A single small stone weighing 15 g was found in the Dhofar region of Oman. Dhofar 378 is a fresh meteorite that was almost completely covered with black fusion crust. Analyses performed at the MaxPlanckInstitute in Germany, and the Chiba Institute of Technology in Japan, have revealed that this meteorite consists predominantly of ferroan clinopyroxenes (49 vol%) and plagioclase in the form of maskelynite (47 vol%). An unnamed mass of 209.1 g is reported to be paired.
Throughout this meteorite, large vesicles (~1 mm diameter) occur within areas of blackened maskelynite. This maskelynite exhibits flow textures and recrystallized fibrous, crystalline, needle-like textures (Mikouchi and McKay, 2003). It often exhibits zoning, from K-, Na-rich cores to Ca-rich rims. In addition, rare areas of K-, Na-rich clear glass are accompanied by quenched rims and recrystallized plagioclase, along with "bubble-like pores" (Ikeda et al., 2006). The presence of these petrographic features, which are similar in many respects to those observed in the martian meteorite NWA 4797 (Walton et al., 2009), is indicative of strong impact-shock effects. Experiments have demonstrated that these features are consistent with a shock force of 5575 GPa (Ikeda et al., 2006), heating to temperatures of >1,000°C, and a post-shock cooling rate of ~2.5°C/hour (Mikouchi et al., 2006). Some of the minor minerals present in Dhofar 378 include ilmenite, phosphates, hedenbergite, fayalite, silica, Ti-rich magnetite, sulfide, and pyroxferroite.
The chemistry and mineralogy of Dhofar 378 is consistent with its formation through a late-stage fractional crystallization process, and is quite similar to that of Los Angeles 001/002. A severe impact-shock event on Mars resulted in the maskelynization of plagioclase and other shock-induced features. In a subsequent event, at initially high temperatures, the recrystallization of some plagioclase grains ensued, promoted by slow cooling conditions. Although the Dhofar 019 shergottite has similar igneous textures, it is much more magnesian than Dhofar 378, and the two are not paired. Noble gas data also confirm that these two are not paired, in that Dhofar 019 has a CRE age of ~16.7 m.y. while that of Dhofar 378 is ~2 m.y.
Cosmic ray exposure ages have now been determined for many martian meteorites, and Mahajan (2015) compiled a chart based on the reported CRE ages for 53 of them. He concluded that together these 53 meteorites represent 10 distinct impact events which occurred 0.92 m.y., 2.12 m.y., 2.77 m.y., 4.05 m.y., 7.3 m.y., 9.6 m.y., 11.07 m.y., 12.27 m.y., 15 m.y., and 16.73 m.y.see his chart here. It was argued that Dhofar 378 was launched from Mars during the 2.12 m.y.-old impact event. In a subsequent review based on multiple criteria, Irving et al. (2017 [#2068]) made a new determination of the number of separate launch events associated with the known (101 at the time of their study) martian meteorites. They speculate that the number could be as few as twenty, and suggest that Dhofar 378 and at least 26 other enriched olivine-free mafic shergottites were probably ejected in a few separate impact events ~25 m.y. ago.
Utilizing SmNd chronometry, Nyquist et al. (2018) determined a crystallization age for Dhofar 378 of 157 (±23) m.y., which is within uncertainty of the age determined for samples of Los Angeles 001 and 002 (172 [±8] m.y. and 173 [±24] m.y., respectively). The εNd values for the three meteorite samples are also identical within error (6.5 [±0.3], 6.3 [±0.2], and 6.1 [±0.6], respectively). In addition, RbSr age data were calculated for all three meteorites and found to be similar among them within uncertainties, corresponding to ages of ~159 m.y., 165 (±7) m.y., and 158 (±15) m.y. for Dhofar 378, Los Angeles 001, and Los Angeles 002, respectively. The ArAr age calculated for Dhofar 378 of ~141 m.y. probably dates the severe impact-induced shock-heating event, while various scenarios have been proposed to explain how this chronometer avoided being reset at the time of ejection ~2 m.y. ago (Park and Bogard, 2006). Noble gas data (21Ne and 38Ar) for Dhofar 378 was obtained by Park and Nagao (2006) to ascertain the CRE age. Their results suggest that this meteorite may have been ejected in a different impact event from that of either Los Angeles or the compositionally similar Ksar Ghilane 002 shergottite.
Satake et al. (2010) utilized the MELTS software program to better understand the parental magma composition of the enriched shergottites Dhofar 378 and Los Angeles, both of which are plagioclase-rich melt rocks. They determined that the starting melt composition of Dhofar 378 and Los Angeles could have been derived from the residual melt of the non-poikilitic areas within the enriched poikilitic shergottite NWA 4468 at 1090°C, suggesting that all these shergottites might share a common magmatic origin.
The specimen of Dhofar 378 shown above is a 0.11 g partial slice containing dark areas and veins composed of maskelynite. Along the top of the specimen the fusion crust is interrupted by a mass of clear glass. The photo below shows a larger slice of this unique martian meteorite, courtesy of S. and E. Haiderer.
∗ Recent geochemical research on the martian shergottites has led to new petrogenetic models and classification schemes. read more >>