Metamorphic transformations of opaque minerals in some eucrites

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Eucrites have undergone varying degrees of metamorphism on their parent body. We investigated mineralogical constraint on the eucrite metamorphism, especially from opaque mineralogy. Three eucrites with distinct degrees of metamorphism were studied : Y-75011,84 is the least equilibrated basaltic clast, Juvinas shows complex histories of shock and thermal metamorphism, and A-881388 is an extensively annealed and unbrecciated. Three eucrites with different metamorphic histories show distinct opaque mineralogy. In Y-75011,84,ilmenite and troilite are found crystallized in the mesostasis and opaque precipitates within pyroxenes are barely recognized. Juvinas has complicated texture, consisting of a variety of lithologies : Minute precipitates within pyroxene area and recrystallized mesostases. In A-881388,during extensive prolonged annealing episode, opaque phases, such as ilmenite, troilite, and chromite are almost completely separated from silicate phases and segregated, resulting in a formation of opaque nodules. Several geothermometers, experimental results and the textural appearance of A-881388 suggest that it might have slowly cooled from near 1000-1050℃ to ∿800℃ during a prolonged annealing episode. While chromite is not observed in Y-75011,84,Juvinas and A-881388 contain chromite in recrystallized mesostases and in an nodule respectively. This implies that chromite could be a metamorphic product.
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