Abstract
We have investigated nucleosynthetic Mo isotope anomalies in 38 different bulk iron meteorites from 11 groups, to produce by far the largest and most precise dataset available to date for such samples. All magmatic iron groups were found to display deficits in s-process Mo isotopes, with essentially constant anomalies within but significant variations between groups. Only meteorites of the non-magmatic IAB/IIICD complex revealed terrestrial Mo isotopic compositions.The improved analytical precision achieved in this study enables two isotopically distinct suites of iron meteorites to be identified. Of these, the r=p suite encompasses the IC, IIAB, IIE, IIIAB, IIIE and IVA groups and exhibits relatively modest but ‘pure’ s-process deficits, relative to Earth. The second r>p suite includes groups IIC, IIIF and IVB. These iron meteorites show larger s-process deficits than the r=p suite, coupled with an excess of r-process relative to p-process components.Comparison of the results with data for other elements (e.g., Cr, Ni, Ru, Ti, Zr) suggests that the Mo isotope variability is most likely produced by thermal processing and selective destruction of unstable presolar phases. An updated model is proposed, which relates the iron meteorite suites to different extents of thermal processing in the solar nebula, as governed by heliocentric distance. In detail, the r=p suite of iron meteorite parent bodies is inferred to have formed closer to the Sun, where the extent of thermal processing was similar to that experienced by terrestrial material, so that the meteorites exhibit only small s-process deficits relative to Earth. In contrast, the r>p suite formed at greater heliocentric distance, where more subtle thermal processing removed a smaller proportion of r- and p-process host phases, thereby generating larger s-process deficits relative to the terrestrial composition. In addition, the thermal conditions enabled selective destruction of p- versus r-isotope carrier phases, to produce the observed divergence of r- and p-process Mo isotope abundances.
Highlights
The solar system formed from the collapse of a molecular cloud of interstellar dust and gas featuring isotopically diverse material produced by nuclear reactions in various pre-existing stellar sources
All Mo isotope results for normalisation to 98Mo/96Mo and 97Mo/95Mo are summarised in Fig. 1, with corresponding data presented in Table 1 and Table 2
The more precise Mo isotope data of this study enable the two distinct iron meteorite groupings to be resolved with more confidence than was previously possible
Summary
The solar system formed from the collapse of a molecular cloud of interstellar dust and gas featuring isotopically diverse material produced by nuclear reactions in various pre-existing stellar sources. Whilst presolar grains in primitive meteorites show this cloud was isotopically heterogeneous at the grain-size level BS8 1RJ, UK. (Zinner, 2007), it was initially thought planetary bodies evolved from a hot solar nebula that was well-mixed on larger scales. Various more recent studies identified isotopic variations in bulk meteorites for a number of refractory elements, which are interpreted to reflect planetary-scale heterogeneities in presolar matter that place critical constraints on the physical conditions within the solar nebula. The isotopic heterogeneities found for elements including Ba, Ca, Cr, Mo, Nd, Ni, Ru, Ti and Zr thereby stand in contrast to the isotopic homogeneity exhibited by other refractory elements such as Hf and Os (see Dauphas and Schauble, 2016 and Yokoyama and Walker, 2016 for a thorough overview). Molybdenum is ideally suited as a tracer of planetary-scale isotopic heterogeneity because it has seven isotopes that were produced by distinct nucleosynthetic processes.
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