Jean-Louis Birck

Widespread 54Cr Heterogeneity in the Inner Solar System

Short-lived radionuclides can be used as high-resolution chronometers for establishing timescales of planetary formation provided that they were homogeneously distributed in the accretion disk. However, isotopic anomalies observed in meteorites bear evidence of incomplete mixing in the early solar system. High-precision thermal ionization mass spectrometry (TIMS) now enables the determination of isotopic anomalies as small as 12 parts per million in the neutron-rich isotope 54Cr. Here, we report systematic deficits in 54Cr relative to Earth in differentiated molten planetesimals (the parent bodies of eucrites, diogenites, mesosiderites, pallasites, angrites, and Mars) and even in some chondritic material (ordinary chondrites). In combination with variable 54Cr excesses in the carbonaceous chondrites, this implies that at least two nebular reservoirs coexisted for differentiated and chondritic bodies. Preservation of the 54Cr heterogeneity in space and time (several million years) motivates us to speculate that late stellar input(s) could have been significant contributions to inner nebula Cr reservoirs or that the 54Cr cosmic memory was well preserved by the mineralogy of the carriers. The consequent spatial, dynamical, and temporal implications regarding solar system formation and dating models are explored further.

Osmium-strontium-neodymium-lead isotopic covariations in mid-ocean ridge basalt glasses and the heterogeneity of the upper mantle

Abstract Osmium, strontium, neodymium, and lead isotopic data have been obtained for 30 hand picked samples of basaltic glass from the Pacific, Atlantic and Indian mid-oceanic ridges. Large variations in Os isotopic ratios exist in the glasses, from abyssal peridotite-like values to radiogenic compositions similar to oceanic island basalts (187Os/186Os and 187Os/188Os ratios range from 1.06 to 1.36 and from 0.128 to 0.163, respectively). Os isotopic and elemental data suggest the existence of mixing correlations. This relationship might be ascribed to secondary contamination processes; however, such a hypothesis cannot account for the negative correlation observed between Os and Nd isotopes and the existence of complementary covariations between Os and Sr Pb isotopes. In this case, Os Sr Nd Pb isotopic variations are unrelated to late post-eruption or shallow level contamination. These relationships provide strong evidence that the Os isotopic composition of the samples are derived from the mantle and thus implies a global chemical heterogeneity of the oceanic upper mantle. The results are consistent with the presence of recycled oceanic crust in the mantle sources of mid-ocean ridge basalts, and indicate that the unique composition of the upper mantle below the Indian ocean results from its contamination by a mantle component characterized by radiogenic Os and particularly unradiogenic Nd and Pb isotopic compositions.

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.

Osmium isotope disequilibrium between mantle minerals in a spinel-lherzolite

Rhenium (Re)–osmium (Os) isotope and elemental data have been obtained for coexisting silicates and sulphide from a spinel-lherzolite from Kilbourne Hole, New Mexico. These results confirm that sulphide dominates the Os budget in mantle rocks, but a significant proportion of the Re is located in the silicates. The silicates and interstitial sulphide yield indistinguishable Os isotope compositions suggesting these phases were in equilibrium at the time of xenolith eruption. However, sulphide inclusions in silicate minerals preserve significantly less radiogenic Os isotope compositions, suggesting that they have been shielded from reaction or diffusion by their silicate hosts. Diffusion rates in silicates and sulphides are likely to be rapid at mantle temperatures. However, for the sulphide inclusions diffusion through the silicate host is likely to be impaired by the high partition coefficient for Os between sulphide and silicate phases, in the range of 104 to 106. Consequently, sulphide inclusions trapped in silicates may be unable to equilibrate either with other minerals or with an intergranular melt, and in principle may preserve significantly older Re–Os age information than coexisting phases.

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 isotope variations in the oceans recorded by Fe-Mn crusts

This study presents osmium (Os) isotope data for recent growth surfaces of hydrogenetic ferromanganese (Fe‐Mn) crusts from the Pacific, Atlantic and Indian Oceans. In general, these data indicate a relatively uniform Os isotopic composition for modern seawater, but suggest that North Atlantic seawater is slightly more radiogenic than that of the Pacific and Indian Oceans. The systematic difference in the Os isotopic composition between the major oceans probably reflects a greater input of old continental material with a high Re =Os ratio in the North Atlantic Ocean, consistent with the distribution of Nd and Pb isotopes. This spatial variation in the Os isotope composition in seawater is consistent with a residence time for Os of between 2 and 60 kyr. Indian Ocean samples show no evidence of a local source of radiogenic Os, which suggests that the present-day riverine input from the Himalaya‐Tibet region is not a major source for Os. Recently formed Fe‐Mn crusts from the TAG hydrothermal field in the North Atlantic yield an Os isotopic composition close to that of modern seawater, which indicates that, in this area, the input of unradiogenic Os from the hydrothermal alteration of oceanic crust is small. However, some samples from the deep Pacific (4 km) possess a remarkably unradiogenic Os isotope composition ( 187 Os= 186 Os ratios as low as 4.3). The compositional control of Os incorporation into the crusts and mixing relationships suggest that this unradiogenic composition is most likely due to the direct incorporation of micrometeoritic or abyssal peridotite particles, rather than indicating the presence of an unradiogenic deep-water mass. Moreover, this unradiogenic signal appears to be temporary, and local, and has had little apparent effect on the overall evolution of seawater. These results confirm that input of continental material through erosion is the dominant source of Os in seawater, but it is not clear whether global Os variations are due to the input of mantle or meteoritic material, or simply indicate that the continental source itself is not uniform.

The distribution and behaviour of rhenium and osmium amongst mantle minerals and the age of the lithospheric mantle beneath Tanzania

Rhenium–osmium (Re–Os) isotope and elemental abundances have been obtained for primary mantle minerals, metasomatic phases, and a range of mantle rock types from xenoliths in recent volcanics in northern Tanzania. Re and Os abundances for sulphide and coexisting silicates in garnet lherzolites from Lashaine confirm that sulphide dominates the Os budget, but also show that Re is almost exclusively sited in silicate phases. Silicate minerals from two different samples yield 187Re–188Os ages of 15.4±6.1 and 31.4±6.3 Myr, respectively. Comparison with 232Th–208Pb (267.1±4.4 Myr) 147Sm–143Nd (164±18 Myr) and 87Rb–87Sr (in equilibrium at the present-day) ages for the same silicate minerals suggests differential closure between these isotope systems, and a closure temperature of ≥670°C for the Re–Os system. Remarkably, sulphide inclusions were not affected by diffusional equilibration between the silicates, and preserve significantly older age information. Model calculations suggest that sulphide–silicate equilibration ceased some 200–300 Ma, and the Os isotope composition of the sulphide (187Os/188Os=0.10432±0.00013) suggests a minimum age of 3.4 Gyr. Most xenoliths possess Os isotope compositions that are less radiogenic than the present-day chondritic mantle indicating that they experienced Re-loss some time ago. Samples showing evidence for modal metasomatism have high Re concentrations and Re/Os ratios, but their relatively unradiogenic Os isotope compositions suggests that the metasomatism occurred recently, consistent with data for metasomatic vein minerals. In contrast, some dunites possess both high Re/Os ratios and radiogenic Os isotope compositions. These samples differ from those affected by modal metasomatism in having low Re and exceptionally low Os concentrations. These results provide quantitative constraints on the distribution of Re and Os amongst mantle minerals, highlight the potential of Re–Os isotope dating of sulphide inclusions for establishing the early history of mantle mineral assemblages, and demonstrate that mantle processes themselves (metasomatism and dunite formation) can significantly modify the Os isotope chemistry of mantle rocks.

The compatibility of rhenium and osmium in natural olivine and their behaviour during mantle melting and basalt genesis

Rhenium and osmium (Re–Os) elemental abundances have been obtained for magmatic olivine from a range of host basalt compositions, for mantle olivine and coexisting phases (silicate and sulphide) from a spinel–peridotite, and olivine and Fe–Ni metal from Pallasite meteorites. These data indicate that Re and Os concentrations in olivine are low in both mantle and magmatic environments, and both elements preferentially partition into silicate melt, sulphide or Fe–Ni metal, relative to olivine. For magmatic olivine the partition coefficients for Re and Os correlate with the MgO content of the olivine (like Fe, Mn and Ni), which suggests that the observed partitioning reflects substitution onto crystallographic sites, rather than defects or the presence of included phases. These data indicate that Os is extremely incompatible (that is, excluded from the silicate structure) in magmatic olivine, which suggests that olivine crystallisation alone cannot be responsible for the low Os contents of some oceanic basalts. Rather, olivine crystallisation is itself responsible for sulphide precipitation (in which Os is highly compatible), by producing sulphur saturation of the melt, and it is the coupled crystallisation of these phases that effects the Os–Mg–Ni co-variations observed in oceanic basalts. Rhenium is also incompatible in magmatic olivine but the data suggest that for Fe-rich olivine compositions Re may become compatible, which may explain, at least in part, the compatible behaviour of this element during basalt petrogenesis on other planetary bodies, such as Mars and the Moon. Preliminary data for mantle olivine, not demonstrably contaminated by included phases, suggest that the high Os concentrations (relative to magmatic olivine) relate to partitioning with a sulphide, rather than silicate melt.

182Hf–182W systematics in eucrites: the puzzle of iron segregation in the early solar system

Eucrites are the witnesses of an early magmatic activity in small planetary bodies. Iron meteorites also testify to an early differentiation in planetesimals. Are basaltic magmatism and iron segregation related to each other? Is there any link between eucrites and some groups of iron meteorites? The new 182Hf–182W chronometer is suitable for investigating this problem and for constraining magmatic activity on the eucrite parent body. Here, we present a Hf–W study for eight eucrites and one angrite. Eucrites define a straight line in the Hf–W isotopic diagram which is interpreted as an isochron corresponding to an age of 11.1±1.1 Myr relative to the formation of primitive ordinary chondrites. This time scale is consistent with data from other chronometers. The very high Hf/W ratios for eucrites support the idea that these rocks were derived from a reservoir from which iron was segregated, simultaneously with or not much earlier than the time of basalt genesis. On the other hand, our results suggest that this differentiation occurred much later than the iron differentiation generating all classes of known iron meteorites.

High-precision analysis of chromium isotopes in terrestrial and meteorite samples by thermal ionization mass spectrometry

We present a method for the chemical separation of Cr from meteorite and terrestrial samples for isotopic analysis by thermal ionization mass spectrometry (TIMS). After sample digestion, separation of Cr(III) is achieved by means of a two-column cation-exchange chromatography procedure using AG 50W-x8 resin. In a first column, Cr(III) is isolated from major elements and the majority of trace elements. In a second column, trace amounts of Fe, Al and Ti are further removed. Total procedural yields are > 80%. Cr isotopes are measured by TIMS in the static multicollection mode. Mn/Cr ratios are obtained by multi-collector inductively coupled plasma source mass spectrometry (MC-ICPMS). The accuracy of our protocol was tested by reference to terrestrial analogs and comparison of Cr isotopic data for samples that underwent Cr purification following the cation-exchange chromatography described here and an alternative separation method employing both a cationic and an anionic chromatography step. Using our technique, Mn/Cr ratios reproduce to <2% (2σ) and 53Cr/52Cr and 54Cr/52Cr to 6 ppm and 12 ppm, respectively (2σ). This highly precise procedure allows the variability of Cr isotopes in the inner solar system objects to be addressed. Our method enabled us to document an initial homogeneity for 50,52,53Cr isotopes within 10 ppm, while 20–70 ppm deficits in 54Cr abundances have been resolved for a number of meteorite samples.