Exploring the fractionation of platinum group elements in carbonaceous chondrites
When the ancient Allende meteorite fell over Mexico in 1969, it scattered fragments containing tiny metallic nuggets older than our Solar System. These extraterrestrial rocks—carbonaceous chondrites—preserve a 4.56-billion-year chemical fingerprint of our cosmic neighborhood's birth. Within them, six rare platinum group elements (PGEs): platinum, palladium, rhodium, ruthenium, iridium, and osmium—act as forensic clues. Despite their extreme density (12-22 g/cm³) and tendency to sink into planetary cores, PGEs persist in chondrite matrices. How did these "iron-loving" metals resist complete segregation during planetary formation? Their fractionation patterns reveal a dramatic story of nebular condensation, asteroidal melting, and late meteorite bombardment that shaped terrestrial planets 1 5 .
PGEs in chondrites provide a unique window into the early Solar System's chemical composition and processes.
These meteorites preserve material virtually unchanged since the Solar System's formation 4.56 billion years ago.
PGEs belong to the most refractory metals, condensing from the solar nebula between 1,800–1,500 K. They split into two subgroups:
Earth's mantle displays chondritic PGE ratios, despite most sinking into the core during differentiation. This paradox implies a later delivery of PGE-rich material—a "late veneer"—via chondritic impacts after core formation ended. Pt isotope studies confirm ≤50% of Earth's PGEs existed pre-late veneer 4 5
In carbonaceous chondrites:
This chemical "yin-yang" suggests volatile exchange during chondrule formation, not pristine nebular dust
PGEs concentrate anomalously in terrestrial anorthosite complexes (e.g., Bushveld). Their formation may replicate chondritic fractionation—possibly via sulfide melt extraction—but debates persist about nebular vs. planetary origins 5
Crushed fragments of carbonaceous chondrites (Allende CV3, Orgueil CI1) separated into magnetic (metal-rich) and non-magnetic (silicate) fractions
| Element | Condensation Temp (K) | Chondritic Abundance (ppb) | Role in Fractionation |
|---|---|---|---|
| Os | 1,800 | 500 | Core formation chronometer |
| Ir | 1,600 | 480 | Volatility tracer |
| Pt | 1,400 | 990 | Late veneer indicator |
| Pd | 1,320 | 560 | Sulfide affinity marker |
| Sample Type | δ¹⁹⁸Pt (‰) | Pt (ppb) | Scientific Implication |
|---|---|---|---|
| CI Chondrites | 0.00 ± 0.02 | ~1,000 | Solar system baseline |
| Achondrites | –0.41 | 4–10 | Magmatic differentiation |
| Earth (Archean) | +0.12 | 0.14 | Pre-late veneer mantle |
| Earth (Modern) | 0.00 | 7.6 | Full late veneer mixing |
Essential reagents and methods for PGE geochemistry:
| Reagent/Method | Function | Critical Insight |
|---|---|---|
| Carius Tube | Sealed digestion of refractory alloys | Prevents Os loss during dissolution |
| ¹⁹⁰Os-¹⁸⁵Re Spike | Isotope dilution quantification | Corrects for incomplete recovery |
| NTIMS | High-precision Os isotope measurement | Detects <0.1% ratio variations |
| Laser Ablation ICP-MS | In situ matrix/chondrule analysis | Maps micron-scale PGE zoning |
| Nitric Acid (HNO₃) | Selective sulfide dissolution | Releases PGEs from pentlandite |
Platinum group elements in carbonaceous chondrites are more than cosmic oddities—they're isotopic archives of the Solar System's violent infancy. From Re-Os chronometers proving Earth's late veneer delivery to Pt anomalies hinting at pre-impact heterogeneity, these metals force us to rethink how planets "inherit" nebular chemistry. Next-gen tools like high-resolution LA-ICP-MS now probe sub-micron PGE carriers in chondrite matrices, potentially revealing how presolar grains survived condensation. As asteroid missions (OSIRIS-REx, Hayabusa2) return fresh samples, the fractionation fingerprints of PGEs will keep decoding the chaos and order of planet building 4 5 .
"Chondrites are the Rosetta Stones that speak in the language of metals forged before worlds."