Abdelmouhcine Gannoun

In situ Os isotopes in abyssal peridotites bridge the isotopic gap between MORBs and their source mantle.

Abyssal peridotites are assumed to represent the mantle residue of mid-ocean-ridge basalts (MORBs). However, the osmium isotopic compositions of abyssal peridotites and MORB do not appear to be in equilibrium, raising questions about the cogenetic relationship between those two reservoirs. However, the cause of this isotopic mismatch is mainly due to a drastic filtering of the data based on the possibility of osmium contamination by sea water. Here we present a detailed study of magmatic sulphides (the main carrier of osmium) in abyssal peridotites and show that the 187Os/188Os ratio of these sulphides is of primary mantle origin and can reach radiogenic values suggesting equilibrium with MORB. Thus, the effect of sea water on the osmium systematics of abyssal peridotites has been overestimated and consequently there is no true osmium isotopic gap between MORBs and abyssal peridotites.

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.

Late Accretion on the Earliest Planetesimals Revealed by the Highly Siderophile Elements

Coming Late to the Planetesimal Highly siderophile (iron-loving) elements (Re, Os, Ir, Ru, Rh, Pt, Pd, and Au) must have been added to the mantles of Earth, the Moon, and Mars after their iron cores formed; otherwise the mantles would be devoid of these elements, which tend to be segregated to the core. Dale et al. (p. 72) report data on highly siderophile elements in rocks from different planetary bodies, including asteroid 4 Vesta and other differentiated asteroids, which are representative of the planetesimals from which the solar system planets formed. Like the larger planetary bodies, differentiated asteroids, which formed over the first few million years of the solar system, bear the evidence of the late addition of highly siderophile elements to their mantles. Thus, this process was not unique to Earth, the Moon, and Mars and happened over an extended period of time in the inner solar system. Analysis of meteorites shows that unprocessed material was accreted to both planets and asteroids 150 million years after the start of the solar system. Late accretion of primitive chondritic material to Earth, the Moon, and Mars, after core formation had ceased, can account for the absolute and relative abundances of highly siderophile elements (HSEs) in their silicate mantles. Here we show that smaller planetesimals also possess elevated HSE abundances in chondritic proportions. This demonstrates that late addition of chondritic material was a common feature of all differentiated planets and planetesimals, irrespective of when they accreted; occurring ≤5 to ≥150 million years after the formation of the solar system. Parent-body size played a role in producing variations in absolute HSE abundances among these bodies; however, the oxidation state of the body exerted the major control by influencing the extent to which late-accreted material was mixed into the silicate mantle rather than removed to the core.

Rapid, simultaneous separation of Sr, Pb, and Nd by extraction chromatography prior to isotope ratios determination by TIMS and MC-ICP-MS

A straightforward separation scheme is described for the separation of Sr, Pb, and Nd from silicate rocks. It allows the concomitant isolation, without any intervening evaporation, of these three elements of great interest in radiogenic isotope geology and cosmochemistry. Following digestion with HF–HNO3, the sample residue is dissolved in 1 M HNO3. After addition of ascorbic acid to reduce Fe(III) to Fe(II), this solution is passed through two tandem columns containing 250 μL of Sr Spec and TRU Spec extraction chromatography resins, respectively. The upper Sr Spec column extracts Sr and Pb, while the lower TRU Spec column extracts the LREE. Sr and Pb are back-extracted with 0.05 M HNO3 and 6 M HCl, respectively. The LREE are eluted directly onto a longer column containing 300 mg of the HDEHP-based EXC material Ln Spec, to obtain, through sequential elution with 0.25 M HCl, a Nd fraction free of any Sm contribution. The whole procedure is achieved within a single working day. The Sr and Nd fractions separated in this way are ready for isotope ratio measurements by TIMS (Sr, Nd) or MC-ICP-MS (Nd). The Pb fraction is converted to the nitrate form before isotopic analysis by MC-ICP-MS. The potential of this method is exemplified by analysis of different powder aliquots of several iron-rich, international standard rocks of basaltic composition.

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.

High precision osmium elemental and isotope measurements of North Atlantic seawater

This study presents new Os isotope and elemental data of seawater from the North Atlantic Ocean. Two techniques have been optimised and used to achieve tracer–seawater equilibration: (i) heating of a seawater–tracer mixture at 100 °C with Br2, CrO3 and H2SO4 in Teflon bombs and solvent extraction of OsO4 with Br2; and (ii) heating of a seawater–tracer mixture to 300 °C with CrO3 and H2SO4 in a sealed glass tube (HPA-S-s) at 125 bars and back extraction with Br2. Both techniques yield similar total procedural blanks in the range of 14–18 fg, and have a total Os yield that ranges from 86 to 92%. For the 200 fg Os standard, the effective ionisation efficiency is between 6 and 10% demonstrating that single filament ionization is capable of achieving significant Os sensitivity. To assess the external reproducibility on the 187Os/188Os ratio, both methods have been systematically applied to a single sample from a water depth of 2000 m and yield an indistinguishable 187Os/188Os ratio of 1.026 ± 0.017 (n = 11) and 1.020 ± 0.013 (n = 3), for techniques (i) and (ii), respectively. The external reproducibility on the 187Os/188Os ratio and Os concentration is 1.6% and 3.7% respectively, demonstrating the capability of both methods for analysing Os in natural water. Both procedures have been applied to a seawater profile from the North East Atlantic Ocean (IB13: 60°29.4′ N, 19°59.2′ W, 2650 m water depth). The results show little measurable variability in either Os concentration or isotope composition with depth and the overall average for the whole profile is 1.024 ± 0.031 (3%) for the 187Os/188Os ratio and 9.68 ± 0.48 (4.9%) pg kg−1 for the Os concentration. Therefore, for this locality at least, Os appears to behave conservatively.

Control of source fertility on the eruptive activity of Piton de la Fournaise volcano, La Réunion

The eruptive activity of basaltic hotspot volcanoes displays major fluctuations on times scales of years to decades. Theses fluctuations are thought to reflect changes in the rate of mantle melt supply. However, the crustal filter generally masks the mantle processes involved. Here, we show that the cyclic and generally increasing activity of the Piton de la Fournaise volcano (La Réunion) since the mid 20th century is tightly linked to the fertility of its source, as recorded by 87Sr/86Sr and incompatible trace elements ratios of lavas. We identify a twofold control of source fertility on eruptive activity: melt extraction from fertile, incompatible element-enriched veins initiates decadal-scale eruptive sequences, so that vein distribution in the plume source directly controls the cyclic activity. Indirectly, reactive flow of enriched melts increases mantle porosity and promotes melts extraction from the peridotite matrix. This process is thought to have caused a fourfold increase in magma supply between 1998 and 2014 at Piton de la Fournaise, and could also explain magma surges at other frequently active hotspot volcanoes, such as Kilauea, Hawaii. The short-term eruptive activity of hotspot volcanoes appears to be ultimately linked to the distribution and size of lithological heterogeneities in mantle plumes.