An anomalous eucrite, Dhofar 007, and a possible genetic relationship with mesosiderites

Abstract

Abstract-We studied the texture, mineralogy, and bulk chemical composition of Dhofar 007, a basaltic achondrite. Dhofar 007 is a polymict breccia that is mostly composed of coarse-grained granular (CG) clasts with a minor amount of xenolithic components, such as a fragment of Mg-rich pyroxene. The coarse-grained, relict gabbroic texture, mineral chemistry, and bulk chemical data of the coarse-grained clast indicate that the CG clasts were originally a cumulate rock crystallized in a crust of the parent body. However, in contrast to monomict eucrites, the siderophile elements are highly enriched and could have been introduced by impact events. Dhofar 007 appears to have experienced a two-stage postcrystallization thermal history: rapid cooling at high temperatures and slow cooling at lower temperatures. The presence of pigeonite with closely spaced, fine augite lamellae suggests that this rock was cooled rapidly from higher temperatures (>0.5 °C/yr at ∼1000 °C) than typical cumulate eucrites. However, the presence of the cloudy zone in taenite and the Ni profile across the kamacite-taenite boundaries indicates that the cooling rate was very slow at lower temperatures (∼1-10 °C/Myr at <600-700 °C). The slow cooling rate is comparable to those in mesosiderites and pallasites. The two-stage thermal history and the relative abundance of siderophile elements similar to those for metallic portions in mesosiderites suggest that Dhofar 007 is a large inclusion of mesosiderite. However, we cannot rule out a possibility that Dhofar 007 is an anomalous eucrite.

Asteroid (4) Vesta: I. The howardite-eucrite-diogenite (HED) clan of meteorites

“…Preliminary data on unbrecciated cumulate gabbro EET 92023 and basaltic achondrite Emmaville, both classified as eucrites, show that they have O isotope compositions that are close to those of A-881394 and Bunburra Rockhole . Dhofar 007 is a polymict breccia composed mostly of cumulate gabbro debris and is rich in metal and siderophile elements (Dale et al, 2012;Yamaguchi et al, 2006). It contains materials with differing 17 O within the range found for Bunburra Rockhole .…”

40Ar/39Ar dating and cosmic‐ray exposure time of desert meteorites: Dhofar 300 and Dhofar 007 eucrites and anomalous achondrite NWA 011

“…Although initially classified as monomict, Yamaguchi et al (2003) found xenolithic components in Dhofar 007 (an impact melt clast, a Mg-rich orthopyroxene fragment, and recrystallized plagioclase), thereby demonstrating the polymict nature of this rock. They also demonstrated that Dhofar 007 has a high abundance of FeNi metal grains and a very high bulk Ni and Co contents compared with other eucrites.…”

“…The abundance of FeNi metal varies from one clast to another. Some clasts are very rich in metal (see also Yamaguchi et al 2003). Veins of impact melt discordantly cross the meteorite, causing local darkening.…”

“…Based on mineralogical and petrographic observations and correlations of Mg# and Sm content, Afanasiev et al (2000) showed that Dhofar 007 is a cumulate eucrite, while Yamaguchi et al (2003) discussed pros and cons of an origin from the mesosiderite or eucrite parent body. Although initially classified as monomict, Yamaguchi et al (2003) found xenolithic components in Dhofar 007 (an impact melt clast, a Mg-rich orthopyroxene fragment, and recrystallized plagioclase), thereby demonstrating the polymict nature of this rock.…”

Oxygen isotopic constraints on the origin and parent bodies of eucrites, diogenites, and howardites

“…However, a ten-fold increase in the precision of ∆ 17 O measurements has allowed the angrites and main-group pallasites to be clearly resolved from HEDs (Greenwood et al, 2005(Greenwood et al, , 2008b. Whether the mesosiderites and HEDs, which have overlapping oxygen isotopic compositions, come from the same or closely related bodies is not known with any certainty (Rubin and Mittlefehldt, 1993;Greenwood et al, 2006;Yamaguchi et al, 2006). Oxides in IIIAB irons have not yet been analyzed with high precision, but these irons are unlikely to come from the HED or main-group pallasite body as they cooled faster than main group pallasites underneath a layer of silicate that was only a few kilometers thick (Goldstein et al, 2009).…”