92Nb-(92)Zr and the Early Differentiation History of Planetary Bodies

Determination of the Isotopic Composition of Zirconium Using MC-ICPMS and a Regression Model for Mass Bias Correction

Measurements of zirconium isotopes provide insights into the formation of planetary bodies, dating nuclear explosions, or maintenance of nuclear reactors. Although many comparative measurements for the isotopic composition of zirconium have been performed using mass spectrometry, there is a lack of zirconium isotopic reference materials and only a single calibrated measurement has been reported to date by total evaporation thermal ionization mass spectrometry (TE-TIMS). In this study, we report an independent measurement of full zirconium isotopic composition in a new zirconium isotopic reference material by multicollector inductively coupled plasma MS using the regression method and two independent certified isotopic reference materials, NIST SRM 984 Rb and NIST SRM 987 Sr. Our results agree well with the recent TE-TIMS measurements and suggest a revision of the standard atomic weight of zirconium.

Precise initial abundance of Niobium-92 in the Solar System and implications for p-process nucleosynthesis

The niobium-92-zirconium-92 (⁹²Nb-⁹²Zr) decay system with a half-life of 37 Ma has great potential to date the evolution of planetary materials in the early Solar System. Moreover, the initial abundance of the p-process isotope ⁹²Nb in the Solar System is important for quantifying the contribution of p-process nucleosynthesis in astrophysical models. Current estimates of the initial ⁹²Nb/⁹³Nb ratios have large uncertainties compromising the use of the ⁹²Nb-⁹²Zr cosmochronometer and leaving nucleosynthetic models poorly constrained. Here, the initial ⁹²Nb abundance is determined to high precision by combining the ⁹²Nb-⁹²Zr systematics of cogenetic rutiles and zircons from mesosiderites with U-Pb dating of the same zircons. The mineral pair indicates that the ⁹²Nb/⁹³Nb ratio of the Solar System started with (1.66 ± 0.10) × 10^-5, and their ⁹²Zr/⁹⁰Zr ratios can be explained by a three-stage Nb-Zr evolution on the mesosiderite parent body. Because of the improvement by a factor of 6 of the precision of the initial Solar System ⁹²Nb/⁹³Nb, we can show that the presence of ⁹²Nb in the early Solar System provides further evidence that both type Ia supernovae and core-collapse supernovae contributed to the light p-process nuclei.

Constraining the astrophysical origin of the p-nuclei through nuclear physics and meteoritic data

A small number of naturally occurring, proton-rich nuclides (the p-nuclei) cannot be made in the s- and r-processes. Their origin is not well understood. Massive stars can produce p-nuclei through photodisintegration of pre-existing intermediate and heavy nuclei. This so-called γ-process requires high stellar plasma temperatures and occurs mainly in explosive O/Ne burning during a core-collapse supernova. Although the γ-process in massive stars has been successful in producing a large range of p-nuclei, significant deficiencies remain. An increasing number of processes and sites has been studied in recent years in search of viable alternatives replacing or supplementing the massive star models. A large number of unstable nuclei, however, with only theoretically predicted reaction rates are included in the reaction network and thus the nuclear input may also bear considerable uncertainties. The current status of astrophysical models, nuclear input and observational constraints is reviewed. After an overview of currently discussed models, the focus is on the possibility to better constrain those models through different means. Meteoritic data not only provide the actual isotopic abundances of the p-nuclei but can also put constraints on the possible contribution of proton-rich nucleosynthesis. The main part of the review focuses on the nuclear uncertainties involved in the determination of the astrophysical reaction rates required for the extended reaction networks used in nucleosynthesis studies. Experimental approaches are discussed together with their necessary connection to theory, which is especially pronounced for reactions with intermediate and heavy nuclei in explosive nuclear burning, even close to stability.

Quantum State-Resolved Study of the Four-Atom Reaction OH- (XΣ+) + D2 (X1Σg+, v = 0) → HOD (X1A‘, v‘) + D- (1S)

The D+ transfer reaction between OH- (X1sigma+) and D2 was studied with crossed molecular beam experiments and quantum chemical calculations at collision energies of 89 and 68 kJ/mol. The D- product ions were observed and measured for the first time in the crossed beam experiments. The center-of-mass (c.m.) flux distributions of the D- product ions exhibit significant asymmetry, and their maxima are close to the velocity and direction of the precursor D2 beam. The data are consistent with a direct mechanism that occurs on a time scale significantly less than a rotational period of the transient complex formed by approaching reactants. The D+ transfer results primarily in the excitation of the H-O-D bending vibrational mode of the molecular product. The experimental observation is in agreement with theoretical results showing that, during the D+ transfer, the H-O-D bond angle changes significantly.

Niobium-zirconium chronometry and early solar system development

Niobium-92 (92Nb) decays to zirconium-92 (92Zr) with a half-life of 36 million years and can be used to place constraints on the site of p-process nucleosynthesis and the timing of early solar system processes. Recent results have suggested that the initial 92Nb/93Nb of the solar system was high (>10(-3)). We report Nb-Zr internal isochrons for the ordinary chondrite Estacado (H6) and a clast of the mesosiderite Vaca Muerta, both of which define an initial 92Nb/93Nb ratio of approximately 10(-5). Therefore, the solar system appears to have started with a ratio of <3 x 10(-5), which implies that Earth's initial differentiation need not have been as protracted as recently suggested.

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.