Exogenous copper sulfide in returned asteroid Itokawa regolith grains are likely relicts of prior impacting body

Katherine D. Burgess,Rhonda M. Stroud

doi:10.1038/s43247-021-00187-7

Katherine D. Burgess, Rhonda M. Stroud

Open Access

https://doi.org/10.1038/s43247-021-00187-7

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Abstract

Samples from asteroid 25143 Itokawa returned by the Hayabusa mission have been identified as LL4-6 ordinary chondrite materials and have shown it to be a rubble pile that aggregated after break-up of a parent body. Here we investigate particle RB-CV-0038 from the Itokawa regolith using scanning and transmission electron microscopy and energy dispersive spectroscopy. We identify a cubanite-chalcopyrite-troilite-pyrrhotite assemblage, the phases and structure of which are indicative of low-temperature, aqueous alteration. Cubanite is stable only at temperatures below around 250 °C and has thus far only been identified in CI carbonaceous chondrites and the comet 81P/Wild2 sample suite. Chalcopyrite is also very rare in the meteorite record and is found mostly in R chondrites and some CK chondrites. Because the Itokawa parent body experienced significant thermal alteration with little evidence of low-temperature equilibration or aqueous alteration, we propose that the assemblage we identify is most likely exogenous and represents a component of an impacting body.

Highlights

Samples from asteroid 25143 Itokawa returned by the Hayabusa mission have been identified as LL4-6 ordinary chondrite materials and have shown it to be a rubble pile that aggregated after break-up of a parent body
We conclude that this grain, and the other Cu-sulfides noted on Itokawa, likely originated in a primitive, hydrated asteroid source such as a CI chondrite-like parent body or D- or P-type asteroids, which have been proposed as the source of other unique meteorites and microxenoliths that experienced lowtemperature aqueous alteration[20,21]
There is a large conchoidal fracture on the olivine, as well as some wavy and stepped surfaces[3], and the scanning transmission electron microscopy (STEM) data show a lack of any distinct space weathering rim

Summary

Introduction

Samples from asteroid 25143 Itokawa returned by the Hayabusa mission have been identified as LL4-6 ordinary chondrite materials and have shown it to be a rubble pile that aggregated after break-up of a parent body. We identify a cubanite-chalcopyrite-troilite-pyrrhotite assemblage, the phases and structure of which are indicative of low-temperature, aqueous alteration. We present data from Itokawa particle RB-CV-0038 showing it contains a cubanite-chalcopyrite-pyrrhotite-troilite assemblage consistent with low-temperature aqueous alteration. Such an assemblage is inconsistent with the thermal metamorphism experienced by the Itokawa parent body and the material must have become part of Itokawa after thermal alteration. We conclude that this grain, and the other Cu-sulfides noted on Itokawa, likely originated in a primitive, hydrated asteroid source such as a CI chondrite-like parent body or D- or P-type asteroids, which have been proposed as the source of other unique meteorites and microxenoliths that experienced lowtemperature aqueous alteration[20,21]

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