"Rock of the Month # 104, posted February 2010" ---

The Dhofar 485 meteorite from Zufar, Oman, found in January 2001, total known weight 1554 grams. Note the mm-scale angular clasts of igneous lithologies in a grey impact-hardened matrix. Sample procured by Blaine Reed.

The figured sample, above left, is a 2.8-g slice of pale grey breccia, 23x17x2 in size, containing medium grey angular mm-scale clasts and trace black chromite. A polished thin section was prepared from this slice. The photomicrograph at right depicts the clearly-ophitic texture of plagioclase and clinopyroxene in a mm-scale eucrite clast. Magnification 50x, long-axis field of view 1.7 mm, in plane-polarized transmitted light. Larger crystals in the granular matrix of the clasts are clinopyroxene and plagioclase, plus lesser orthopyroxene, including orthopyroxenite (diogenite) clasts. Accessory phases include troilite, chromite, secondary goethite, trace kamacite (up to 50 µm) and Fe-Ti oxides (ilmenite and hematite).

Basaltic eucrite clasts, as large as 4x4 mm, are often darkened, as if composed in part of devitrified glass enclosing feldspar crystallites. A larger number are holocrystalline clasts of variable aspect, e.g., 1) 70% plagioclase plus 30% clinopyroxene, texture intergranular rather than ophitic (thus, "microgabbro" rather than "diabase"), 2) 50:50 intergrown plagioclase and clinopyroxene, with classic ophitic texture, and 3) plagioclase-dominated clasts with trace interstitial fine-grained troilite. These eucrite fragments affirm diverse lithology in the source region of the breccia: glassy, feldspathic, intergranular and ophitic variants can be distinguished.

Dhofar 485 is a howardite, one of a class of meteorites interpreted as regolith breccias from the surface of asteroid 4 Vesta. In this sample, mm-scale clasts of gabbro, diabase and lesser pyroxenite lithologies are cemented in a dense, granular matrix. The broader HED achondrite family have been attributed to Vesta, one of the so-called basaltic asteroids located 2.2-2.5 astronomical units from Earth. Vesta, the fourth asteroid to be observed, was discovered in 1807. It has a diameter of ~555 km. A range of data collated from space missions, remote sensing technology and meteorite mineralogy and chemistry has led to this widely-accepted explanation (see, e.g., Drake, 1979; Binzel and Xu, 1993; Hiroi and Pieters, 1998; Drake, 2001; and Burbine et al., 2001).

References

Binzel,RP and Xu,S (1993) Chips off of asteroid 4 Vesta: evidence for the parent body of basaltic achondrite meteorites. Science 260, 186-191.

Burbine,TH, Buchanan,PC, Binzel,RP, Bus,SJ, Hiroi,T, Hinrichs,JL, Meibom,A and McCoy,TJ (2001) Vesta, Vestoids, and the howardite, eucrite, diogenite group: relationships and the origin of spectral differences. Meteoritics & Planetary Science 36, 761-781.

Drake,MJ (1979) Geochemical evolution of the eucrite parent body: possible nature and evolution of asteroid 4 Vesta? In `Asteroids' (Gehrels,T editor), University of Arizona Press, Tucson, 1181pp., 765-782.

Drake,MJ (2001) The eucrite/Vesta story. Meteoritics & Planetary Science 36, 501-513.

Hiroi,T and Pieters,CM (1998) Origin of Vestoids suggested from the space weathering trend in the visible reflectance spectra of HED meteorites and lunar soils. Antarctic Meteorite Research 11, National Institute of Polar Research, Tokyo, 209pp., 163-170.