G. Manhes

UThPb systematics of the eucrite “Juvinas”: Precise age determination and evidence for exotic lead

The U-Th-Pb isotope systematics of the eucrite “Juvinas” have been studied in whole rock fragments as well as in plagioclases and pyroxenes. The results show that this monomict breccia crystallized with a very high UPb initial ratio at T = 4.539 ± 0.004 AE ago. There is evidence for a less radiogenic Pb component (206Pb204Pb = 13.0; 207Pb204Pb = 13.5; 208Pb204Pb = 32.71) interpreted as “exotic lead” induced by a meteoritical impact at the surface of the Juvinas parent body, 1.92 ± 0.06 AE ago.

Lead isotope study of basic-ultrabasic layered complexes: Speculations about the age of the earth and primitive mantle characteristics

Abstract In order to explore mantle heterogeneity through geological time, Pb isotopic compositions have been determined for 8 differentiated-layered intrusions whose ages are between 2.7 and 0.05 b.y.. The Pb-Pb ages of these intrusions and the U-Pb characteristics of their parent sources are discussed. The Pb-Pb dating method is found to be applicable for this type of basic or ultrabasic rocks and agrees satisfactorily with other available methods. Significant differences are found between the calculated values for the parent bodies of these intrusions. This could reflect either mantle heterogeneity since Archean times, or contamination of some of the bodies by continental crust. Discrimination between these hypotheses can be proposed from the positions of the initial Sr ratios of these massives in respect to the supposed “terrestrial” evolution line. The two intrusions which plot on that closed system Sr evolution line (Muntsche Tundra, U.S.S.R., and Skaergaard, Greenland) belong also to a simple two-stage evolution model for Pb, with a low first-stage μ value of 7.8. If these two bodies are considered as pieces of a “primitive” closed-system mantle, a 4.55 ± 0.01 age of the earth can be calculated from their Pb initial ratios.

An optimized procedure for boron separation and mass spectrometry analysis for river samples

An optimized procedure for the separation of boron from natural river samples and an improved mass spectrometry determination of boron isotopic ratio are presented. The chemical procedure, based on the use of the boron-specific resin Amberlite IRA 743, is especially efficient in separating boron from natural organic matter-rich samples like river waters. The properties of Amberlite IRA 743 have been investigated. The two factors important in determining the boron affinity for the resin are: the pH value and the ionic strength of the solution from which B is to be extracted. A logarithmic relationship between B partition coefficients and pH values is found. High ionic strength significantly lowers the fixation of B onto the Amberlite resin. The knowledge of the factors controlling the affinity of the resin Amberlite IRA 743 for boron enables us to design a simple and miniaturized chemical separation procedure characterized by (i) three chromatographic steps using, respectively, 50, 10 and 3 μl of resin, (ii) no evaporation step between each column, and (iii) final separation of boron from residual organic matter by sublimation of boric acid at 75 °C. Boron isotopic ratios are measured using an improved cesium metaborate technique, with graphite and mannitol. Adequate loading conditions enable us to obtain typical signal intensities of 5×10−12 A for 250 ng of boron. No in-run isotopic fractionation is observed, the external reproducibility for standards processed through the entire chemical procedure, as well as for samples, corresponds to 0.35‰ (±2σ). According to this precision, a slight, but reproducible isotopic fractionation of 0.4‰ is observed for standards processed through the entire chemical procedure whose origin is discussed, but is still unclear.

U-Pb systematics in iron meteorites: Uniformity of primordial lead

Pb isotopic compositions and U-Pb abundances were determined in the metal phase of six iron meteorites: Canyon Diablo IA, Toluca IA, Odessa IA, Youndegin IA, Deport IA and Mundrabilla An. Prior to complete dissolution, samples were subjected to a series of leachings and partial dissolutions. Isotopic compositions and abundances of the etched Pb indicate a contamination by terrestrial Pb which is attributable to previous cutting of the meteorite. Pb isotopic compositions measured in the decontaminated samples are identical within 0.2% and essentially confirm the primordial Pb value defined by Tatsumotoet al. (1973). These data invalidate more radiogenic Pb isotopic compositions published for iron meteorites, which are the result of terrestrial Pb contamination introduced mainly by analytical procedure. Our results support the idea of a solar nebula which was isotopically homogeneous for Pb 4.55 Ga ago. The new upper limit for U-abundance in iron meteorites, 0.001 ppb, is in agreement with its expected thermodynamic solubility in the metal phase.

Reexamination of the 187 Re bound on the variation of fundamental couplings

We reconsider the

Method for isotope ratio drift correction by internal amplifier signal synchronization in MC-ICPMS transient signals

{}^{187}\mathrm{Re}

Transient signal isotope analysis using multicollection of ion beams with Faraday cups equipped with 1012 Ω and 1011 Ω feedback resistors

bound on the variation of the fine-structure constant. We combine the meteoritic measurement with several present-day lab measurements to firmly establish the quantitative upper limit to the time variation over the age of the solar system. We find that the relative change of the fine-structure constant between its present value and

Deconvolution of the isotopic drift in LC-MC-ICPMS coupling: a new tool for studying isotope fractionation induced by sample introduction techniques

\ensuremath{\alpha}

Transient signal isotope analysis: validation of the method for isotope signal synchronization with the determination of amplifier first-order time constants†

of

A reply to the discussion of the [I,S] diagram by V.M. Oversby

\ensuremath{\sim}4.6\mathrm{Gyr}