DHOFAR 125

A single stone covered with black fusion crust and weighing 2,697 g was found in the Dhofar region of Oman. Sometime later, a 354 g stone named Dhofar 312 was recovered ~500 m away; analyses indicate that they may be paired. Dhofar 125 is a highly equilibrated acapulcoite having a recrystallized texture with 120° triple junctions. Dhofar 125 was the largest acapulcoite when found, and is composed of low-Ca pyroxene (35.6 vol%), olivine (27.5 vol%), plagioclase (14.0 vol%), troilite (7.7 vol%), augite (6.5 vol%), FeNi-metal (5.3 vol%), and minor chlorapatite and chromite. While the oxygen isotope, compositional, and thermal equilibrium data are consistent with the other acapulcoites, the average grain size in Dhofar 125 is significantly smaller than that of other acapulcoites. Dhofar 125 has a shock stage of S1 and a weathering grade that varies from W1 in the core to W2 near the rim.

The thermal history of the acapulcoites was explored in a study by El Goresy et al. (2005), in which they examined the diversity of isotopic compositions of graphite in the Acapulco meteorite. They identified various accessory graphite morphologies having a number of distinctive C- and N-isotopic signatures, which inferred an origin from at least three pre-accretionary reservoirs. The graphite form which contains the lightest N, the exsolution veneers between kamacite and taenite, is thought to preserve the most primordial C- and N-isotopic signatures from pre-accretionary FeNi-metal. In addition, they found rare graphite grains within metal–sulfide spherules with isotopic compositions similar to those of graphite grains in the matrix, while the spherules themselves have a spatial distribution within silicates suggesting a formation by partial melting of matrix metal. Furthermore, the occurrence of entrapped orthopyroxene–troilite symplectites with quench textures supports a history of partial melting and in situ recrystallization, perhaps without melt migration.

The chondritic Hf–W ratio is consistent with minimal migration of partial melts. The Hf–W isochron provides an age of ~6 m.y. after CAI formation, and when this is coupled with Pb–Pb data, it indicates that cooling rates were more rapid than those of H6 chondrites, but similar to those of H4 chondrites (Kleine et al., 2007). Considering their similar cooling rates and the significantly higher thermal metamorphism experienced by acapulcoites compared to H4 chondrites, an earlier commencement of accretion and/or a smaller parent body size for the acapulcoites is a reasonable inference. Another possibility for the early rapid cooling of the acapulcoite parent body may be the fact that it experienced a collisional disruption early in its history forming sub-km- to multi-km-sized fragments, which eventually succumbed to gravitational reassembly.

Acapulcoites and lodranites formed on a common parent body and share similar thermal histories. The calculated age of formation for Dhofar 125 based on the Hf–W chronometer is 5 (±2) m.y. after Allende CAIs. In a comparison with other chronometers, the Hf–W age was shown to be older, possibly reflecting the onset of thermal metamorphism rather than peak temperatures (Touboul et al., 2006). Dhofar 125 has a CRE age of 5.7 m.y, which is within the narrow range of virtually all of the acapulcoites (4–7 m.y.). Similar to E chondrites, Dhofar 125 contains an excess of radiogenic 129Xe, which indicates the early incorporation of 129I into the rock (Patzer et al., 2003). They suggest that this noble gas data and other mineralogical similarities may indicate that a genetic relationship exists between the acapulcoite/lodranite and E chondrite parent bodies.

For more complete and current formation scenarios of the acapulcoite–lodranite parent body, visit the Monument Draw and Lodran pages. The Dhofar 125 specimen shown above is a 0.95 g interior slice.

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