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PI : Jemma Davidson

Title : An O-isotopic study of chromite grains from asteroid Itokawa: Constraining formation and alteration conditions

Abstract

The goal of this proposed study is to maximize the information gained from individual minerals, specifically chromite (┬▒Cr-rich spinel) and co-existing olivine, with the aim of increasing the scientific value of Hayabusa-returned samples from asteroid 25143 Itokawa. Chromite, a spinel mineral, records both pre- and post-accretionary histories of a parent asteroid. The elemental compositions, morphologies, and O-isotopic compositions of chromites are determined by their primary formation conditions (temperature, time, oxygen fugacity) and secondary alteration conditions (e.g., thermal metamorphism and shock). Chromite elemental and isotopic compositions vary between meteorite groups, and morphologies change during thermal metamorphism. We will investigate the compositional, morphological, and O-isotopic characteristics of chromite (┬▒olivine) mineral grains in Itokawa samples returned by the Hayabusa mission and compare these characteristics with those of chromite grains in a suite of ordinary and carbonaceous chondrites. Previous work on Hayabusa-returned Itokawa samples shows that they are consistent with LL4 to 6 ordinary chondrite material, implying that they have experienced varying degrees of thermal metamorphism. We will compare the compositions, textures, and O-isotopic compositions of chromites from LL3 to 6 chondrites with those from chromite-bearing Hayabusa samples via electron microscopy (quantitative analysis, electron imaging) and secondary ion mass spectrometry (O-isotope analysis), to investigate correlations between formation mechanism, oxygen fugacity, and formation/alteration temperatures (using olivine-spinel geothermometry). We will also compare with the characteristics of chromite from carbonaceous chondrites to identify potential material from foreign impactors. By studying chromite in Hayabusa particles, we will constrain the parent body formation and alteration conditions of asteroid 25143 Itokawa.

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