Timing of metal–silicate differentiation in the Eagle Station pallasite parent body

Abstract

The time of the metal–silicate differentiation of the Eagle Station pallasite (ESP) parent body was investigated using the 26Al–26Mg short-lived chronometer (half-life of 0.72Myr). The Mg isotope ratios were measured in ESP olivines by both MC–SIMS and HR-MC–ICPMS, allowing us to check the consistency between the results given by two different analytical protocols and data reduction processes. Results show that the two datasets are consistent, with a (δ26Mg*)av. value of –0.003 (±0.005)‰ (2 s.e., n=89). Such a value, associated with data from the 182Hf–182W short-lived systematics (half-life of 8.9Myr), indicates an ESP parent body metal–silicate differentiation occurring most likely at least at ∼2Ma, but possibly 4Ma, after CAI formation. From the 27Al/24Mg ratios measured in ESP olivines using MC–SIMS, the duration of the olivine crystallization process was inferred to have lasted over ∼275kyr if the core has differentiated as early as 2Ma after CAIs, while in the case of a core differentiation occurring 4Ma after CAIs, the silicate–silicate differentiation should have lasted for another 4Myr.