Primitive achondrite, ungrouped
Found January 24, 2000
18° 23.2' N., 54° 12.2' E.
A single stone weighing 116 g was found in the desert region of Dhofar, Oman. Analysis was conducted at the Vernadsky Institute, Moscow (Lorenz and Ivanova), and it was classified as an ungrouped primitive achondrite. This is a highly metamorphosed, recrystallized, polymict breccia, consisting of three separate lithologies within a pyroxene–plagioclase matrix:
Minor constituents include Ca-rich augite, Cr-rich troilite (20 vol% in ol-rich lithology), chromite (pyx–plag lithology), and rare FeNi-metal. The modal content of the combined metallic component is only 0.3%, significantly less than the acapulcoite/lodranite clan and the winonaite–IAB iron complex meteorites. The Cr-enrichment of troilite suggests formation occurred in a more reducing environment than most other primitive achondrites. This meteorite has been significantly weathered (W4) and shocked (S4).
The olivine and orthopyroxene compositions of Dhofar 500 are similar to those of the winonaite–IAB iron complex meteorites, while the olivine also shows similarities to the ureilites (Lorenz et al., 2003). However, pyroxene has a chemical composition that differs from other achondrites; i.e., it is more magnesian and higher in TiO2 than that of the acapulcoite/lodranite clan, ureilites, and brachinites. In addition, the Fe/Mn ratio in pyroxene is close to that of ordinary chondrites, but differs from that of the acapulcoite/lodranite clan and ureilites. Although Dhofar 500 has a high Cr content in troilite similar to that of ureilites, possibly a result of reduction processes, its higher plagioclase content and more limited silicate compositional range distinguish it from ureilites.
Oxygen isotope studies have determined that Dhofar 500 does not plot among any of the known meteorite groups; however, it does plot near the trend lines of both the silicate component of the winonaite–IAB iron complex meteorites and the ureilites. Notably, Dhofar 500 has olivine and troilite compositions that are similar to those of polymict ureilites, but without a carbon-rich matrix.
The Ar–Ar age calculations for Dhofar 500 reflect a complex history with ages of 4,454 (±11) m.y. and 4,307 (±11) m.y., consistent with the range of ages for winonaites and silicates from IAB complex irons and some ordinary chondrites (Korochantseva et al., 2011). Another partial degassing event is indicated at an age younger than 3.2 b.y.
The photo of Dhofar 500 shown above is a 0.43 g slice with fusion crust. The photo below is an in situ view courtesy of the Comet Meteorite Shop.
