No abstract

Rocky asteroids and planets display nucleosynthetic isotope variations that are attributed to the heterogeneous distribution of stardust from different stellar sources in the solar protoplanetary disk. Here we report new high precision palladium isotope data for six iron meteorite groups, which display smaller nucleosynthetic isotope variations than the more refractory neighbouring elements. Based on this observation we present a new model in which thermal destruction of interstellar medium dust results in an enrichment of s-process dominated stardust in regions closer to the Sun. We propose that stardust is depleted in volatile elements due to incomplete condensation of these elements into dust around asymptotic giant branch (AGB) stars. This led to the smaller nucleosynthetic variations for Pd reported here and the lack of such variations for more volatile elements. The smaller magnitude variations measured in heavier refractory elements suggest that material from high-metallicity AGB stars dominated stardust in the Solar System. These stars produce less heavy s-process elements (Z ≥ 56) compared to the bulk Solar System composition.The protoplanetary disk from which our Solar System formed incorporated dust that was inherited from the collapsing molecular cloud. A few per cent of this dust formed around stars with active nucleosynthesis and retained the extreme isotopic fingerprint of its formation environment 1,2 . This dust, which is isotopically anomalous compared to Solar System compositions, is here termed stardust. However, it is often also referred to as presolar grains when found in meteorites. Most stardust in primitive meteorites originates from asymptotic giant branch (AGB) stars, the site of s-process nucleosynthesis, with only small contributions from supernovae environments 3 . The majority of the dust in the solar protoplanetary disk grew in the interstellar medium (ISM) from a well-mixed gas phase as mantles on pre-existing nuclei. These mantles likely inherited the composition of the local ISM, i.e., a near solar isotopic composition 1 .Nucleosynthetic isotope variations, relative to Earth, are well established for a range of elements in bulk meteorites 4 . These variations mostly reflect the heterogeneous distribution of isotopically distinct dust in the protoplanetary disk. It is generally thought that this heterogeneity was established, at least in part, due to processes occurring in the protoplanetary disk itself. Physical sorting of grains, either by mineralogical type 5 or size 6 , and selective destruction of stardust by thermal processing in the protoplanetary disk 7-9 or by aqueous alteration on parent bodies 10 , have all been proposed as possible mechanisms to generate isotope heterogeneity. While these individual processes can explain nucleosynthetic variations for specific elements, it is debated whether a unifying explanation for all elemental trends exists 11 . Therefore, considerable uncertainty remains as to which processes were important for dust processing in the protoplanet...

The short-lived 182 Hf-182 W decay system is a powerful chronometer for constraining the timing of metal-silicate separation and core formation in planetesimals and planets. Neutron capture effects on W isotopes, however, significantly hamper the application of this tool. In order to correct for neutron capture effects, Pt isotopes have emerged as a reliable in-situ neutron dosimeter. This study applies this method to IAB iron meteorites, in order to constrain the timing of metal segregation on the IAB parent body.The ε 182 W values obtained for the IAB iron meteorites range from −3.61 ± 0.10 to −2.73 ± 0.09. Correlating ε i Pt with ε 182 W data yields a pre-neutron capture ε 182 W of −2.90 ± 0.06. This corresponds to a metal-silicate separation age of 6.0 ± 0.8 Ma after CAI for the IAB parent body, and is interpreted to represent a body-wide melting event. Later, between 10 and 14 Ma after CAI, an impact led to a catastrophic break-up and subsequent reassembly of the parent body. Thermal models of the interior evolution that are consistent with these estimates suggest that the IAB parent body underwent metalsilicate separation as a result of internal heating by short-lived radionuclides and accreted at around 1.4 ± 0.1 Ma after CAIs with a radius of greater than 60 km.

The winonaites are primitive achondrites which are associated with the IAB iron meteorites. Textural evidence implies heating to at least the Fe, Ni-FeS cotectic, but previous geochemical studies are ambiguous about the extent of silicate melting in these samples. Oxygen isotope evidence indicates that the precursor material may be related to the carbonaceous chondrites. Here we analysed a suite of winonaites for modal mineralogy and bulk major- and trace-element chemistry in order to assess the extent of thermal processing as well as constrain the precursor composition of the winonaite -IAB parent asteroid. Modal mineralogy and geochemical data are presented for eight winonaites. Textural analysis reveals that, for our sub-set of samples, all except the most primitive winonaite (Northwest Africa 1463) reached the Fe, Ni-FeS cotectic. However, only one (Tierra Blanca) shows geochemical evidence for silicate melting processes. Tierra Blanca is interpreted as a residue of small-degree silicate melting. Our sample of Winona shows geochemical evidence for extensive terrestrial weathering. All other winonaites studied here (Fortuna, Queen Alexander Range 94535, Hammadah al Hamra 193, Pontlyfni and NWA 1463) have chondritic major-element ratios and flat CI-normalised bulk rare-earth element patterns, suggesting that most of the winonaites did not reach the silicate melting temperature. The majority of winonaites were therefore heated to a narrow temperature range of between ~1220 (the Fe, Ni-FeS cotectic temperature) and ~1370 K (the basaltic partial melting temperature). Silicate inclusions in the IAB irons demonstrate partial melting did occur in some parts of the parent body (Ruzicka and Hutson, 2010), thereby implying heterogeneous heat distribution within this asteroid. Together, this indicates that melting was the result of internal heating by short-lived radionuclides. The brecciated nature of the winonaites suggests that the parent body was later disrupted by a catastrophic impact, which allowed the preservation of the largely unmelted winonaites. Despite major-element similarities to both ordinary and enstatite chondrites, trace-element analysis suggests the winonaite parent body had a arbonaceous chondrite-like precursor composition. The parent body of the winonaites was volatile -depleted relative to CI, but enriched compared to the other carbonaceous classes. The closest match are the CM chondrites, however, the specific precursor is not sampled in current meteorite collections

No abstract