviate (MSWD), demonstrating that the uncertainty on individual data points are generally over estimated. The external two standard deviation uncertainty on individual data points is estimated by Monte Carlo simulation as <0.075‰ (about a factor of two improvement on early publications of Mg isotope data).The consistency of the standard addition estimates of δ26Mg in silicate rocks imply that if any residual matrix effects are present, then they must be less than the spread of the data (0.11‰) given the diverse range of matrices in each of the samples. The δ26Mg values of the silicate rocks suggest that Mg isotope ratios in silicate material may only have a very restricted range. The δ26Mg values of silicate material in the present study falls between the average values reported by Teng et al. [Teng, F.Z., Wadhwa, M., Helz, R.T., 2007. Investigation of magnesium isotope fractionation during basalt differentiation: implications for a chondritic composition of the terrestrial mantle. Earth and Planetary Science Letters 261, 84–92. doi:10.1016/j.epsl.2007.06.004] and Wiechert and Halliday [Wiechert, U., Halliday, A.N., 2006. Non-chondritic magnesium and the origins of the inner terrestrial planets. Earth and Planetary Science Letters 256, 360–371. doi:10.1016/j.epsl.2007.01.007] and given the spread of published δ26Mg values for chondritic material, a chondritic composition for terrestrial Mg cannot be ruled out. We suggest that some of the small discrepancies between our data and analysis of the same samples in earlier studies, may have arisen because the chemical purification of Mg prior to analysis can easily induce analytical artifact. This method could be expanded to the isotope ratios of other elements, which also rely on correcting for mass bias using the standard-sample bracketing method, where similar analytical discrepancies may also exist.

Rhenium (Re) is one of the least abundant elements in Earth, averaging 0.28 ppb in the primitive mantle. The unique occurrence of rheniite Res, (74.5 wt% of Re) in Kudryavy volcano precipitates raises questions about recycling of Re-rich reservoirs within the Kurile-Kamchatka volcanic Island are setting. The sources of this unique Re enrichment have been inferred from studies of Re-Os isotope systematic and trace elements in volcanic gases, sulphide precipitates and host volcanic rocks. The fumarolic gas condensates are enriched in hydrophile trace elements relative to fluid-immobile elements and exhibit high Ba/Nb (133-204), Rb/Y (16-406) and Th/Zr (0.01-0.25) ratios. They are characterised by high Re (7-210 ppb) and Os abundances (0.4-0.9 ppb), with Os-187/Os-188 ratios in a range 0.122-0.152. This Os isotopic compositional range is similar to that of the peridotite xenoliths from the metasomatised mantle wedge above the subducted Pacific plate, the radiogenic isotopic signature of which is probably due to radiogenic addition from a slab-derived fluid. Re- and Os-rich sulphide and oxide minerals precipitate from volcanic gases within fumarolic fields. Molybdenite (MoS2) powellite (CaMoO4) and cannizzarite (Pb4Bi6S13) contain 1.5-1.7 wt%, 10 ppm, and 65-252 ppb of Re, respectively. Both molybdenite and rheniite contain normal Os concentrations, with total Os abundances in a range from 0.6 to 3.1 ppm for molybdenite, and 2.3-24.3 ppb for the rheniite samples. Repeated analyses of osmium isotope ratios for two rheniite samples form a best-fit line with an initial Os-187/Os-188 ratio of 0.32 +/- 0.15 and an age of 79 +/- 11 yr, which is the youngest age ever measured in natural samples. The high Re contents in molybdenite and rheniite led to high radiogenic Os-187 values, even in the limited period of time, with Os-187/Os-188 ratios up to 3.3 for molybdenite and up to 4.4 for rheniite. The Os isotopic compositions of andesite-basaltic rocks from the Kudryavy volcano (Os-187/Os-188 Up to 0.326) are more radiogenic than those of residual peridotites and fumarolic gas condensates that are mainly constituted from magmatic vapor. Such radiogenic values can be attributed either to the addition of a radiogenic Os-rich subduction component to the depleted mantle, or to the assimilation of older dacitic caldera walls (Os-187/Os-188 = 0.6) during arc magma ascent and emplacement. The latter hypothesis is supported by the correlation between Os-187/Os-188 ratio and indicators of fractionation such as MgO or Ni, and by low contents of potentially hydrophile trace elements such as Ba, Rb and Tit relative to fluid-immobile elements such as Nb, Zr and Y. The high Re flux in the Kudryavy volcano (estimated at similar to 46 kg/yr) can be explained by remobilisation of Re by Cl-rich water from an underplated mantle wedge and subducted organic-rich sediments of the Pacific plate. (c) 2007 Elsevier Ltd. All rights reserved.

The Re-Os distribution and isotopic composition have been studied within different ore facies, host-rocks and sediments from the Alexandrinka volcanogenic hydrothermal massive sulphide deposit, Southern Urals, Russia. The osmium contents increase and the initial (OS)-O-187/(OS)-O-188 isotopic compositions decrease in the ore facial range: sulphide chimneys-stockwork zone-seafloor massive and elastic sulphides-metal I iferous sediments. This range reflects variable degrees of reduced hydrothermal fluid-oxidized seawater mixing during the hydrothermal ore-forming process. The Os isotopic composition of the hydrothermal sulphide chimney (Os-187/(188)s= 1.3) is estimated to be a minimum value of the Devonian hydrothermal fluid, which is an intermediate between initial Os isotopic compositions of island-arc volcanics and interlayered sediments. The initial Os isotopic composition of metalliferous sediments ((OS)-O-187/(OS)-O-188 similar to 0.17-0.2) possibly reflects that of the Devonian seawater. The low rhenium concentrations in metalliferous sediments (about 1-2 ppb) could indicate oxic formation conditions.The Re-Os isotope data define a best-fit line corresponding to a Late Devonian age of 355 +/- 15 Ma (2 sigma) with initial (OS)-O-187/(OS)-O-188 of 0.12 +/- 0.19. This age could indicate a late Os isotope reequilibration due to ongoing hydrothermal fluid flow from the Givetian (stratigraphic age similar to 375 Ma) until the closure of the Ural paleoocean in the Late Devonian, The Os contents are higher and Re/Os ratio is lower within Palaeozoic island-arc hosted Urals VHMS deposits compared with TAG deposit in MOR setting. (c) 2007 Elsevier B.V All rights reserved.

Abstract— It is now established that a large extraterrestrial object hit the Earth at the end of the Cretaceous period, about 65 Ma ago. We have investigated Re‐Os, Hf‐W, and Mn‐Cr isotope systems in sediments from the Cretaceous and the Paleogene in order to characterize the type of impactor. Within the Cretaceous‐Tertiary (K‐T) boundary layer, extraterrestrial material is mixed with terrestrial material, causing a dilution of the extraterrestrial isotope signature that is difficult to quantify. A phase essentially composed of Ni‐rich spinel, formed in the atmosphere mainly from melted projectile material, is likely to contain the extraterrestrial isotopic signature of the impactor. We show that the analysis of spinel is indeed the best approach to determine the initial isotope composition of the impactor, and that W and Cr isotopes confirm that the projectile was a carbonaceous chondrite.

Re-Os and Sm-Nd isotope systematics for the Nurali and the Mindyak lherzolite massifs have been determined in conjunction with their whole-rock major and trace element contents. The data suggest that the peridotites represent residues after the extraction of up to 25% of partial melt from the fertile mantle protolith. Later melt percolation and associated fluid-rock reaction events have modified the Sm/Nd, Re/Os and Pd/Os ratios of peridotites, but didn't affect significantly their major element compositions.The Re-Os and Sm-Nd isotope data strongly suggest that several magmatic events are involved in the evolution of these bodies. The mantle sections of these complexes formed during Proterozoic times, represent the first reported evidence for Precambrian peridotites in the Southern Urals.The oldest Re-Os age of 1250 +/- 80 Ma for the Nurali cumulates records the separation from the convective upper mantle. Multiple partial melting of the peridotites followed by fractional crystallisation produced layered cumulates which were subsequently stored in the sub-continental lithosphere over similar to 0.8 Ga. The age of the Nurali ophiolite coincides with the development of an epicontinental rift basin on the passive margin of the Baltica proto-continent.The younger Sm-Nd age of Mindyak peridotites (882 83 Ma) and Re-Os age of associated gabbros (804 +/- 37 Ma) record another tectonic event responsible for the separation of the Mindyak massif from convective mantle. Between similar to 850 and 650 Ma, the margins of the paleo-Asian ocean became the site of island-arc formation. This ophiolite then evolved in an intra-oceanic island-arc setting. The Mindyak lherzolite massif could be the first record of a Neoproterozoic Cadomian are in the Southern Urals.A later island-arc formation event has then affected both massifs in different ways. The Mindyak massif was incorporated into a rifting zone at similar to 500 Ma, producing a second partial melting event of peridotites, cross-cutted by mafic dykes. The Nurali massif has also been cross-cut by gabbro-diorite dykes at Devonian time during the subduction event leading to Urals island-arc formation.The combination of. Re-187-Os-187 and Sm-147-Nd-143 systematics for peridotites, mafic-ultramafic cumulates and mafic dykes reveals the complex history of ophiolite complexes, including isolation from the convective upper mantle at Proterozoic time, storage in sub-continental lithospheric mantle and re-activation during later tectonic events, such as island-arc formation. (c) 2007 Elsevier B.V. All rights reserved.

Os equilibrium solubilities were determined at 1350°C over a wide range of oxygen fugacities (−12<logfO2<−7) applying the mechanically assisted equilibration technique (MAE) at 105Pa (=1bar). Os concentrations in the glass samples were analysed using ID-NTIMS. Additional LA-ICP-MS and SEM analyses were performed to detect, visualize and analyse the nature and chemistry of “nanonuggets.” Os solubilities determined range at a constant temperature of 1350°C from 0.63±0.04 to 37.4±1.16ppb depending on oxygen fugacity. At the highest oxygen fugacities, Os3+ can be confirmed as the main oxidation state of Os. At low oxygen fugacities (below logfO2=−8), samples are contaminated by nanonuggets which, despite the MAE technique, were still not removed entirely from the melt. However, the present results indicate that applying MAE technology does reduce the amount of nanonuggets present significantly, resulting in the lowest Os solubility results reported to date under these experimental conditions, and extending the experimentally accessible range of fO2 for these studies to lower values. Calculated metal/silicate melt partition coefficients are therefore higher compared to previous studies, making Os more siderophile. Neglecting the as yet unknown temperature dependence of the Os metal/silicate melt partition coefficient, extrapolation of the obtained Os solubilities to conditions for core-mantle equilibrium, results in a DOsFeliq,∞/sil=1.5×105, while metallic alloy/silicate melt partition coefficients range from 1.4×106 to 8.6×107, in agreement with earlier findings. Therefore DOsmet/sil remains too high by 2–4 orders of magnitude to explain the Os abundance in the Earth’s mantle as result of core-mantle equilibrium during core formation.

Rhenium loss through magma degassing could be partly balanced by rhenium enrichment in fumarolic magmatic gases and Re-bearing precipitates, as may be the case for the Kudriavy volcano associated with an active subduction zone. The relatively unradiogenic (187)Os/(188)Os isotope ratios (0.122 up to 0.152) and high Os contents (averaging 0.6 ppb) of fumarolic gas condensates imply that significant Re and Os are remobilised from depleted MORB mantle. Involvement of a Re-rich component is evident from high Re concentrations in high-temperature gas condensates, ranging from 7 to 200 ppb. Indeed, Re-rich Os-poor components such as organic-rich subducted sediments and volcanic rocks do not significantly shift the isotopic composition of fumarolic products. The relatively radiogenic composition of the dacite-andesite-basaltic arc volcanics ((187)Os/(188)Os ratio up to 0.58), however, could result from significant Os (and Re) input from subducted sediments.

The isotopic compositions of mid-ocean-ridge basalts (MORB) from the Indian Ocean have led to the identification of a large-scale isotopic anomaly relative to Pacific and Atlantic ocean MORB1. Constraining the origin of this so-called DUPAL anomaly2 may lead to a better understanding of the genesis of upper-mantle heterogeneity. Previous isotopic studies3,4,5,6,7,8,9,10 have proposed recycling of ancient subcontinental lithospheric mantle or sediments with oceanic crust to be responsible for the DUPAL signature. Here we report Os, Pb, Sr and Nd isotopic compositions of Indian MORB from the Central Indian ridge, the Rodriguez triple junction and the South West Indian ridge. All measured samples have higher 187Os/188Os ratios than the depleted upper-mantle value11,12 and Pb, Sr and Nd isotopic compositions that imply the involvement of at least two distinct enriched components in the Indian upper-mantle. Using isotopic and geodynamical arguments, we reject both subcontinental lithospheric mantle and recycled sediments with oceanic crust as the cause of the DUPAL anomaly. Instead, we argue that delamination of lower continental crust may explain the DUPAL isotopic signature of Indian MORB.

In order to study the partitioning of Re and Os between liquid iron-rich alloy and magnesiowüstite at high pressure, multi-anvil experiments have been performed on samples of Fe–Ni–Os–Re–O (4–8wt.% Os and 4–12wt.% Re) metal contained in MgO single crystal capsules. The range of pressure–temperature conditions was 5–10GPa and 1900–2200°C with experimental run durations of 6–30min. During the experiments, the MgO reacted with the liquid metal to form magnesiowüstite. Compositions of the quenched liquid metal and the FeO, MgO and NiO contents of magnesiowüstite were determined by electron microprobe. Re and Os concentrations in magnesiowüstite were determined by LA–ICP–MS using a Re–Os-doped silicate glass standard. Based on the experimental results and assuming a valence of +2 for both Re and Os in magnesiowüstite, liquid metal–magnesiowüstite distribution coefficients (KDmet/mw) are 60–240 for Re and 1.3×104 to 3.1×104 for Os. Within the uncertainties, there is no observable effect of either temperature or pressure on the partitioning of Re and Os over the range of experimental conditions. However, the values are very low compare to metal–silicate KDmet/mw values determined at 1bar and 1400°C (3×109 for Re and 7×106 for Os [Geochim. Cosmochim. Acta 65 (2001) 2161; Am. Mineral. 85 (2000) 912]). KDmet/mw values, assuming core–mantle equilibrium, are estimated to be ∼68 for both elements. Thus, although mantle concentrations of Re may be explained by core–mantle equilibration at high pressure and temperature, the experimentally determined distribution coefficients for Os are several orders of magnitude too high. Our results are therefore consistent with the “late veneer” hypothesis as an explanation for the mantle concentrations of highly siderophile elements. However, a consequence of the late veneer would be domains in the deep mantle with suprachondritic Re/Os ratios.

This paper presents a two‐stage anion‐exchange procedure for tungsten extraction, an improved mass spectrometric procedure for tungsten analysis and a simplified chemical separation and TIMS procedure for the determination of Hf concentrations. The chemical separation of tungsten is based on its complexing properties with HF and H 2 O 2 . The blank level for a sample size of 300 mg is about 80 pg for tungsten. The procedure is designed for the high sensitivity of negative thermal ionisation mass spectrometry (NTIMS) provided by the use of Mg oxide as an emitter on Ir filaments. Tungsten can be readily measured with a high precision in various meteoritical material and especially in small W‐poor silicate fractions. Samples containing as little as a few ng g ‐1 tungsten can be analysed reliably with this method.

Os isotope ratios and Os and Re concentrations were measured in 56 lavas coming from 10 different subduction zones. Samples span a large range of major element concentrations (from basalts to dacites) and Mg# (from 0.32 to 0.81). The 10 subduction zones, namely the Lesser Antilles, Java, Papua New Guinea, the Philippines, Izu–Bonin, Kamchatka, the Aleutians, Mexico, Colombia and Peru–Chile, have a range of geodynamic settings. Measured 187Os/188Os ratios range from 0.130 to 1.524 and Os concentrations range from 0.05 to 46 ppt. Re concentrations range from 24 to 915 ppt. Os initial isotope ratios are systematically positively and linearly correlated with the inverse of Os concentrations in arc lavas from a given volcano, indicating that the Os isotopic compositions always reflect a binary mixing process. Similar mixing relationships are also seen at the sample scale. All trends converge towards unradiogenic compositions similar to those of upper mantle peridotites. These mixing relationships might be ascribed to a general contamination process; however, a single shallow-level process of crustal assimilation is hardly reconciled with the diversity of basements (from oceanic crust to continental crust compositions) of the selected arc volcanoes, the occurrence of the mixing lines for both primary and differentiated samples, and the absence of covariations between Os contents, isotope ratios, and indices of contamination and differentiation. On the other hand, because subducted components are very radiogenic and differ from one zone to another, the radiogenic components may be explained by varying amounts and natures of oceanic crust and sediments in the source of arc lavas. However, this explanation implies two disequilibrium processes, first during magma formation in order to produce heterogeneous lavas, and second during magma ascent to the surface to preserve slab signatures.

Rhenium–Osmium (Re–Os) isotope and elemental data are presented for mafic–ultramafic rocks from the central region of the Lewisian Archean terrain in northwest Scotland. These results give a best estimate for the time of emplacement of the mafic–ultramafic bodies of 2686.7±14.7 Myr (2σ). The initial 187Os/188Os isotope ratio of 0.10940±0.00076 indicates that such material possessed a chondritic Os isotope composition, which suggests that these rocks were formed by direct melting of mantle material, consistent with major and trace element constraints on their formation. Nevertheless, the Re–Os systematics of some of the mafic–ultramafic rocks in the Lewisian have been significantly disturbed, such that the original age information has been lost. These rocks lie on a regression line that defines an age of ∼3260 Myr, and a negative initial Os isotope composition, suggesting perturbation of the Re–Os system, either through assimilation or post-emplacement elemental exchange. Such a process also appears to have affected the Sm–Nd systematics in the same samples. Crustal assimilation can account for the observed Os and Nd isotope variations but only if the assimilated material possessed 187Os/188Os values of ca. 25 at ∼2687 Myr. In contrast, the surrounding gneisses and metasediments preserve present-day measured 187Os/188Os values of between 3 and 16. Rather, the spatial variation of initial Os and Nd isotope compositions suggests that isotope perturbation was caused by local sub-solidus element exchange between different lithologies, consistent with major element data and petrographic observations. Taken together, these results highlight the utility of the Re–Os isotope system for obtaining precise ages for Archean mafic–ultramafic rocks, and as a sensitive petrogenetic tracer capable of discriminating between assimilation or elemental exchange.

A suite of basaltic rocks sampled over a vast exposure and stratigraphic thickness in the Deccan traps has been investigated for Os isotopic systematics. The results plot on a very well defined Re–Os isochron corresponding to an age of 65.6±0.3 Ma (2σ uncertainty). This age is in excellent agreement with previous K–Ar and Ar–Ar data. Os data also imply a short duration of volcanism, which should have important implications on mantle geodynamics. The 187Os/188Os initial ratio is typically chondritic: 0.12843±0.00047 (2σ) and indicates that metasomatism and crustal contamination played only a very minor role in the Re–Os budget during formation of the Deccan traps.