Th/U variability in Allende chondrules

· by · in ,

Janne Blichert-Toft, Christa Göpel, Marc Chaussidon, F. Albarède

Geochimica et Cosmochimica Acta
In Press, Journal Pre-proof, Available online 17 April 2020

LINK

“Lead isotope compositions were measured on both single and combined chondrules from the CV3 carbonaceous chondrite Allende with the goal of determining the range of Th/U implied by the radiogenic 208Pb/206Pb values. All samples were aggressively acid step-leached to separate radiogenic from primordial lead. It is found that apparent Th/U varies both between individual chondrules and between the different leaching fractions of each chondrule or group of chondrules. Specifically, the apparent Th/U ratio deviates from the planetary value (3.876), varying spectacularly from 0.65 to 14.6. Variations between leachates and residues disclose the existence of internal heterogeneities, while inter-chondrule variations reveal the presence of external heterogeneities. Three main explanations for the observed Th-U fractionation that are not mutually exclusive prevail: (1) uranium species, notably UO and UO2, coexisted in the nebular gas at high temperature, whereas Th existed exclusively as ThO2; (2) chondrules interacted with an exotic oxidized vapor; and (3) chondrules represent melt of dust of different origins, a hypothesis dictated by the evidence of internal heterogeneity. The extent to which the measured apparent Th/U variability is due to each of these particular processes is difficult to assess, but the existence of substantial Th/U heterogeneity, especially within, but also among, single (or pooled) chondrules from the same chondrite calls for caution when Pb-Pb linear arrays, or mixing lines, are assigned isochronous significance.”

Related posts (automatically generated):

  1. Hf-W Chronometry of Allende Chondrules and Matrix
  2. Hafnium-Tungsten Age of Allende Chondrules and Matrix
  3. Uranium isotopic composition and absolute ages of Allende chondrules
  4. In situ Si isotope and chemical constraints on formation and processing of chondrules in the Allende meteoriteOPEN ACCESS 
  5. Multiple CV Chondrule Precursors Originating from the Inner and Outer Solar System: Evidence from Cr-Ti-O Isotope Systematics of Allende Chondrules
  6. Oxygen isotope signatures in bulk chondrules: Implications for the aqueous alteration and thermal metamorphism on the Allende CV3 parent body
  7. Complementary nucleosynthetic isotope anomalies of Mo and W in chondrules and matrix in the Allende carbonaceous chondrite: The case for hydrothermal metamorphism and its implications
  8. Cr isotopes in physically separated components of the Allende CV3 and Murchison CM2 chondrites: Implications for isotopic heterogeneity in the solar nebula and parent body processesOPEN ACCESS 
  9. In Situ Argon Isotope Analyses of Chondrule-Forming Materials in the Allende Meteorite: A Preliminary Study for 40Ar/39Ar Dating Based on Cosmogenic 39ArOPEN ACCESS 
  10. A unifying model for the accretion of chondrules and matrixOPEN ACCESS 
Tags: , , ,