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Rapid cooling of planetesimal cores has been inferred for several iron meteorite parentbodies based on metallographic cooling rates, and linked to the loss of their insulating mantles during impacts. However, the timing of these disruptive events is poorly constrained. Here, we used the short-lived 107 Pd-107 Ag decay system to date rapid core cooling by determining Pd-Ag ages for iron meteorites. We show closure times for the iron meteorites equate to cooling in the timeframe ~7.8-11.7 Myr after CAI, and indicate that an energetic inner Solar System persisted at this time. This likely results from the dissipation of gas in the protoplanetary disk, after which the damping effect of gas drag ceases. An early giant planet instability between 5-14 Myr after CAI could have reinforced this effect. This correlates well with the timing of impacts recorded by the Pd-Ag system for iron meteorites. Main TextIron meteorites are thought to represent the once heated interior portions of planetesimals and are some of the earliest-formed bodies in our Solar System 1 . As such, they are survivors of the many dynamical processes that have shaped Solar System architecture, including dissipation of the protoplanetary disk, and runaway growth, migration and reorganisation of the giant planets 2,3 . Importantly, high and variable metallographic cooling rates among many iron meteorite groups, including the IIAB, IIIAB and IVAs, indicate that these metals cooled in bodies that had been stripped of their insulating silicate mantles 4,5 . Dating core crystallisation can therefore elucidate the timing of impact processes, which in turn constrains the evolution of our early Solar System. The short-lived 107 Pd-107 Ag decay system (t1/2 ~6.5 Myr) provides a tool for this. Palladium, which is highly siderophile, partitions into the metal phase during crystallisation, and chalcophile Ag is strongly sequestered into sulfides. This leads to Pd-Ag fractionation during crystallisation of a cooling core resulting in samples from a single body with variable Pd/Ag ratios, thereby allowing us to derive parent-body initial 107 Pd/ 108 Pd ratios from isochron slopes. The ages for parent-body cooling through the closure temperature of the Pd-Ag system (~820-970 K) 6 can be derived from these initials.Early studies of iron meteorites using the Pd-Ag chronometer did not fully account for the exposure of samples to galactic cosmic rays (GCR) [7][8][9] , which can disturb the system through secondary neutron capture reactions by Pd isotopes that lead to a decrease in 107 Ag/ 109 Ag. This occurs primarily via neutron capture on 108 Pd and subsequent βdecay to 109 Ag, and requires correction to achieve meaningful ages 10 . After GCR correction, initial 107 Pd/ 108 Pd ratios of individual meteorites from the iron meteorite groups IIAB, IIIAB and IVA imply core cooling between ~7.2 Myr and 10.7 Myr after the formation of calcium-aluminium rich inclusions (CAIs) 6,11,12 , relative to the Solar System initial (SSI) 107 Pd/ 108 Pd determined from carbonaceous chon...

Operating an Agilent 7700X ICP-MS spectrometer under robust plasma conditions (1550 W) with a He-filled octopole collision cell and analysing solutions (<2000 μg•g −1 total dissolved solids) still suffered analyte peak suppression due to matrix effects. International reference rocks BCR-1, BHVO-1, AGV-1, G-2 and BCR-2 all showed count rate reductions for 36 elements (mass range 7 Li to 238 U) averaging ~10% but with no dependence on isotope mass. Use of an internal standard (103 Rh) and/or using a tenfold dilution of sample solutions reduced these effects but problems with reduced count rates combined with larger errors for some elements introduced other problems. The best approach was to normalise the count rates for each element in the other samples against those for BCR-1 as an external standard; thus the count suppression due to the matrix effect is corrected for each individual element. This approach provides standardization "traceability" in line with the ERM ISO/IEC requirement. Experiments are also reported on quantifying the proportions of Ba and selected REE oxide/hydroxide components versus parent isotopes (XO/X and XOH/X). This information is essential for correcting peak interferences on higher mass number REE for the rock samples, and equations are developed to use measured CeO/Ce and CeOH/Ce ratios to predict such values for any other member of the REE suite. Concentrations obtained show excellent agreement with recommended values for the international reference materials especially for the REE. Robust data are also provided for two other standard rocks: nepheline syenite STM-1 and quartz syenite CAAS-1; the latter shows exceptional enrichments of Zr, REE, Th, and U.