Measurement of the half-life of 176Lu

18F half-life measurement using a high-purity germanium detector

The half-life of (18)F has been measured using HPGe detectors with a (137)Cs reference source. The counting ratio of 511 keV γ-rays from (18)F to 622 keV γ-rays from (137)Cs was fitted for the half-life with a weighted least-square method. Uncertainties due to the systematic effects arising from the measurement of a high activity (18)F source were studied in detail. The half-life of (18)F was found to be (109.72±0.19) min. The result is in a good agreement with the recommended value of (109.728±0.019) min evaluated at the Laborotaire National Henri Becquerel (LNHB).

Combined U-Pb and Lu-Hf isotope analyses by laser ablation MC-ICP-MS: methodology and applications

The Lutetium-Hafnium isotopic system represents one of the most innovative and powerful tools for geochronology and isotopic studies. Combined U-Pb and Lu-Hf in situ analyses on zircon by LA-MC-ICP-MS permit to characterize isotopically the host magma from which it crystallized furnishing significant information for sediment provenance and crustal evolution studies. In this paper e describe the Lu-Hf systematic by LA-MC-ICP-MS developed in the laboratory of Geochronology of the University of Brasilia and report the results obtained by repeated analyses of 176Hf/177Hf isotopic ratio of three zircon standards: GJ-1 = 0.282022 ± 11 (n=56), Temora 2 = 0.282693 ± 14 (n=25) and UQZ = 0.282127 ± 33 (n=11). The 176Hf/177Hf ratio (0.282352 ± 22, n=14) of gem quality zircon used as in-house standard have been also characterized. As a geological application, we analyzed two complex zircons selected from a migmatitic rocks from the Borborema Province, NE Brazil. On the basis of U-Pb and Lu-Hf data, two main crystallization events have been identified in both studied zircons. An older event at ca. 2.05 Ga recognized in the inherited cores represents a well-characterized paleoproterozoic magmatic event that affected the whole Borborema Province. A second crystallization event at ~ 575 Ma, recognized at the rims, represents a Neoproterozoic (Brazilian) high grade metamorphic-magmatic event.

Half-life of 176Lu

The half-life of 176Lu, measured using gamma-ray spectrometry, has been found to be (3.56+/-0.07)x10(10)yr. Comparison has been made between present results and literature data.

Improved limit on the electron capture decay branch of 176Lu

We have performed searches for the electron-capture decay branches of 176Lu to the ground state and first excited state of 176Yb. No evidence of either decay mode was observed. From these measurements we have established upper limits on both of these possible branches that are each > 20 times more stringent than the single previously published limit.

Early history of Earth's crust-mantle system inferred from hafnium isotopes in chondrites

The 176Lu to 176Hf decay series has been widely used to understand the nature of Earth's early crust-mantle system. The interpretation, however, of Lu-Hf isotope data requires accurate knowledge of the radioactive decay constant of 176Lu (lambda176Lu), as well as bulk-Earth reference parameters. A recent calibration of the lambda176Lu value calls for the presence of highly unradiogenic hafnium in terrestrial zircons with ages greater than 3.9 Gyr, implying widespread continental crust extraction from an isotopically enriched mantle source more than 4.3 Gyr ago, but does not provide evidence for a complementary depleted mantle reservoir. Here we report Lu-Hf isotope measurements of different Solar System objects including chondrites and basaltic eucrites. The chondrites define a Lu-Hf isochron with an initial 176Hf/177Hf ratio of 0.279628 +/- 0.000047, corresponding to lambda176Lu = 1.983 +/- 0.033 x 10-11 yr-1 using an age of 4.56 Gyr for the chondrite-forming event. This lambda176Lu value indicates that Earth's oldest minerals were derived from melts of a mantle source with a time-integrated history of depletion rather than enrichment. The depletion event must have occurred no later than 320 Myr after planetary accretion, consistent with timing inferred from extinct radionuclides.

Calibration of the lutetium-hafnium clock

Well-defined constants of radioactive decay are the cornerstone of geochronology and the use of radiogenic isotopes to constrain the time scales and mechanisms of planetary differentiation. Four new determinations of the lutetium-176 decay constant (lambda176Lu) made by calibration against the uranium-lead decay schemes yield a mean value of 1.865 +/- 0.015 x 10(-11) year(-1), in agreement with the two most recent decay-counting experiments. Lutetium-hafnium ages that are based on the previously used lambda176Lu of 1.93 x 10(-11) to 1.94 x 10(-11) year(-1) are thus approximately 4% too young, and the initial hafnium isotope compositions of some of Earth's oldest minerals and rocks become less radiogenic relative to bulk undifferentiated Earth when calculated using the new decay constant. The existence of strongly unradiogenic hafnium in Early Archean and Hadean zircons implies that enriched crustal reservoirs existed on Earth by 4.3 billion years ago and persisted for 200 million years or more. Hence, current models of early terrestrial differentiation need revision.

The oldest rocks on Earth: time constraints and geological controversies

Ages in the range 3.6–4.0 Ga (billion years) have been reported for the oldest, continental, granitoid orthogeneisses, whose magmatic precursors were probably formed by partial melting or differentiation from a mafic, mantle-derived source. The geological interpretation of some of the oldest ages in this range is still strongly disputed.

The oldest known supracrustal (i.e. volcanic and sedimentary) rocks, with an age of 3.7–3.8 Ga, occur in West Greenland. They were deposited in water, and several of the sediments contain 13C-depleted graphite microparticles, which have been claimed to be biogenic.

Ancient sediments (c. 3 Ga) in western Australia contain much older detrital zircons with dates ranging up to 4.4 Ga. The nature and origin of their source is highly debatable. Some ancient (magmatic) orthogneisses (c. 3.65–3.75 Ga) contain inherited zircons with dates up to c. 4.0 Ga. To clarify whether zircons in orthogneisses are inherited from an older source region or cogenetic with their host rock, it is desirable to combine imaging studies and U-Pb dating of single zircon grains with independent dating of the host rock by other methods, including Sm-Nd, Lu-Hf and Pb/Pb.

Initial Nd, Hf and Pb isotopic ratios of ancient orthogneisses are essential parameters for investigating the degree of heterogeneity of early Archaean mantle. The simplest interpretation of existing isotopic data is for a slightly depleted, close-to-chondritic, essentially homogeneous early Archaean mantle; this does not favour the existence of a sizeable, permanent continental crust in the early Archaean.

By analogy with the moon, massive bolide impacts probably terminated on Earth by c. 3.8–3.9 Ga, although no evidence for them has yet been found. By c. 3.65 Ga production of continental crust was well underway, and global tectonic and petrogenetic regimes increasingly resembled those of later epochs.

Hf-Nd isotope evidence for a transient dynamic regime in the early terrestrial mantle

Modern basalts have seemingly lost all 'memory' of the primitive Earth's mantle except for an ambiguous isotopic signal observed in some rare gases. Although the Earth is expected to have reached a thermal steady state within several hundred million years of accretion, it is not known how and when the initial chemical fractionations left over from planetary accretion (and perhaps a stage involving a magma ocean) were overshadowed by fractionations imposed by modern-style geodynamics. Because of the lack of samples older than 4 Gyr, this early dynamic regime of the Earth is poorly understood. Here we compare published Hf-Nd isotope data on supracrustals from Isua, Greenland, with similar data on lunar rocks and the SNC (martian) meteorites, and show that, about 3.8 Gyr ago, the geochemical signature of the Archaean mantle was partly inherited from the initial differentiation of the Earth. The observed features seem to indicate that the planet at that time was still losing a substantial amount of primordial heat. The survival of remnants from an early layering in the modern deep mantle may account for some unexplained seismological, thermal and geochemical characteristics of the Earth as observed today.