Francis Albarède

THE LU-HF ISOTOPE GEOCHEMISTRY OF CHONDRITES AND THE EVOLUTION OF THE MANTLE-CRUST SYSTEM

Abstract We report analyses of the176Hf/177Hf ratio for 25 chondrites from different classes of meteorites (C, O, and E) and the176Lu/177Hf ratio for 23 of these as measured by plasma source mass spectrometry. We have obtained a new set of present-day mean values in chondrites of176Hf/177Hf= 0.282772 ± 29 and176Lu/177Hf= 0.0332 ± 2. The176Hf/177Hf ratio of the Solar System material 4.56 Ga ago was 0.279742 ± 29. Because the mantle array lies above the Bulk Silicate Earth in a143Nd/144Nd versus176Hf/177Hf plot, no terrestrial basalt seems to have formed from a primitive undifferentiated mantle, thereby casting doubt on the significance of high3He/4He ratios. Comparison of observedHf/Nd ratios with those inferred from isotopic plots indicates that, in addition to the two most prominent components at the surface of the Earth, the depleted mantle and the continental crust, at least one more reservoir, which is not a significant component in the source of oceanic basalts, is needed to account for the Bulk Silicate Earth Hf-Nd geochemistry. This unaccounted for component probably consists of subducted basalts, representing ancient oceanic crust and plateaus. The lower continental crust and subducted pelagic sediments are found to be unsuitable candidates. Although it would explain the Lu-Hf systematics of oceanic basalts, perovskite fractionation from an early magma ocean does not account for the associated Nd isotopic signature. Most basalts forming the mantle array tap a mantle source which corresponds to residues left by ancient melting events with garnet at the liquidus.

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.

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.

Volatile accretion history of the terrestrial planets and dynamic implications.

Accretion left the terrestrial planets depleted in volatile components. Here I examine evidence for the hypothesis that the Moon and the Earth were essentially dry immediately after the formation of the Moon—by a giant impact on the proto-Earth—and only much later gained volatiles through accretion of wet material delivered from beyond the asteroid belt. This view is supported by U–Pb and I–Xe chronologies, which show that water delivery peaked ∼100 million years after the isolation of the Solar System. Introduction of water into the terrestrial mantle triggered plate tectonics, which may have been crucial for the emergence of life. This mechanism may also have worked for the young Venus, but seems to have failed for Mars.

The growth of continental crust

Abstract The petrological and geochemical composition of the mantle-derived igneous products that will eventually form the continental crust (protolith), the episodic nature, and the geodynamic sites of crustal growth are discussed. Models in which crustal growth takes place at converging boundaries from orogenic magmas contrast with those in which basaltic plume material is involved (underplating, loose-plate loading, oceanic plateaus). Because some chemical components of the crust are either preferentially returned to the mantle at subduction zones (Mg, Ca) or sequestered in the crust (Si, Al, Na, K), the composition of the crust and that of its protolith are probably very different. Continental crust may therefore form from basaltic magmas and not necessarily from intermediate (e.g., andesitic) magmas. Because subduction is a continuous process, the episodic pattern of crust formation ages is a strong argument against crustal growth at converging boundaries. The preferred model is based on major mantle instabilities (superplumes) and their surface expression, the oceanic plateaus where thick piles of plume basalts rapidly erupted on the ocean floor reach the buoyancy threshold that defines the status of continental crust. The plateaus are accreted against the continents, and the felsic magmas that stand out as the most conspicuous feature of continental crust chemistry, are produced subsequently upon subduction erosion and possibly by gravitational instability of thin hot young lithospheric plates.

Ion-exchange fractionation of copper and zinc isotopes

Abstract Whether transition element isotopes can be fractionated at equilibrium in nature is still uncertain. Standard solutions of Cu and Zn were eluted on an anion-exchange resin, and the isotopic compositions of Cu (with respect to Zn) of the eluted fractions were measured by multiple-collector inductively coupled plasma mass spectrometry. It was found that for pure Cu solutions, the elution curves are consistent with a 63Cu/65Cu mass fractionation coefficient of 0.46‰ in 7 mol/L HCl and 0.67‰ in 3 mol/L HCl between the resin and the solution. Batch fractionation experiments confirm that equilibrium fractionation of Cu between resin and 7 mol/L HCl is ∼0.4‰ and therefore indicates that there is no need to invoke kinetic fractionation during the elution. Zn isotope fractionation is an order of magnitude smaller, with a 66Zn/68Zn fractionation factor of 0.02‰ in 12 mol/L HCl. Cu isotope fractionation results determined from a chalcopyrite solution in 7 mol/L HCl give a fractionation factor of 0.58‰, which indicates that Fe may interfere with Cu fractionation. Comparison of Cu and Zn results suggests that the extent of Cu isotopic fractionation may signal the presence of so far unidentified polynuclear complexes in solution. In contrast, we see no compelling reason to ascribe isotope fractionation to the coexistence of different oxidation states. We further suggest that published evidence for iron isotopic fractionation in nature and in laboratory experiments may indicate the distortion of low-spin Fe tetrahedral complexes. The isotope geochemistry of transition elements may shed new light on their coordination chemistry. Their isotopic fractionation in the natural environment may be interpreted using models of thermodynamic fractionation.

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.

An improved U-Th-Pb age calculation for electron microprobe dating of monazite

Abstract Determination of U-Th-Pb ages using the electron probe microanalyser (EPMA) is an inexpensive alternative method for dating monazite. The method is rapid and reliable, both for simple monogenenetic monazite and for complex polygenetic monazite having undergone metamorphic events involving fluid interaction and recrystallization. The main limitation of the method is its rather poor precision, i.e., ± 45 to ± 120 Ma for ages ranging from 300 to 3000 Ma calculated on each spot. The precision is limited by Pb content and by counting statistics, which cannot give precision better than 2% for individual determinations on U, Th, and Pb even at high levels. A procedure that uses the new Th/Pb vs. U/Pb diagram to improve the calculated precision on U-Th-Pb ages gives results to within ± 5 to ± 15 Ma for ages ranging from 300 to 3000 Ma. With complex polygenetic monazite, in which either the points show large scattering indicated by a large MSWD or the regression line exhibits a slope very different from neighbour theoretical isochrons, the procedure must be applied separately on homogeneous domains only. This makes it possible to distinguish between events separated by a gap of ∼20 to 60 Ma, according to the range of ages concerned (i.e., 300 to 3000 Ma). Several examples are given to illustrate these systematics.

Hawaiian hot spot dynamics as inferred from the Hf and Pb isotope evolution of Mauna Kea volcano.

The present work reports multiple collector inductively coupled plasma mass spectrometry (MC-ICPMS) measurements of the isotopic compositions of Hf and Pb in the first 3 km of the deep core retrieved by the Hawaii Scientific Drilling Project. The measurements cover all the samples from the standard geochemical reference set, glasses from the deep hole, and replicates from the pilot hole. Both Hf and Pb are less radiogenic in Mauna Loa compared to Mauna Kea. The transition between Mauna Kea and Mauna Loa lavas in the deep core is progressive for eHf and 208Pb/204Pb, but a sharp discontinuity is observed for 208Pb*/206Pb*. There is no correlation between the alkalinity of the samples and isotopic composition. In detail, the Hf isotope compositions of samples from the pilot hole are not all identical to those of the HSDP-2 core for samples retrieved from a similar depth, suggesting that steep topography existed at the time of emplacement or that a different eruptive sequence was recorded. The strong correlation between 208Pb*/206Pb* and 3He/ 4He (He data from M. D. Kurz et al. (Rapid helium isotopic variability in Mauna Kea shield lavas from the Hawaiian Scientific Drilling Project, submitted to Geochemistry Geophysics Geosystems, 2002)) requires the episodic incorporation of a component that resembles the basalts erupted by either Kilauea or the Loihi eruptive centers (this component is referred to as K/L). The data suggest that some 500 kyr ago, Mauna Kea was tapping a mantle source similar to that tapped by Kilauea today. Isotopic variability of Pb and He cannot be accounted for by radiogenic ingrowth in a closed system, but requires the mixing of mantle source components with distinct outgassing histories. The time series of isotopic and concentration data in Mauna Kea samples spanning about 350,000 years of age indicate the recurrence of geochemical patterns in the melting column. Ignoring the most recent alkalic samples, we find that the dominant fluctuations of eHf and 207Pb/204Pb correspond to a period of 50,000 years. For La/ Yb, Zr/Nb, 87Sr/ 86Sr, 206Pb/204Pb, 207Pb/ 206Pb, and 208Pb/206Pb, a dominant period of ca. 18,000 years is obtained. Once provision is made for the existence of harmonics, the consistency between the isotopic spectrum of the pilot hole and the HDSP-2 core is very good. The input of the K/L component does not seem to be periodic. We use these recurrence intervals in conjunction with the upwelling rate deduced from buoyancy flux and seismic evidence of the maximum dimension of scatterers to constrain the radius of the Hawaiian plume conduit to be in the range of 10-50 km and the upwelling velocity to be in the range of 0.13-3 m/yr. Plausible vertical length scales of heterogeneities in the conduit are 6.5-160 km.