C. J. Allegre

Noble gases in basalt glasses from a Mid-Atlantic Ridge topographic high at 14°N: geodynamic consequences

We present a complete noble gas study of mid-oceanic ridge basalt glasses (MORB) from a small ridge segment, centered on an along-strike topographic elevation of the Mid-Atlantic Ridge at about 14°N. We have found the highest 40Ar/36Ar ratio ever observed for a MORB glass, i.e. 28,150 ± 330 for sample 2ΠD40, correlated with high 129Xe/130Xe ratios and the highest noble gas concentrations in a so-called popping-rock, labeled 2ΠD43. The latter sample displays a 4He/40Ar* ratio of 2.0–2.7, which is close to the production ratio in the mantle due to the radioactive decay of U, Th and K. Hence, this sample probably best represents the elemental noble gas ratios in the mantle, from which we have computed the4He concentration in the mantle source of MORB to be 1.5 × 10−5 cm3 STP g−1. High 4He/3He ratios in two of the samples from the summit of the topographic high indicate the presence of a U, Th-rich component in the mantle source, possibly old subducted oceanic crust and/or sediments, which could originate in the so-called mesosphere boundary layer.

The influence of rivers on marine boron isotopes and implications for reconstructing past ocean pH

Ocean pH is particularly sensitive to atmospheric carbon dioxide content. Records of ocean pH can therefore be used to estimate past atmospheric carbon dioxide concentrations. The isotopic composition of boron (δ11B) contained in the carbonate shells of marine organisms varies according to pH, from which ocean pH can be reconstructed. This requires independent estimates of the δ11B of dissolved boron in sea water through time. The marine δ11B budget, however, is still largely unconstrained. Here we show that, by incorporating the global flux of riverine boron (as estimated from δ11B measurements in 22 of the worlds main rivers), the marine boron isotope budget can be balanced. We also derive ocean δ11B budgets for the past 120 Myr. Estimated isotope compositions of boron in sea water show a remarkable consistency with records of δ11B in foraminiferal carbonates, suggesting that foraminifera δ11B records may in part reflect changes in the marine boron isotope budget rather than changes in ocean pH over the Cenozoic era.

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 2.5×104 and 5.4×104 with a mean of 3.5×104 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.

Boron isotope systematics in large rivers: implications for the marine boron budget and paleo-pH reconstruction over the Cenozoic

Abstract The chemical composition of the oceans and long-term climate changes are believed to be linked. Reconstruction of seawater pH evolution is desirable as pH may be related to atmospheric pCO2, and hence to climate evolution. Boron isotopes in oceanic carbonates have been suggested to be a proxy for oceanic paleo-pH reconstruction. Nevertheless, the calculation of paleo-pH values over geological periods requires a precise knowledge of the boron isotopic composition of the oceans when calcite precipitated. We present the systematics of boron isotopic composition of the worlds main rivers. We deduce a continental boron flux to the oceans of 38×1010 gB/year with a mean isotopic composition of +10‰. These results lead to a balanced boron budget in the oceans and allow the development of a model for the marine boron secular evolution over the past 100 Myr. It is shown that the oceanic boron cycle is mainly controlled by the boron continental discharge and the boron uptake from the oceans during low temperature alteration of oceanic crust. However, the recent important increase of the clastic sediment supply, linked to the Himalayan erosion, impacts the oceanic boron budget by enhancing significantly the boron uptake by adsorption on sediments. We predict a boron isotopic composition in the oceans lower during the Cenozoic and slightly higher during the Cretaceous than today. The modelled values for the marine boron isotopes follow the variations of boron isotopes in carbonates over the Cenozoic era provided by previous studies, suggesting that the variations of the seawater pH may not have been important on this time scale. If this is the case, it involves that buffering mechanisms occur in the oceans to maintain seawater pH at a roughly constant value against past atmospheric pCO2 variations.

Osmium isotopic constraints on the nature of the DUPAL anomaly from Indian mid-ocean-ridge basalts

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 MORB. Constraining the origin of this so-called DUPAL anomaly may lead to a better understanding of the genesis of upper-mantle heterogeneity. Previous isotopic studies 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 value 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.

Age of the Deccan traps using 187Re–187Os systematics

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: 3M a (2 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 187 Os= 188 Os initial ratio is typically chondritic: 0:12843 0:00047 (2 ) and indicates that metasomatism and crustal contamination played only a very minor role in the Re‐Os budget during formation of the Deccan traps.

Osmium isotopic compositions from oceanic basalts

We report Os isotopic analyses of certain oceanic basalts as well as Re and Os concentrations of oceanic and continental mafic igneous rocks.187Os/186Os ratios of OIB from Loihi (1.10), Mauna Loa (1.13) and Reunion (1.11) are slightly more radiogenic than estimates of the bulk silicate earth ( ∼ 1.06) and the majority of published values for peridotites. An ankaramite from Iceland has a187Os/186Os of 1.07. These values preclude the possibility that these OIB are derived either totally or largely by the fusion of ancient subducted oceanic crust, as such material would have extremely high187Os/186Os ratios. Mixing calculations show that a basalt with a187Os/186Os of 1.11 could have been generated from a source containing ∼ 8% of 3 Ga crust, assuming that the depleted mantle had chondritic ReOs systematics. However, such an interpretation of the more radiogenic OIB cannot be considered unique. The fact that the Iceland sample has a lower187Os/186Os than samples from Reunion and Hawaii suggests that the age of the lithosphere through which the magmas passed may be a factor. During ascent, OIB magmas could have been contaminated with either the oceanic crust itself, which had evolved highly radiogenic187Os/186Os ratios since its formation at the ridge, or withhigh-Re/Os components of the lithospheric mantle. However, one cannot exclude the possibility that the difference in the Os isotopic compositions between the more radiogenic OIB and those of continental and oceanic peridotites reflects the more depleted history of the uppermost mantle (implying, in one interpretation, that the187Os/186Os of the bulk silicate earth could be greater than 1.06).

Rare gas systematics in Red Sea Ridge basalts

From south to north, the 4He/³He ratios of submarine basalts from the Red Sea and Gulf of Tadjoura increase from 45,000 (R/Ra=15) in the Gulf of Tadjoura to 84,000 (R/Ra=8.6) in the Mabahiss deep. Our helium isotopic results indicate that the mantle plume proposed in the north of the Red Sea, on the basis of the Sr and Pb isotopic ratios, does not posess a high ³He signature. The neon and Ar isotopic ratios of the Red Sea glasses are explained by mixing between AIR, MORB and a plume with a solar like isotopic Ne signature and low 40Ar/36Ar, similar to that of Loihi.

Osmium behavior in estuaries: the Lena River example

The behavior of dissolved osmium at the river/ocean interface was studied in the Lena River estuary. Dissolved osmium removal is observed at very low salinities. The loss is estimated to be 28% of the dissolved concentration of the river. The removal cannot be related to the flocculation of iron oxide–organic matter colloids, but occurs simultaneously with the loss of aluminum. The proposed mechanism is the adsorption of dissolved osmium on suspended particles in the maximum turbidity zone. If this is correct, then the removal of osmium in estuaries is probably a common phenomenon. No contribution of osmium from the sediments is detected but neither the samples nor the stratified nature of the estuary are favorable to the study of a flux from sediments. If this finding can be generalized, the estimated global riverine flux of osmium to the ocean has to be recalculated and the osmium residence time in the ocean would change from 3.3×104 yr to 4.6×104 yr.

Re–Os isotopic constraints on the genesis and evolution of the Dergamish and Ivanovka Cu (Co, Au) massive sulphide deposits, south Urals, Russia

Abstract Rhenium and osmium elemental and isotopic data have been obtained for the two mafic–ultramafic hosted volcanogenic massive sulphide (VMS) deposits of Dergamish and Ivanovka from the south Urals. The associated ophiolitic blocks belong to the Main Uralian Fault (MUF) melange zone considered to represent obducted early Palaeozoic oceanic crust. Despite their close geographical proximity, the two ore bodies are morphologically, mineralogically and isotopically quite different. Sulphides from Ivanovka possess higher Ni and Os and lower Re and Cu relative to those from Dergamish. The Re and Os isotope data for Dergamish define a best-fit line corresponding to a Late Devonian age of 366±2 Ma (2 σ ) with an MSWD of 4.6. This age is some 40 My younger than the inferred Silurian crystallisation age of the associated mafic–ultramafic rocks, but in good agreement with the previously published Rb–Sr and Ar–Ar ages of 360–380 Ma corresponding to the high-pressure metamorphic age of the adjacent Maksyutov metamorphic complex. These data suggest that Re–Os systematics of the Dergamish sulphide deposit were reset, either by diffusion or recrystallisation, during high-pressure metamorphism or subsequent cooling. The preservation of unradiogenic Os isotopic ratios in some of the Ivanovka samples and the near chondritic initial Os isotopic composition obtained for the Dergamish samples indicates that most of the Os in the massive sulphides was ultimately derived from the mantle. The corresponding tectonic setting equates to an area with submarine high-level mantle rocks. In contrast, sulphides from Ivanovka have experienced continued re-equilibration and have been modified by post-depositional processes at least some of which occurred relatively recently.