Late metal-silicate separation on the IAB parent asteroid: Constraints from combined W and Pt isotopes and thermal modelling

TL;DR

This study uses W and Pt isotopes combined with thermal modeling to date metal-silicate separation on the IAB parent asteroid, revealing a core formation around 6 million years after CAIs and subsequent collisional events.

Contribution

It introduces a method combining W and Pt isotopes to accurately date metal-silicate separation in meteorites, accounting for neutron capture effects.

Findings

Metal-silicate separation occurred at 6.0 ± 0.8 Ma after CAI.

The IAB parent body experienced a major breakup between 10-14 Ma after CAI.

Thermal models suggest rapid accretion and internal heating by short-lived radionuclides.

Abstract

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 $\epsilon^{182}$W values obtained for the IAB iron meteorites range from -3.61 $\pm$ 0.10 to -2.73 $\pm$ 0.09. Correlating $\epsilon^{\mathrm{i}}$Pt with $^{182}$W data yields a pre-neutron capture $^{182}$W of -2.90 $\pm$ 0.06. This corresponds to a metal-silicate separation age of 6.0 $\pm$ 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 metal-silicate separation as a result of internal heating by short-lived radionuclides and accreted at around 1.4 $\pm$ 0.1 Ma after CAIs with a radius of greater than 60 km.