DHOFAR 125


Acapulcoite
Acapulcoite–Lodranite Clan
standby for dhofar 125 photo
Found January 26, 2000
18° 59.20' N., 54° 36.03' E.

A single stone covered with black fusion crust and weighing 2,697 g was found in the Dhofar region of Oman. Sometime later, the 354 g Dhofar 312 was recovered ~500 m away; analyses indicate they might be paired. Dhofar 125 is a highly equilibrated acapulcoite having a recrystallized texture with 120° triple junctions. Dhofar 125 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-isotopic, 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 indicated an origin from at least three pre-accretionary reservoirs. The form of graphite that contains the lightest N, which exists as 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.

Hidaka et al. (2013) report a nearly chondritic composition for this and other acapulcoites with respect to the non-magnetic component. The chondritic Hf–W ratio is consistent with minimal migration of partial melts. The Hf–W isochron provides a well-defined age of 4.5631 (±0.0008) b.y., or ~6 m.y. after CAI formation. 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). When the Hf–W chronometry is coupled with Pb–Pb data (4.5559 [±0.0006] b.y.), 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 that it experienced a collisional disruption early in its history forming sub-km- to multi-km-sized fragments, which eventually succumbed to gravitational reassembly.

Similar to E chondrites, Dhofar 125 contains an excess of radiogenic 129Xe, which is indicative of early incorporation of 129I into the rock (Patzer et al., 2003). It was suggested that this noble gas data and other mineralogical similarities between these meteorite groups indicated a genetic relationship exists. An I–Xe age of 4.5571 (±0.0005) b.y. was determined for Dhofar 125 (Schönbächler et al., 2014). The meteorite 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.).

Notably, the H4 chondrite GRV 020043 is both mineralogically and O-isotopically most similar to primitive achondrites and may represent the chondritic precursor of the primitive acapulcoite–lodranite clan (Li et al., 2010). The differences that do exist, such as in the elements V, Cr, and Se, may be related to specific characteristics of the precursor material (Hidaka et al., 2012). Conversely, based on siderophile element abundances in magnetic components and lithophile element abundances in non-magnetic components, Hidaka et al. (2012) concluded that the precursor material of the acapulcoite–lodranite clan was most similar to EL chondrites. Regardless of the specific chemical composition of the precursor, acapulcoites and lodranites formed on a common parent body and share similar thermal histories. 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. The photo below is an excellent petrographic thin section micrograph of Dhofar 125, shown courtesy of Peter Marmet.

standby for dhofar 125 ts photo
click on photo for a magnified view
Photo courtesy of Peter Marmet