Tungsten Nuclear Anomalies in Planetesimal Cores

Abstract

Use of the extinct 182 Hf-182 W chronometer to constrain the timing of planetary accretion and differentiation rests on the assumption that the solar nebula had homogeneous tungsten isotopic composition. Here, we report deficiencies of $0.1 part in 10,000 in the abundance of 184 W in group IVB iron meteorites relative to the silicate Earth. These are most likely due to incomplete mixing at the planetesimal scale (2Y4 km radius bodies) of the products of slow (s-) and rapid (r-) neutron-capture nucleosynthesis in the solar nebula. The correction that must be applied to the 182 Hf-182 W model age of core formation in IVB irons due to the presence of these nuclear anomalies is $0.5 Myr.

Nucleosynthetic Tungsten Isotope Anomalies in Acid Leachates of the Murchison Chondrite: Implications for Hafnium-Tungsten Chronometry

“…However, the s-process path in the Hf-Ta-W-Re-Os region of the nuclide chart is not well understood (e.g., Ávila et al 2012), making such predictions uncertain. In contrast, the new W isotopic data presented here provide the first direct measurement of correlated nucleosynthetic effects on the different W isotopes, and as such provide a powerful means for quantifying nucleosynthetic anomalies on ε 182 W. Qin et al (2008b), using Maxwellian-averaged cross sections (MACS) from Bao et al (2000) Qin et al (2008b) also noted that the MACS of Bao et al (2000) led to a predicted r-process 182 W residual abundance that was too high, and suggested that this overproduction problem may be resolved if the MACS value of 182 W was reduced by ≈20%. Using the modified value, a slope of ≈0.5 was obtained for the ε 182 W-ε 184 W correlation line.…”

“…A heterogeneous distribution of these different phases would produce nucleosynthetic W isotopic anomalies, which may affect the use of the 182 Hf-182 W system to infer the timescales of early solar system processes. However, with the exception of small 184 W deficits in IVB iron meteorites (Qin et al 2008b) Qin et al 2008b). However, the s-process path in the Hf-Ta-W-Re-Os region of the nuclide chart is not well understood (e.g., Ávila et al 2012), making such predictions uncertain.…”

THE ELUSIVE60Fe IN THE SOLAR NEBULA

“…The parent body of angrites is 4564.42 ± 0.12 Ma old (Amelin 2008) and Quitté et al (2010) determined that its initial 60 Fe/ 56 Fe ratio is ≈(3.1 ± 0.8) × 10 −9 . Although tungsten isotope evidence (Schersten et al 2006;Qin et al 2008) indicates that most iron meteorites are formed within a few Ma of CAIs, they show no evidence of having 60 Ni excesses (Markowski et al 2006). The exact ages of these samples, however, remain poorly determined.…”

Discovery of non-radiogenic tungsten isotopic anomalies in the Allende CV3 chondrite

“…After the Arlandini et al (1999) paper was published, the neutron capture cross section of 182 Ta was revised downward by 50%, which increased the production of 182 W by 40%. This has been discussed in (Qin et al, 2008). We discuss the issue further below (Fig.…”

“…However, a series of recent studies using high precision multiple collector-inductively-coupled-plasma-mass spectrometry (MC-ICP-MS) and thermal ionization mass spectrometry (TIMS) have revealed that the isotopic homogeneity for many elements in meteoritic materials are not warranted (e.g., Mo, Yin et al, 2002b;Dauphas et al, 2002;Becker and Walker, 2003;Ba, Hidaka et al, 2003;Ranen and Jacobsen, 2006;Carlson et al, 2007;Nd, Sm, Andreasen and Sharma, 2006;Carlson et al, 2007;Cr, Trinquier et al, 2007;Ni, Regelous et al, 2008;W, Qin et al, 2008;Os, Brandon et al, 2005;Yokoyama et al, 2007). Qin et al (2008) first reported the significant level of 184 W deficiencies iñ 0.1ε units from the IVB iron meteorites. The resulting variations in the isotopic composition of 184 W found in the IVB iron meteorites was attributed to reflect the heterogeneous distribution of s-and r-process nuclides at the nebula scale for the source materials of parent body of the IVB irons accreted (an asteroid of ~2-4 km radius; Haack et al, 1990).…”