Claude-Jean Allègre

143Nd/146Nd, a natural tracer: an application to oceanic basalts

The possibility of using 143Nd/146Nd as a natural tracer was examined by analyzing two Hercynian granitic rocks, one carbonatite, three alkali basalts and nine tholeiites. 143Nd/146Nd ratios are notably different for these rock types: 0.7068–0.7073 for granitic rocks, 0.7078 for the carbonatite, 0.7080 for alkali basalts and 0.7083 for tholeiites. The isotopic difference between tholeiites and alkali basalts suggests that they are derived from parts of the mantle which have been chemically different. Comparison of the measured 143Nd/146Nd ratio of tholeiites and the isotopic growth in a simple two-stage model system suggests that the source of the tholeiite magmas has had light-REE-depleted pattern, being consistent with Gasts (1968) contention that ridge tholeiites are derived from a part of the mantle which is a residue left after a previous melting event.

Quantitative models of trace element behavior in magmatic processes

Abstract It was the deep-seated conviction of Paul Gast that the trace element approach was at least as powerful as experimental petrology to solve the major petrological problems. In this paper, we review the present status of the utilization of trace elements in igneous processes, which acknowledges his opinion. After a study of the historical development of trace element geochemistry which includes a brief discussion of the analytical methods presently available, we review the various models which describe the trace element behavior. A few of them are added, which concern mixing processes, and a set of graphical notations is proposed, which can be extended to any model. Partition coefficients, the methods for their analytical determinations, and their effective dependence on temperature, pressure and composition are then considered. We discuss various methods for the recognition of the process responsible for the variation of trace element concentrations in a suite of rocks. Once this “process identification” is achieved, the approach of inverse problems allows the determination of the characteristic parameters of this process. We detail this approach, exemplified by the fractional crystallization process, as applied to a series of alkali basalts (Terceira Island). We conclude that the solution of such inverse problems and further determinations of partition coefficients based on ion probe data might be the future development of trace element geochemistry.

Comparative uranium-thorium-lead and rubidium-strontium study of the Saint Sèverin amphoterite: consequences for early solar system chronology

Pb-Pb and Rb-Sr internal isochrons have been determined in the Saint Severin amphoterite. The Sr age and initial ratio are, respectively, 4.61 ± 0.15 b.y. (λ = 1.39 × 10−11 yr−1) and 0.69903 ± 0.00020 (2σ errors). Evidence is presented for acceptance of radiogenic Sr from whitlockite, possibly from troilite, during events younger than 3.1 b.y., thus giving too high initial ratios. The Pb-Pb age, determined from plagioclase, whole rock and very radiogenic whitlockite is 4.543 ± 0.019 b.y. (2σ errors), using the new U decay constants. Both U-Pb and Th-Pb ages of whitlockite are concordant at 4.54 b.y. The range of the Th/U ratio of whitlockite is 9–11, in agreement with the value of Crozaz [Earth Planet. Sci. Lett. 23 (1974) 164–169]. The results are compared with the other Rb-Sr and U-Pb data on chondrites and achondrites.

Rare gas systematics on Mid Atlantic Ridge (37–40°N)

Rare gas concentrations and isotopic compositions in 19 glasses from the North Atlantic ridge have been analyzed. Sixteen of them are located between 36.8 and 39.9°N on the Azores plateau, three others are located between 21.25 and 30.2°N and may represent normal North Atlantic Mid Ocean Ridge basalts. Helium concentrations decrease by three orders of magnitude between 37° and 38.5°N. These low He concentrations (10−8 cm3 STP/g) are attributed to degassing before eruption. The 4He/3He ratios decrease from 90 000 at 37°N to 75 000 at 38.5°N and then increase to 100 000 at 40.5°N. The low 4He/3He ratio measured on the ridge at 38.5°N indicates a plume–ridge interaction with the Terceira–Pico–Sao Jorge plume, which has a 4He/3He lower than 50 000 (R/Ra>15). More radiogenic 4He/3He ratios (up to 110 000) can also be found in this segment of the ridge and may indicate either in situ production of 4He by radioactive decay or atmospheric contamination of very low helium content samples. The 20Ne/22Ne ratios decrease from a mantle-like value (>12.0) between 20 and 37°N to the atmospheric ratio (9.8) at 38.5°N. The 40Ar/36Ar ratios show the same tendency as the neon isotopic ratios (atmospheric-like at 38.5°N). This may indicate contamination in shallow degassed magma chambers. However, because the maximum measured 20Ne/22Ne ratios are strongly correlated to the 206Pb/204Pb, 87Sr/86Sr and 143Nd/144Nd ratios, an alternative explanation for the atmospheric 20Ne/22Ne is the presence of an atmospheric component in the source. However, due to the large He loss (and therefore Ne and Ar), atmospheric contamination is the more probable explanation.

Correlated Os–Pb–Nd–Sr isotopes in the Austral–Cook chain basalts: the nature of mantle components in plume sources

Osmium (Os), strontium (Sr), neodymium (Nd) and lead (Pb) isotopes have been measured on a suite of aphyric basalts from 12 islands of the Austral–Cook island archipelago, an area which exhibits a range in Pb isotope compositions that encompasses almost the entire range displayed by ocean island basalts (OIB). Although the samples have Os concentrations (1.69–34.80 ppt) at the lower end of the range measured for OIB, they display a range of initial 187Os/188Os ratios (between 0.1279 and 0.1594) similar to that defined by olivine–phyric, Os-rich OIB. Positive Os–Nd, Os–Pb and negative Os–Sr isotope correlations are documented, indicating that the isotopic arrays represent various proportions of mixing between a HIMU-type end-member represented by Mangaia and Tubuai islands and characterized by radiogenic Os and Pb isotopic compositions, and an end-member represented by Rarotonga island which is characterized by unradiogenic Os and intermediate Sr, Nd and Pb isotopic compositions. The HIMU signature of the mantle component involved in Tubuai–Mangaia mantle sources requires long-term enrichments of U and Th relative to Pb and Re relative to Os, without associated increase in Rb/Sr, that are consistent with recycled oceanic crust. The end-member represented by Rarotonga basalts shows Os, Sr, Nd, and Pb isotopic signatures similar to those presumed for the ‘bulk silicate earth’ (BSE), which cannot be obtained by mixing the four mantle components (DMM, HIMU and EM1 and 2) generally used to circumscribe the Sr–Nd–Pb isotopic data of OIB. The primitive-like isotopic characters of this end-member might be ascribed to the presence of undepleted material from a lower segment of the mantle in the source of the Austral–Cook island basalts (and more specifically Rarotonga basalts); however, such a hypothesis is challenged by both the absence of a primordial 3He signature and the non-primitive Ce/Pb and Nb/U values for the Austral–Cook island basalts. Alternatively, assuming that the primitive-like isotopic composition of the Rarotonga samples reflects mixing proportions between the HIMU component and a mantle component characterized by unradiogenic Os, Nd and Pb and radiogenic Sr isotopic composition relative to BSE, the involvement of recycled, old subcontinental lithosphere in the genesis of this component should then be considered.

Rb-Sr systematics in granite from central Nepal (Manaslu): Significance of the Oligocene age and high 87Sr/86Sr ratio in Himalayan orogeny

We report a precise Rb-Sr whole-rock isochron, obtained from the Manaslu granite of central Nepal, that is indicative of an age of 28 m.y. The initial 87 Sr/ 86 Sr ratio is very high, above 0.740, indicating crustal anatexis as the origin of the granite.

The Earth as a Chemical System

The solar system is composed of the Sun, which provides 99.8% of the total mass, the planets, and the ‘planetary objects’ (dust, comets, asteroids).

The Equilibria of Phases in the Lithosphere

The matter found on the Earth’s surface can be divided into two categories: (1) that formed under the conditions of temperature and pressure prevailing at the surface, and (2) that formed at much higher pressure and temperature. Among the latter, two chief types may be distinguished:

The Fractionation of the Light Isotopes

Modern mass spectrometers make it possible to measure variations in isotopic abundances to the order of 1 part in 104. With this sort of equipment, one might undertake to measure the isotopic composition of various elements in various natural mineral compounds, for example, the isotopic composition of S in natural sulfides, in native S, in S found in oils and coals, etc.

The Geochemical Fractionation of Trace Elements

Just as the major elements do, the trace elements also suffer dispersion or fractionation in the course of geochemical processes. They become distributed among phases of which they are not the essential constituents and which they did not create. They can take part in catalyses but this role, which may be quite important, is still totally unknown.