Potential Chronological Disturbance of the D’Orbigny Angrite Inferred from Discordant 26Al Ages

Abstract

Angrites originate from the early-formed differentiated angrite parent body. The pristine volcanic angrite D’Orbigny is devoid of brecciation, shock effects, or any evidence of secondary processes and is thus key for studying the early stages of planetary accretion and differentiation. However, chronometers used to establish the formation chronology of angrites (including D’Orbigny) yield discordant ages, either (i) suggesting that secondary processes could have disturbed the apparent formation ages or (ii) being taken as evidence of heterogeneous distribution of ^26 Al in the early solar system. Yet spinel is minimally susceptible to secondary parent body processes and therefore a reliable target for establishing precise ^26 Al– ^26 Mg ages. Here, we present the first in situ ^26 Al– ^26 Mg analyses of spinel and plagioclase in D’Orbigny. Individual mineral assemblages provide distinct ages: olivine–spinel shows a well-defined isochron with an initial Al ratio ([ ^26 Al/ ^27 Al] _i ) of (5.39 ± 0.85) × 10 ^−6 , indicating formation at 2.35 $\tfrac{+0.25}{-0.22}$ Myr after the formation of calcium–aluminum-rich inclusions (CAIs), whereas plagioclase–olivine defines an isochron with [ ^26 Al/ ^27 Al] _i = (7.46 ± 1.87) × 10 ^−7 , implying formation at 4.40 $\tfrac{+0.44}{-0.38}$ Myr after CAIs, consistent with previous MC-ICP-MS studies. This temporal gap can be attributed to secondary processes such as metamorphic or impact-generated diffusion. Thus, D’Orbigny and other angrites do not represent an immaculate anchor for chronometric comparison. This complexity should be considered in future studies, especially when using D’Orbigny as an anchor to discuss the chronology of the early solar system.

Keywords

• Cosmochronology
• Achondrites
• Isotopic abundances
• Meteorites