Osmium Isotopic Compositions Research Articles

Osmium isotopic compositions by vapor phase sample introduction using a multi-collector ICP-MS

Osmium isotopic compositions have been measured using a vapor phase sample introduction technique in which volatile OsO4 is sparged directly into a multi-collector ICP-MS for analysis. Replicate analyses of an in-house solution standard yield an average 188Os/192Os value that is within 10 ppm of the accepted natural value when corrected for mass discrimination using 189Os/192Os, with a reproducibility of ±0.5 permil (2 SE, n = 9) for samples containing 10 ng of Os. The average 187Os/192Os measured for this solution agrees with the value determined by negative thermal ionization mass spectrometry to <1 permil, with a precision of ±1.5 permil (2 SE). Vapour phase analysis of Os isotopic compositions by MC-ICP-MS is applicable to a variety of geological and environmental studies.

Precise isotope analysis of natural and enriched osmium samples using different ICP-MS instruments

The application of inductively coupled plasma sector field mass spectrometers (ICP-SFMS), inductively coupled plasma quadrupole mass spectrometers with hexapole collision cell (HEX-ICP-MS) and multiple collector ICP-MS (MC-ICP-MS) for the determination of isotopic composition of highly enriched 187 Os and osmium samples is described. Good agreement of isotopic abundance was found in the three applied methods. A precision (2RSD) of 0.21–0.001, 0.83–0.09 and 5.5–0.083% for osmium isotope ratios, depending on isotope abundance in an interlaboratory metallic osmium sample, was observed in MC-ICP-MS, ICP-SFMS and HEX-ICP-MS measurements, respectively. A (NH 4) 2OsCl 6 (specpure Johnson Matthey Chemicals, JMC) sample was also analyzed by the MC-ICP-MS. The measured osmium isotope composition of the metallic osmium and JMC sample corresponds to natural osmium known in literature, with 187 Os abundance of 2.3 at.%. 187 Os abundance in three highly enriched samples of not identified, but assumed to be from special geological Precambrian formation of Russian origin varied between 98.934 and 99.53%. In addition, the 186 Os / 192 Os , 188 Os / 192 Os , 189 Os / 192 Os and 190 Os / 192 Os isotope ratios in all three enriched samples were close to the natural isotopic composition. This may indicate radioactive decay of 187 Re to 187 Os in rhenium-rich osmium samples in which the original osmium with a low initial concentration was of natural isotopic composition.

Osmium isotope constraints on lower crustal recycling and pluton preservation at Lassen Volcanic Center, CA

Osmium isotope compositions of intermediate- to silicic-composition calc-alkaline volcanic rocks from the Lassen volcanic region of the Cascade arc are significantly more radiogenic (γOs=+23 to +224) than typical mantle. These evolved arc rocks in the Lassen region have unradiogenic Sr, Nd, and Pb isotope compositions which overlap with those of contemporaneous mafic lavas. Crystal fractionation of mafic- to intermediate-composition magmas produces Re/Os ratios that are high enough to evolve to very radiogenic Os isotope compositions in only a few million years, providing a potential fingerprint for detecting the involvement of such young, relatively mafic crust in magmatic systems. However, the Sr, Nd, and Pb isotope compositions will remain constant over such short time intervals due to relatively low parent/daughter enrichment during magmatic evolution. The radiogenic Os isotope compositions in typically evolved Lassen rocks are interpreted to reflect significant interaction with lower crustal material that has radiogenic Os isotope compositions. Beneath this section of the Cascade arc, large amounts of such high-γOs lower crust may have formed and been isolated from MASH zone mixing and homogenization processes during the Pliocene or Late Miocene. The results from this study indicate that Os isotopes may provide a unique glimpse into lower crustal processes, such as recycling, in primitive orogenic arcs.

西南日本大山玄武岩, デイサイトのオスミウム同位体比: 沈み込み帯でのオスミウムの挙動

西南日本大山玄武岩, デイサイトのオスミウム同位体比: 沈み込み帯でのオスミウムの挙動

The osmium isotopic composition of convecting upper mantle deduced from ophiolite chromites

Chromites separated from the upper mantle or lower crustal portions of 18 ophiolites ranging in age from 900 Ma to 50 Ma are examined for Re-Os isotopic systematics. The ophiolites include both MORB and back arc types, although most are from supra-subduction zone (SSZ) settings. The chromites are robust indicators of the initial Os isotopic compositions of the systems sampled. There is very limited range in calculated initial γ Os values, with the entire group averaging +1.31. Least squares linear regression of the age of chromite formation (in Ga) versus initial 187Os/ 188Os of a filtered suite yields a slope of −0.0058±0.0019 (2σ) and a present day intercept of 0.12809±0.00085 (2σ), equivalent to a γ Os value of +0.9±0.6. Of the suite of 51 samples analyzed, 68% lie within ±1% of this evolution trajectory. Although most of the samples formed in SSZ environments, there is little evidence to suggest modification of the mantle Os isotopic composition via radiogenic melts or fluids derived from subducting slabs. The ophiolite data are interpreted as representative of the convecting upper mantle and suggest that the present isotopic composition of the convecting upper mantle averages approximately 1.2% less radiogenic than the estimated minimum composition of the primitive upper mantle of 0.1296±8 (Meisel et al., 2001). The most likely explanation for the difference is the formation, subduction and isolation of some portion of the mafic oceanic crust. Using models based on the assumption that the convecting upper mantle comprises 50% of the total mass of the mantle, and that the average isolation period for subducted oceanic crust is 1.5 to 2.0 Ga, it is estimated that approximately 2 to 3% of the total mass of the mantle is composed of subducted mafic oceanic crust that remains isolated from the convecting upper mantle. Because the isotopic compositions of the DMM and PUM overlap within uncertainties, however, the results do not require any isolated slab component.

An alternative explanation for the distribution of highly siderophile elements in the Earth.

The presence of highly siderophile elements (Ru, Rh, Pd, Re, Os, Ir, Pt, and Au) in the mantle has been a long standing enigma in the Earth sciences. Highly siderophile elements exhibit extremely low silicate/metal partition coefficients and so should have partitioned into the core, leaving the mantle depleted and fractionated compared with precursor material. Late accretion of undifferentiated material after completion of core formation may have overprinted the residue inherited from metal-silicate equilibrium partitioning. Here, we present a model based on the osmium isotopic composition of the mantle that sheds new light on the distribution of highly siderophile elements in Earth. As the Earth grew from the accretion of chondritic material of unspecified nature, gravitational and radioactive heating permitted early segregation of metal from silicate. This resulted in fractionation of highly siderophile element abundances in the residual mantle relative to chondritic abundances. After completion of core formation, the model supposes that a late carbonaceous veneer delivered biogenic and highly siderophile elements to the Earth. This late veneer was mixed inhomogeneously with the fractionated residue left over after core formation. Part of the deep mantle was isolated early from shallow convection and thus preserved primordial noble gas and highly siderophile element signatures. In this scenario, the contrast between the 187Os/188Os ratio of the carbonaceous late veneer and that of fertile lherzolites places strict constraints on the coupled silicate/metal partition coefficients of Re and Os (DRe = 1.6 ± 1.2 × 10-4 + DOs). Similarly, the high 187Os/188Os and 186Os/188Os ratios observed in certain plumes impose restrictions on the coupled silicate/metal partition coefficients of Pt and Os (DPt = 8.0 ± 6.2 × 10-4 + DOs)

Residence time of osmium in the oceans

Estimates of osmium residence time in the oceans that are based on oceanic mass balance calculations (35–50 kyr) appear irreconcilable with those inferred from the recent evolution of the osmium isotope composition of seawater (3–4 kyr). It is argued that the osmium budget of the oceans is currently close to steady state and thus that the estimates made by the two methods should agree. As the inventory of osmium in the oceans is relatively well constrained, these disparate residence time estimates imply wildly different osmium input fluxes to the oceans. An osmium residence time of 8–10 kyr is proposed by evaluating the uncertainties and limitations of both methods, and it is argued that osmium inputs to the ocean are currently underestimated by a factor of ∼3. This reflects in part the underestimation of the river input of osmium to the oceans owing to a bias within the existing data set and in part the probable existence of sources of osmium to the oceans that have not yet been identified. The very short residence time of 3–4 kyr inferred from the postglacial change in seawater composition (assuming a single step change in input flux) is rejected as it implies unreasonably high osmium input fluxes to the oceans. It is concluded that a postglacial spike in osmium flux, associated with a meltwater event, must have driven part of the change in seawater composition. However, it is also shown that such a spike cannot be the dominant cause of the most recent shift in seawater 187Os/188Os.

The Os and Nd isotopic systematics of c. 2.44 Ga Akanvaara and Koitelainen mafic layered intrusions in northern Finland

We have determined Os and Nd isotopic compositions of various rock types in and adjacent to the c. 2.44 Ga Akanvaara and Koitelainen layered intrusions which are part of the extensive Paleoproterozoic, ultramafic to felsic, mantle plume-related magmatism in the Fennoscandian Shield. The initial ε Nd in the layered sequences of both intrusions fall within a limited range around -2.0, being compatible with results from other coeval layered intrusion in the shield. The initial Os isotopic ratios in chromitites are near-chondritic: the total range of γ Os( T) is from −1 to +6 with the highest values reported for low-Os chromites. No systematic differences are found in the osmium isotopic compositions of the chromites between the intrusions or relative to the stratigraphic position of the sample. The studied intrusions share similar isotopic and geochemical features with rocks from younger flood basalt provinces, and also the same problem of the origin of the lithospheric source component, viz. crustal assimilation versus melt generation within lithospheric mantle. The observed Nd–Os isotopic systematics of the intrusions require that the magmas had negative ε Nd values already before emplacement and can best be accounted for by 15–25% deep-crustal contamination of a komatiitic melt initially having a near-chondritic Os isotopic composition, coupled with additional minor in situ assimilation of country rock material. This hypothesis is supported by the existence of coeval, crustally contaminated komatiitic volcanism on the Fennoscandian Shield.

Osmium isotopic compositions of mantle xenoliths: a global perspective

The Re-Os isotopic systematics of spinel- and garnet-bearing mantle xenoliths from North and Central America, Europe, southern Africa, Asia, and the Pacific region were determined to define more precisely the isotopic composition of a hypothetical primitive upper mantle (PUM). When plotted against Al 2O 3 or other melt depletion indicators, the 187Os/ 188Os ratios of all xenolith suites display positive trends toward a uniform composition at a fertile mantle composition. A 187Os/ 188Os value for PUM of 0.1296 ± 0.0008 (level of confidence 95%) was defined on the basis of 117 spinel-bearing xenoliths from this work and data from the literature, including data for massif peridotites. The 187Os/ 188Os ratio of the PUM is similar to the range of compositions defined by ordinary and enstatite chondrites, not carbonaceous chondrites. Spinel-bearing mantle peridotites sampled by volcanism and peridotite massifs appear to have been extracted from a common fertile source (PUM) between 1 and 2 Ga ago. These peridotites now form part of the subcontinental lithospheric mantle underlying continental crust of similar or greater formation age.

Sources and sinks of unradiogenic osmium runoff from Papua New Guinea

The Fly and Sepik are two of the largest rivers draining the island of New Guinea and among the world’s large rivers in discharge and sediment yield. Sediment from the upper Fly River and its delta have low 187Os/ 188Os of 0.511–0.560, and sediments from the upper and lower Sepik River have even lower ratios of 0.294–0.418. Filtered waters (<0.45 μm) from the Fly and Sepik rivers have 187Os/ 188Os of 0.616 and 0.335, respectively. Osmium isotopic compositions of surficial sediments from the Gulf of Papua ( 187Os/ 188Os of 0.394–0.961) further demonstrate that the sediment currently supplied to the oceans from New Guinea is unradiogenic. The particulate and dissolved loads supplied by these rivers are significantly less radiogenic than that contributed to the oceans by rivers draining regions of old continental crust ( 187Os/ 188Os>1.2). Burial of Os of both seawater and riverine derivation occurs in conjunction with organic carbon burial in the Gulf of Papua. There is a negative correlation between the Os isotopic composition of the bulk sediment and the fraction of organic matter of terrestrial origin based on C isotopes. The systematic relationship between C and Os isotopic compositions in these samples indicates that organic matter is the major host of Os in tropical estuarine sediments. The Os/C org of Gulf of Papua surface sediments is intermediate between that of other modern and ancient sediments. Approximately 10 11 mol/yr of C org and 20–57 mol/yr of Os are buried in the inner shelf (<50 m depth) of the Gulf of Papua over an area of only 21 000 km 2, corresponding to 1–3% of the global marine Os burial flux in 0.006% of the total sea surface area. These results suggest that a large proportion of the global burial flux of Os may take place in conjunction with the deposition of low-C org sediments. Lowering of the Os isotopic composition of ambient seawater due to a large flux of unradiogenic riverine Os may be indicated by the Os isotopic composition of water from the Coral Sea ( 187Os/ 188Os=0.844) and marine sediments within the Gulf of Papua. The island of New Guinea is estimated to provide 4–5% of the global riverine Os flux. Given the very large particulate and dissolved flux to the oceans from rivers in the wet tropics, the effect of the weathering of young arc-related crust in the SW Pacific is an important source of unradiogenic Os to modern seawater.

Diffusion of osmium in pyrrhotite and pyrite: implications for closure of the Re–Os isotopic system

In order to better constrain the extent to which common sulfide minerals will retain their osmium isotopic composition subsequent to crystallization, we have conducted experiments to quantify the diffusion behavior of osmium in pyrite and pyrrhotite. Experiments consisted of either (1) isothermal soaking of diffusion couples consisting of natural pyrite or pyrrhotite crystals packed against powdered Os-bearing Fe-sulfide or (2) ‘relaxation’ of initially high near-surface osmium concentrations produced in the latter experiments (pyrite only). Osmium penetration into samples was characterized by depth profiling using Rutherford backscattering spectroscopy (RBS) (pyrite) or electron microprobe analyses across sectioned run products (pyrrhotite). Results of the first type of diffusion experiment involving pyrite show only limited osmium penetration into sample surfaces, with the extent of penetration uncorrelated with run duration. Images of pyrite samples using atomic force microscopy show roughening of initially smooth surfaces as a consequence of step formation and suggest that osmium incorporation into the near-surface occurred by solute uptake during step growth and not by volume diffusion. Prolonged (1000 + h) ‘relaxation’ experiments revealed no additional osmium penetration into pyrite surfaces and based on the depth resolution for RBS, a maximum diffusion coefficient of 2.5×10 −23 m 2/s at 500°C was calculated. Experiments involving pyrrhotite over the temperature range of 950–1100°C showed extensive osmium uptake and osmium concentration gradients that conform with Fickian diffusion behavior. We found that pyrrhotite Fe/S could be varied by changes in the composition of the starting material and osmium source and over the range of Fe/S produced in experiments (molar Fe/S=0.83–0.90), we observed no systematic variation in the osmium diffusion coefficient. Diffusion coefficients measured parallel to the a crystallographic axis were on average ∼1.4× higher than values measured parallel to c and regression of the c-axis data yielded the Arrhenius relation: D(T)=1.31±0.16×10 −5 m 2 / s exp −211.5±14.7 kJ/ mol RT The application of these diffusion data to simple models of diffusive exchange during static or polythermal time–temperature histories is used to assess the conditions under which radiogenic osmium will be retained. During isothermal annealing, calculations indicate that the cores of millimeter-sized spherical pyrrhotite crystals undergoing diffusive exchange with an external osmium reservoir will have their initial compositions perturbed in ≤0.5 Ma at temperatures exceeding 400°C. Pyrite undergoing the same process at 500°C requires in excess of 10 Ma before crystal cores are affected. The relatively short ‘core retention’ time-scales for pyrrhotite indicates that this mineral may be prone to isotopic resetting following relatively brief crustal thermal events, thus possibly accounting for the scatter that commonly occurs in Re–Os isochrons generated from massive sulfide samples. Calculated closure temperatures ( T c) for osmium exchange in pyrrhotite yielded values of 300–400°C for grain sizes ranging from 10 to 1000 μm. These values of T c are similar to those calculated for Ar retention in biotite, and considerably lower than for Sr in apatite and plagioclase, for example. Such low closure temperatures for pyrrhotite suggest this mineral will date the final stage in the cooling of a magmatic system and possibly be susceptible to open system osmium exchange in the presence of late-stage hydrothermal fluids. This latter result infers that caution be applied when interpreting elevated initial osmium isotopic ratios as a product of crustal assimilation at the magmatic stage.

Platinum-group elements and Os isotopic characteristics of the lower oceanic crust

We present osmium isotopic compositions and Os, Re and platinum-group element (PGE) contents in the lower oceanic crust gabbros from ODP Site 735B on the Southwest Indian Ridge. The average gabbros are very depleted in PGEs, e.g. the Os concentrations are very low and range from 0.5 to 38 ppt. One troctolite sample is an exception, containing, e.g. 376 ppt Os, 396 ppt Ir and 8 ppb Pd. Re contents are extremely heterogeneous, ranging from 128 ppt to 28 ppb. The present day 187Os/ 188Os varies from 0.140 to 0.467, and correlates with Rb/Cs, suggesting that seawater alteration affected the Os isotopic system. The average Os content in the lower oceanic crust is about 9 ppt while median value for Re is about 487 ppt. Such very high Re/Os is relatively similar to the upper MORB-type oceanic crust, indicating that recycled ocean crust basalts and gabbros will rapidly develop very radiogenic Os isotopic signatures.

The osmium riverine flux and the oceanic mass balance of osmium

The osmium concentration ([Os]) and isotopic composition were determined in a set of 17 of the largest rivers of the world. [Os] varies between 4.6 and 52.1 pg/kg and the 187Os/ 188Os ratio varies between 0.64 and 2.94. Measurement of rainwater samples shows that there is no input of oceanic Os to the continent through rain. Assuming a negligible anthropogenic Os input in the dissolved load, the natural average river water has an Os concentration of 7.9 pg/kg and a mean 187Os/ 188Os ratio of 1.54. The total riverine flux of Os to the ocean is estimated to be 295 kg/yr. The dissolved Os flux from island arcs and oceanic islands represents less than 5% of the total riverine flux and is not further considered. The continental Os flux to the ocean is then represented by the riverine flux, as dissolved Os from eolian dust and glacial sediments is negligible. Assuming steady state, it is possible to estimate a maximum unradiogenic flux to the ocean of 126 kg/yr (cosmic dust or mantle-derived) and an oceanic residence time between 2.5×10 4 and 5.4×10 4 with a mean of 3.5×10 4 year. The estimation of the flux of dissolved cosmic particles shows that their contribution to the seawater Os would be ∼14% of the contribution of the unradiogenic component, which means that the mantle-derived flux should contribute a major part. The first results on water from high temperature axial hydrothermal systems indicate that their input is probably negligible, which would necessitate that dominant contribution from the low temperature alteration of the oceanic crust and/or of the ultramafic exposures contributes dominantly to the input of unradiogenic Os to the seawater. We show that it would be necessary to leach 1.3% of the Os contained in the volume of ultramafic exposures accessible to seawater to account for all of the unradiogenic component contribution. Another simpler but less likely possibility is that the dissolved cosmic dust represents the only source of dissolved unradiogenic Os to the ocean in which case the riverine input represents 94% of the total dissolved flux to the ocean instead of 70%. The modern global dissolved Os flux to the ocean would then have a 187Os/ 188Os ratio of 1.44 instead of 1.06 and the system would be far from steady state.

The osmium isotopic composition change of Cenozoic sea water as inferred from a deep-sea core corrected for meteoritic contributions

A detailed study was made of the oceanic 187Os/ 186Os variation through the Cenozoic by using a hydrogen peroxide leaching procedure on a pelagic clay core from the North Pacific (Long Lines-44 GPC3). A 6% H 2O 2 solution was used. The range of 187Os/ 186Os obtained by this procedure was between 2 and 8.2. A milder leach (0.15% H 2O 2) in three out of four samples yielded a slightly higher 187Os/ 186Os value than the stronger leach implying that the stronger leach released some meteoritic Os with 187Os/ 186Os of 1. Using published 3He concentrations in GPC3 and cosmic dust 3He and Os fluxes to deep-sea sediments, the extraterrestrial Os concentration was estimated for each core segment and subtracted to yield “corrected” values of 187Os/ 186Os. The results based on the milder leaching protocol and the 3He correction protocol yield similar values. The “corrected” ocean water 187Os/ 186Os pattern for the past 25 million years is similar to that obtained by other workers and is compatible with other results for the entire Cenozoic. The variation with time in GPC3, uniquely, provides a statigraphic signature for the Paleogene.

Osmium isotope constraints on ore metal recycling in subduction zones

Veined peridotite xenoliths from the mantle beneath the giant Ladolam gold deposit on Lihir Island, Papua New Guinea, are 2 to 800 times more enriched in copper, gold, platinum, and palladium than surrounding depleted arc mantle. Gold ores have osmium isotope compositions similar to those of the underlying subduction-modified mantle peridotite source region, indicating that the primary origin of the metals was the mantle. Because the mantle is relatively depleted in gold, copper, and palladium, tectonic processes that enhance the advective transport and concentration of these fluid soluble metals may be a prerequisite for generating porphyry-epithermal copper-gold deposits.

Osmium isotopic composition of corals: evidences for multiple sources

Osmium concentrations in corals are very low (<20 ppt) and large isotopic variations (between 2.92 ± 0.05 and 9.01 ± 0.05) are observed indicating sample heterogeneity. These variations are interpreted as a two component mixing for each sample. The unradiogenic component has about the same value for all samples and is of mantle/meteorite composition and concentration. The radiogenic component is interpreted as a composite signal, different for each sample and resulting from the mixing of seawater osmium and continental detrital sediment osmium. Simple calculations show that the micrometeoritic flux of 40 ± 20.10 6 kg/yr is too small by about two orders of magnitude to explain the composition of these samples. In the absence of a potential mantle source for the corals, it is considered that corals, as they filter water for feeding, scavenge meteoritical osmium from seawater.

Rhenium–osmium isotopic investigation of Java subduction zone lavas

Arc lavas from Java have low concentrations of osmium (from 0.10 to 17.18 ppt) and variable isotopic compositions (187Os/188Os ratios between 0.241 and 3.704). Inverse relationships between isotopic compositions and concentrations are observed for two groups of samples with distinct ages and Pb compositions. These results reveal mixing processes between unradiogenic Os from the peridotitic upper mantle and two different radiogenic Os components, reflecting two different crustal contaminants or different proportions of subducted oceanic crust and sediments. Mixing relationships seen in single lavas indicate that radiogenic and unradiogenic Os-bearing phases can coexist at the sample scale without isotopic equilibration.

Re-Os isotope systematics and weathering of Precambrian crustal rocks: implications for the marine osmium isotope record

The effects of weathering of freshly exposed Precambrian rocks on the mobility of osmium were studied on soils developed on a chronosequence of glacial moraines from the western Wind River Range in Wyoming. The Os budget of the Precambrian granitoid source rocks is dominated by Os-rich trace phases, such as magnetite. Amongst the major silicates, biotite, or a trace phase in biotite, carries most of the Os and Re and is also the most radiogenic mineral with 187Os/ 186Os of ∼113 and 187Re/ 186Os of ∼2,000. Re-Os isotope systematics of source rocks and soils indicate that rapid oxidation of magnetite mobilizes Os with an isotopic composition similar to the isotopic composition of the bulk soils. A very radiogenic fraction of Os is mobilized through preferential weathering of biotite. Radiogenic runoff from Precambrian shields, inferred from osmium isotope analyses of freshwater Fe-Mn-nodules suggests that high-latitude Precambrian shields are important source areas of radiogenic Os in seawater. We propose that glacial scouring and weathering of glacial tills exposed after deglaciation of Precambrian shields surrounding the North Atlantic provides a mechanism for the slightly more radiogenic nature of North Atlantic seawater compared to other seawater masses. Glacial-interglacial variations in the osmium isotopic composition of seawater seem plausible and may be triggered by changes in weathering regimes on glacial-interglacial time scales in high-latitude shield areas surrounding the North Atlantic.

Osmium isotopic compositions in iron meteorites: an exploration of isotopic anomaly

Osmium isotopic compositions in iron meteorites: an exploration of isotopic anomaly

Variations in the osmium isotope composition of sea water over the past 200,000 years

The first osmium isotope data are presented for marine sediments spanning the past 200,000 years (two full glacial cycles) and establish that there have been ocean-wide variations in sea-water osmium isotope composition over this time. While the osmium isotope composition of sea water has been relatively invariant over much of this interval ( 187Os/ 186Os of 8.5 to 8.6), excursions to lower isotope ratios ( 187Os/ 186Os ≈ 8.1) occur late in the last two glacial periods at 20 ka and 160 ka. These are times of high global aridity, low global temperature and maximum ice extent. While various interpretations are possible, a likely explanation is that the low sea-water ratios reflect reduced chemical weathering of the continents during times of extreme cold and aridity. There is no evidence that there is a net enhancement of chemical weathering during periods of continental glaciation. The response of ocean composition to changes in input flux constrain the oceanic residence time of osmium to be less than 12,000 years.