Philippe Telouk

Precise analysis of copper and zinc isotopic compositions by plasma-source mass spectrometry

Abstract The stable isotope geochemistry of Cu and Zn is poorly known because of the lack of a suitable analytical technique. We present a procedure for the analysis of Cu and Zn isotope compositions by plasma-source mass spectrometry (Plasma 54) together with a method to purify Cu and Zn from natural samples of silicates, ores, and biological material. A plasma-source mass spectrometer equipped with a magnetic filter and multiple collection can make up for the instability of the ICP source and provide precise Cu and Zn isotope compositions. Instrumental mass fractionation is corrected with respect to the isotopic composition of a standard of a different element added to the sample (Zn for a Cu sample, Cu for a Zn sample) previously purified by anion-exchange chemistry. We have adopted the NIST Cu standard (SRM 976) and a Johnson–Mattey Zn solution as references. This external normalization leads to an internal precision of 20 ppm and an external reproducibility of 40 ppm or 0.04 per mil (95% confidence level). The isotopic compositions can be obtained on as little as 200 ng of element. Isobaric interferences are small enough to be neglected. Isotopic fractionation is observed for Cu on the anion-exchange resin, which requires a full yield to be achieved upon purification. The exponential law of mass fractionation is shown to provide a more consistent correction than the linear and the power law. It is shown that in the mass spectrometer Cu and Zn isotopes do not fractionate to the same extent. The ratio of instrumental mass biases remains constant over one measurement session. This ratio is determined from the analysis of mixed standard solutions, then is used to correct the isotopic composition measured for Cu and Zn samples. Some preliminary results show the existence of isotopic variations of up to several per mil amongst natural samples of silicates, ores, sediments, and biological material, which paves the way for the use of Cu and Zn isotopes as geochemical and biochemical tracers.

A short timescale for terrestrial planet formation from Hf–W chronometry of meteorites

Determining the chronology for the assembly of planetary bodies in the early Solar System is essential for a complete understanding of star- and planet-formation processes. Various radionuclide chronometers (applied to meteorites) have been used to determine that basaltic lava flows on the surface of the asteroid Vesta formed within 3 million years (3 Myr) of the origin of the Solar System. Such rapid formation is broadly consistent with astronomical observations of young stellar objects, which suggest that formation of planetary systems occurs within a few million years after star formation. Some hafnium–tungsten isotope data, however, require that Vesta formed later (∼16 Myr after the formation of the Solar System) and that the formation of the terrestrial planets took a much longer time (62-14+4504 Myr). Here we report measurements of tungsten isotope compositions and hafnium–tungsten ratios of several meteorites. Our measurements indicate that, contrary to previous results, the bulk of metal–silicate separation in the Solar System was completed within <30 Myr. These results are completely consistent with other evidence for rapid planetary formation, and are also in agreement with dynamic accretion models that predict a relatively short time (∼10 Myr) for the main growth stage of terrestrial planet formation.

High-precision analysis of Pb isotope ratios by multi-collector ICP-MS

Abstract We investigated high-precision Pb isotope ratio analysis by multi-collector-inductively coupled plasma-mass spectrometry (MC-ICP-MS) using added thallium as an internal isotopic standard to correct for mass dependent isotopic fractionation. We compared MC-ICP-MS analysis of both an inter-laboratory standard, NBS 981, and geological samples to conventional thermal ionization mass spectrometry (TIMS). As expected, we found that analytical error in the latter was dominated by mass fractionation. In MC-ICP-MS, we found that fractionation appears to follow the exponential law, but that the fractionation coefficients, f, of Tl and Pb were not identical. This difference in fractionation coefficients cannot be compensated for by renormalizing to a different Tl isotopic composition as done in other studies. We found, however, that the fPb/fTl ratio was constant over the course of an analytical session, allowing fPb to be calculated from fTl. An exponential law correction was then applied to the Pb isotope measurements which effectively eliminates errors associated with mass fractionation. Precision for the MC-ICP-MS analyses ranged from a factor of 2 to a factor of 6 better than for TIMS analyses for the 206 Pb / 204 Pb and 208 Pb / 206 Pb ratios respectively. Residual error in the MC-ICP-MS analyses was dominated by error in the analysis of 204 Pb , perhaps in part due to random errors introduced by correcting for a 204 Hg isobaric interference. We also found systematic errors in the MC-ICP-MS analyses compared to TIMS determinations that may be due to uneven background and collector biases in the instrument used. We found that these systematic errors were the same for both NBS 981 and the geological samples, so accurate correction factors could be generated from the standard analyses to correct the sample analyses. MC-ICP-MS has the additional advantages of requiring less preparative chemistry, less instrument time, and considerably less labor overall.

The Lu–Hf dating of garnets and the ages of the Alpine high-pressure metamorphism

It remains controversial whether burial and exhumation in mountain belts represent episodic or continuous processes. Regional patterns of crystallization and closure ages of high-pressure rocks may help to discriminate one mode from the other but, unfortunately, metamorphic geochronology suffers from several limitations. Consequently, no consensus exists on the timing of high-pressure metamorphic events, even for the Alps—which have been the subject of two centuries of field work. Here we report lutetium–hafnium (Lu–Hf) mineral ages on eclogites from the Alps as obtained by plasma-source mass spectrometry. We find that the Lu/Hf ratio of garnet is particularly high, which helps to provide precise ages. Eclogites from three adjacent units of the western Alps give (from bottom to top) diachronous Lu–Hf garnet ages of 32.8 ± 1.2, 49.1 ± 1.2 and 69.2 ± 2.7Myr. These results indicate that the Alpine high-pressure metamorphism did not occur as a single episode some 80–120Myr ago,,,, but rather that burial and exhumation represent continuous and relatively recent processes.

New Lu-Hf and Pb-Pb Age Constraints on the Earliest Animal Fossils

Abstract The Neoproterozoic Doushantuo Formation, South China, preserves a unique assemblage of early multicellular fossils and overlies rocks, which are thought to have formed during an ice age of global extent. The age of this formation is thus critical for understanding the important biological and climatic events that occurred towards the end of the Proterozoic Eon. Until now, direct dating of sedimentary formations such as the Doushantuo has been difficult and associated with large uncertainties. Here, we show that dating of Doushantuo phosphorites by a novel Lu–Hf dating method and conventional Pb–Pb geochronometry independently yield ages of 584±26 Ma and 599.3±4.2 Ma, respectively. These ages are in agreement with bio- and chemostratigraphical observations and show that the Doushantuo animal remains predate diverse Ediacaran fossil assemblages, making them the oldest unambiguous remains of metazoans currently known. Furthermore, the Pb–Pb age for the post-glacial Doushantuo rocks suggests that the Neoproterozoic glaciation in China might predate glacial rocks in Eastern North America commonly associated with the younger (Marinoan) of two major Neoproterozoic glaciations. The combination of Lu–Hf and Pb–Pb dating shows considerable potential for dating other phosphorite successions and future application of these methods could therefore provide further constraints on Proterozoic biological and environmental history.

147Sm–143Nd and 176Lu–176Hf in eucrites and the differentiation of the HED parent body

The 147Sm–143Nd and 176Lu–176Hf systematics of 21 whole-rock eucrites, including five cumulates, have been investigated by MC-ICP-MS. A statistically significant Sm–Nd isochron was obtained on 18 samples with an age of 4464±75 Ma (MSWD=1.26) and an intercept of 0.50680±0.00010. This age clearly is controlled by the cumulate eucrites. The 21 basaltic and cumulate eucrites together do not form a statistically significant Lu–Hf isochron. Basaltic eucrites, however, form an errorchron with an age of 4604±39 Ma (MSWD=4.52), which becomes an acceptable isochron if the error on the Lu/Hf ratio is doubled with respect to the laboratory estimate. The initial 176Hf/177Hf is 0.27966±0.00002. Three of the cumulate eucrites regressed together further yield a statistically significant age of 4470±22 Ma, indistinguishable from their Sm–Nd age. We therefore conclude that cumulate eucrites are ∼100 Myr younger than basaltic eucrites and basaltic eucrites are close in age to the Solar System. There is no evidence in the present data to support a decay constant of 176Lu significantly different from 1.93 10−11 yr−1 [Sguigna, Can. J. Phys., 60 (1982) 361–364]. The broad range of variation and strong correlation of Lu/Hf and Sm/Nd ratios require that eucrites are partial melts and not residual magmas. The relative Lu/Hf and Sm/Nd fractionation is controlled by the plagioclase to mafic mineral ratio in the residue. The steep correlation between these two ratios is explained by the enhancement of plagioclase saturation in the low-gravity field of the eucrite parent body. Basaltic eucrites probably formed as large-degree melts of a chondritic source. The strong Lu/Hf and Sm/Hf fractionation observed in cumulates clearly reflects the presence of residual ilmenite during melting: our preferred interpretation is that cumulates were impregnated by small-degree melts of ilmenite-bearing gabbros. Since pressure on the eucrite parent body never reaches the garnet stability field, this observation questions the ubiquitous presence of residual garnet in the mantle sources of magmas formed on larger planets such as the Moon, Mars, and possibly Earth.

The Lu–Hf isotope geochemistry of shergottites and the evolution of the Martian mantle–crust system

We report the first Lu‐Hf isotopic data from SNC (Martian) meteorites with high-precision analyses of five shergottites by plasma sector mass spectrometry. Hf isotopic compositions indicate the presence of both geochemically enriched and depleted components, reflecting early segregation of Martian crust and mantle. Comparison with Sm‐Nd and U‐Th‐Pb isotopic systems suggests that the enriched component reflects the extraction of very small-degree melts at depths in excess of 200 km in the presence of garnet, while the depleted component could correspond to large-degree melts from mantle already depleted by the extraction of enriched liquids. The general lack of correlation between major element composition and isotopic properties of SNC meteorites argues against the segregation of the lithophile elements into Earth-like continents or Moon-like highlands on Mars, although early magma ocean fractionation may explain many of the primary isotopic properties of SNC meteorites. Recent (0.18‐0.33 Ga old) fractionation of Lu=Hf and Sm=Nd in shergottites is inconsistent with purely magmatic partitioning of these elements and appears more likely to be a product of metasomatic processes involving the circulation of P 2 O 5 -rich fluids, triggered either by distant magmatic activity or, more probably, by impacts.1999 Elsevier Science B.V. All rights reserved.

142Nd evidence for early Earth differentiation

Abstract We measured 142Nd/144Nd in metabasalts and metagabbros from the western and southern parts of the Isua Supracrustal Belt, West Greenland. The samples were selected based on field and thin section criteria so as to minimize metamorphic effects on the isotope systematics. We developed a new multi-stage chemical separation scheme and a new isotope measurement technique using multiple-collector inductively coupled plasma mass spectrometry and, upon replication of measurements, achieved a precision better than 20 ppm. Out of eight samples, three show a positive 142Nd anomaly of 30 ppm consistent with a previous measurement by Harper and Jacobsen [Nature 360 (1992) 728–732]. These samples also define a 147Sm–143Nd isochron age that agrees with the commonly accepted U–Pb zircon age of 3.78 Ga for Isua, while the non-anomalous samples show substantial dispersion. The latter lack of coherence is interpreted as indicating either isotopic heterogeneities in the Isua upper mantle or redistribution of Nd by fluids during late Archean metamorphic events. Scatter among the non-anomalous samples is also observed for the Lu–Hf isotope systematics and confirms the severity of metamorphic remobilization. If the 147Sm–143Nd and 146Sm–142Nd systems are explained by a two-stage model, it is required that the upper mantle went through a major differentiation event within a few tens of millions of years after planetary accretion. If early Archean convection decoupled the two Nd isotope systems, this necessitates that the episode of differentiation took place within no more than 150 Myr of planetary accretion. One interpretation of the high-Sm/Nd reservoir is that it represents the remnants of a stagnant lithospheric lid overlying a magma ocean. Because the lid is shattered by impacts, it is unable to sink. Alternative explanations, such as an ultra-deep magma ocean at the core–mantle boundary or massive subduction of early crust, call for the early burial of enriched material and the ubiquitous presence of a 142Nd anomaly in the early Archean mantle.

Precise and accurate neodymium isotopic measurements by plasma-source mass spectrometry

Abstract The plasma-source mass spectrometer equipped with both a magnetic mass filter and multiple collection (PSMS) of Lyon was tested for high-precision analysis of neodymium isotopic compositions. These can be determined on as little as 70 ng Nd by PSMS with an internal precision of 10 ppm and an external reproducibility of ≈30 ppm. A magnetic mass filter, which provides flat top peaks, multiple collection, which integrates plasma instabilities, and steady-state nebulization, which enables accurate cross-calibration with standards, bring precision of PSMS within the range of that achieved by thermalionisation mass spectrometry. The molecular oxide and hydride peaks are undetectable. The Sm isobaric interferences can be accurately corrected which relieves the requirement of Nd purification and allows the separation procedure to be reduced to a one-step elution on a cation-exchange column. Very fast sample throughput (18 min per analysis) makes this technique a powerful tool for isotopic stratigraphy. This is illustrated by a preliminary study at high resolution of the neodymium isotopic compositions of the basalts from the Piton de la Fournaise volcano (Reunion Island).

Evidence from Sardinian basalt geochemistry for recycling of plume heads into the Earth's mantle.

Up to 10 per cent of the ocean floor consists of plateaux—regions of unusually thick oceanic crust thought to be formed by the heads of mantle plumes. Given the ubiquitous presence of recycled oceanic crust in the mantle source of hotspot basalts, it follows that plateau material should also be an important mantle constituent. Here we show that the geochemistry of the Pleistocene basalts from Logudoro, Sardinia, is compatible with the remelting of ancient ocean plateau material that has been recycled into the mantle. The Sr, Nd and Hf isotope compositions of these basalts do not show the signature of pelagic sediments. The basalts’ low CaO/Al2O3 and Ce/Pb ratios, their unradiogenic 206Pb and 208Pb, and their Sr, Ba, Eu and Pb excesses indicate that their mantle source contains ancient gabbros formed initially by plagioclase accumulation, typical of plateau material. Also, the high Th/U ratios of the mantle source resemble those of plume magmas. Geochemically, the Logudoro basalts resemble those from Pitcairn Island, which contain the controversial EM-1 component that has been interpreted as arising from a mantle source sprinkled with remains of pelagic sediments. We argue, instead, that the EM-1 source from these two localities is essentially free of sedimentary material, the geochemical characteristics of these lavas being better explained by the presence of recycled oceanic plateaux. The storage of plume heads in the deep mantle through time offers a convenient explanation for the persistence of chemical and mineralogical layering in the mantle.