IRON ISOTOPES REVEAL A HIGH TEMPERATURE ORIGIN FOR MAIN-GROUP PALLASITES

The origin of pallasites has been a matter of debate. Studies have shown that olivine and metal are in chemical equilibrium, lending support to the idea that they sample the core-mantle boundary of differentiated asteroids. However, recent thermal studies have suggested that they could be impact-induced samples (Yang et al., 2010). Iron isotopes have been used to argue for a low temperature re-equilibration event as the measured metal-olivine fractionation in some pallasites are large, reflecting equilibrium temperatures as low as 300 °C (Chernonozhkin et al., 2016).

Taking a closer look at existing data, it can be seen that for the same pallasites, the measured δ⁵⁶Fe values in the metal fractions span a range of 0.3‰ across laboratories, calling into question the use of iron isotopes as a reliable thermometer. However, an alternative explanation for this discrepancy is that the metal fractions analyzed do not represent that of bulk Fe-Ni metal, but contains some troilite (~-0.4‰; this study), schreibersite (~+0.2‰; Weyer et al., 2005), and kinetically fractionated taenite (~+0.1‰; Dauphas, 2007).

To re-evaluate this issue, we analyzed clean Fe-Ni metal fractions in 7 main-group pallasites by microdrilling into regions that are homogenous in Fe-Ni concentrations, guided by X-ray maps. Olivine crystals adjacent to the Fe-Ni metals were also microdrilled. These powders were then dissolved, purified, and analyzed using MC-ICPMS. Our results show that Δ⁵⁶Fe_{metal-olivine} spans a small range from 0.01 to 0.05‰ (±0.06‰, 2σ). Assuming Fe isotopic equilibrium, we used NRIXS data for olivine (Dauphas et al., 2014) and for Fe-metal (hcp; Shahar et al., 2016) to estimate metal-olivine equilibration temperatures. Our data are inconsistent with metal-olivine equilibration temperatures below 1000 °C.