Abstract
AbstractThe Emmaville eucrite is a relatively poorly studied basaltic achondrite with an anomalous oxygen isotope signature. In this study, we report comprehensive mineralogical, petrographic, and geochemical data from Emmaville in order to understand its petrogenesis and relationship with the basaltic eucrites. Emmaville is an unusually fine‐grained, hornfelsic‐textured metabasalt with pervasive impact melt veins and mineral compositions similar to those of typical basaltic eucrites. The major and trace element bulk composition of Emmaville is also typical of a basaltic eucrite. Three separated individual lithologies were also analyzed for O isotopes; a dark gray fraction (E1), a shocked lithology (E2), and a lighter gray portion (E3). Fractions E1 and E2 shared similar O isotope compositions to the bulk sample (E‐B), whereas the lighter gray portion (E3) is slightly elevated in Δ17O and significantly elevated in δ18O compared to bulk. No evidence for any exogenous material is observed in the thin sections, coupled with the striking compositional similarity to typical basaltic eucrites, appears to preclude a simple impact‐mixing hypothesis. The O‐isotopes of Emmaville are similar to those of Bunburra Rockhole, A‐881394, and EET 92023, and thus distinct from the majority of the HEDs, despite having similarities in petrology, mineral, and bulk compositions. It would, therefore, seem plausible that all four of these samples are derived from a single HED‐like parent body that is isotopically distinct from that of the HEDs (Vesta) but similar in composition.
Highlights
It has long been known that certain basaltic achondrites share similarities with eucrites
We conclude the following based on our studies: 1. Mineral compositions of Emmaville are consistent with basaltic eucrites 2
Oxygen isotopes of separated lithologies of Emmaville are heterogeneous where the dark gray and shocked lithologies plot near the bulk composition and the light gray lithology being slightly enriched in d17O and significantly enriched in d18O 4
Summary
It has long been known that certain basaltic achondrites share similarities with eucrites. These eucrite-like achondrites have distinct isotopic compositions and petrologic characteristics indicative of formation on a separate parent body from the HED clan (e.g., Ibitira, Northwest Africa [NWA] 011; Yamaguchi et al 2002; Wiechert et al 2004; Floss et al 2005; Greenwood et al 2005; Mittlefehldt 2005). Others show smaller isotopic variations but are otherwise petrologically and compositionally indistinguishable from basaltic eucrites, such as Pasamonte (Wiechert et al 2004; Greenwood et al 2005; Scott et al 2009).
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