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

• Published in: Meteoritics & planetary science 2022-02, Vol.57 (2), p.450-471
• Main Authors: Sanders, Ian S., Scott, Edward R. D., Krot, Alexander
• Format: Article
• Language: English
• Subjects: Anomalies; Carbonaceous chondrites; Chondrites; Chondrule; Depletion; Deposition; Inclusions; Isotopes; Leachates; Metamorphism; Molybdenum; Nuclides; Tungsten
• ISSN: 1086-9379; 1945-5100
• DOI: 10.1111/maps.13742

The remarkable complementary isotopic relationship in the Allende chondrite between chondrules (depleted in s‐process molybdenum and tungsten) and matrix (enriched in these nuclides) has been interpreted as evidence that the anomalies were established during chondrule formation, and that chondrules were, therefore, not made by planetesimal collisions. We question this interpretation, and to better understand the complementary relationship, we review nucleosynthetic isotopic variations of Mo and W in bulk carbonaceous chondrites, their components, and acid leachates extracted from them. Mo isotopic data almost always track a mixing line between pure s‐process Mo and s‐process‐depleted Mo (i.e., with excess p‐process and r‐process Mo in a fixed ratio). Tungsten data track an equivalent mixing line. Guided by our review, we develop a model suggesting how the isotopic variations in Allende’s chondrules and matrix could be attributable to hydrothermal alteration in the parent body. In our model, anomalous Mo and W, both depleted in s‐process isotopes, are easily leached from their carriers in the matrix, then transported in solution and precipitated preferentially in water‐deficient components, such as chondrules, where the aqueous solvent is consumed. The model operates after accretion so does not inform chondrule‐forming mechanisms. It also goes some way to explaining variations of Mo and W isotopes in Ca‐Al‐rich inclusions in Allende, and variations of s‐process Mo in bulk carbonaceous chondrites.